Significance of Various Sensing Mechanisms for Detecting Local and Atmospheric Greenhouse Gases: A Review

Elucidating the capital mechanism for detecting greenhouse gases (GHGs) in the atmosphere, based on sensitivity, performance, and cost‐effectiveness, is challenging, but markedly needed in the presence of global climate change caused by GHG emissions and subsequent feedback. Often measured in units of Global Warming Potential (GWP), the GHGs are linked to climate change, especially due to their intrinsic tendencies to absorb heat energy. Hence, measures for reducing GHG emissions are implemented within the context of improving energy consumption; substituting high‐GHG output fuels for more neutral alternatives; trapping and sequestering carbon; and reconditioning agricultural processes. The extent to which these curtailment methods succeed hinges on GHG detection and quantification mechanisms. However, the universal determination of GHGs is constrained by the availability of sensors; this work, therefore, highlights sensor advantages/disadvantages and potential enrichment strategies. Herein, experimental developments in GHG sensing technologies (i.e., chemiresistive, electrochemical, infrared, optical, acoustic, calorimetric, and gas chromatographic sensors) are evaluated, in terms of approaching desirable features, such as sensitivity, selectivity, stability, accuracy, and low cost. This work underscores ongoing global research to produce universal, cost‐effective methods that, with high sensitivity, proffer accurate GHG readings to allay global warming, through comparisons of recent, up‐and‐coming sensor technologies.


Introduction
As various geological records demonstrate clearly, Earth's temperature and climate vary widely.The greenhouse effect is a natural phenomenon that occurs when gases in Earth's atmosphere trap the heat emitted at Earth's surface. [1]A significant part of the trapped energy is radiated back toward the surface, resulting in much warmer surface temperatures than those that would occur without the greenhouse effect.The greenhouse gases (GHGs) in the troposphere cool the stratosphere by the amount of the energy re-radiated back to the surface.Conversely, the temperature of the atmosphere increases due to the absorbed energy.Electromagnetic waves facilitate the transfer of heat in a patent process that is commonly referred to as radiative heat or energy transfer, whereby, in referring to the dynamics that transpire between the Earth and atmosphere, a greater concentration of GHGs stimulates terrestrial infrared absorption to result in more warming.Interactions between GHGs and the climate.Reproduced with permission. [6]Copyright 1994, The Royal Swedish Academy of Sciences.
Without the greenhouse effect, the global mean temperatures would drop below −18 °C. [2]During the ≈800 000 years before a spike in GHG emissions occurred globally, the total amount of carbon dioxide (CO 2 ) in the atmosphere has ranged between ≈160 to 290 parts per million (ppm), whereas methane (CH 4 ) and nitrous oxide (N 2 O) have ranged between 400 -800 parts per billion (ppb) and 200 -305 ppb, respectively. [3]Since the outset of the industrial era, there has been a surge in CO 2 , CH 4 , and N 2 O levels, respectively, from 280 ppm, 800 ppb, and 270 ppb to over 400 ppm, 1800 ppb, and 320 ppb.Indeed, according to the Intergovernmental Panel on Climate Change (IPCC), since 2011 (measurements reported in AR5), GHG concentrations have continued to increase in the atmosphere, reaching annual averages of 410 ppm for CO 2 ; 1866 ppb for CH 4 ; and 332 ppb for N 2 O in 2019 (Figures 1 and 2(i)). [4]The continued increase in the globalmean temperature has led to scientists generating predictions, pursuant to the AR6 WG1 Summary for policymakers, that the global surface temperature will continue to increase until at least mid-century under all the emissions scenarios considered. [5]As the climate crisis intensifies, it is important to scientifically calculate global GHG emissions at a time when international cooperation and policy promotion, such as the Paris Agreement, are being effectuated.
CH 4 , which is short-lived but potent, is emitted during the production and transport of coal, natural gas, and oil and with certain agricultural practices.Agriculture, land use, industrial activities, and the combustion of fossil fuels and solid waste are also important activities leading to the production of N 2 O (Figure 1).Artificial chemicals commonly used in refrigeration and air conditioning, fire extinguishing, foam production, and medical aerosols include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF 6 ) and nitrogen trifluoride (NF 3 ).The halocarbons (chlorofluorocarbons: CFCs, hydrochlorofluorocarbons: HCFCs, HFCs, and PFCs) are good absorbers of infrared radiation (IR), and many of them absorb energy in the wavelength region where such energy absorption does not occur in CO 2 and water vapor (H 2 O(v)) (within the atmospheric window).The increase in temperature with altitude is caused by the absorption of the Sun's ultraviolet (UV) radiation by the ozone (O 3 ) layer.The radiative effect of CO 2 and H 2 O(v) absorbs radiant energy emitted from the planet's surface without diffusing to the outside and cools the stratosphere by the amount of absorbed energy.Conversely, the temperature of the atmosphere increases due to the absorbed energy.
Although methods to estimate GHG concentrations can be quite accurate, it is difficult to minimize the atmospheric concentrations of GHGs due to the lifetime and reactivity of each gas.National policies often do not contain stipulations for reducing their concentrations.Moreover, current emissions sometimes cannot accurately reflect the amount of naturally occurring GHGs, and only anthropogenic quantities are reflected.Despite that, the amount of naturally occurring GHGs will intensify due to global warming.In addition, since it is currently difficult to obtain accurate data, actual measured GHG concentration information is required to be derived through a real-time, 3D, and low-cost observation network.As the climate crisis deepens, it is important to scientifically calculate global GHG emissions at a Figure 2. In i,ii), components contributing to climate change and their expected consequential influences on organisms to biomes; in iii), UNEP published facts on the pre-and post-industrial GHG concentrations, as well as their sources and GWPs, where the GWP of CO 2 is 1; iv), natural and anthropogenic causes for GHG emissions and climate change are defined.Diagrams in (i) and (ii) adapted with permission. [27]Copyright 2021, Elsevier Inc.Data of (iii) are collected. [28]Copyright 2011, GRID-Arendal and SMI books.Diagram in (iv) reproduced with permission. [27]Copyright 2021, Elsevier Inc.
time when international cooperation and policy promotion, such as the Paris Agreement, unfold.
Currently, there are several distinct methods of measuring concentrations of CO 2 [5,7] or other GHGs in the atmosphere, among them being: infrared analysis and manometry; titrimetry; Fourier transform spectrometry; and methods that involve chemiresistive, electrochemical, non-dispersive infrared, optical, acoustic, calorimetric, or gas chromatographic sensors.However, the limitations of sensors commonplace in GHG detection have constrained their universal determination, and, in order to facilitate the development of a brand-new technology, this review highlights the limitations and potential enrichment strategies for refining the existing technologies.While GHG concentrations are an important factor in global climate change, current GHG concentration monitoring equipment is expensive, bulky, and installed only in some areas.In many instances, the amount of GHGs that are generated is easy to reduce, although the nature of emissions remains unclear, and an active discussion of GHG emissions in policies or systems is warranted.
In the existing observation network of GHGs in some areas, surface detection through towers is being implemented, and while the configuration of this network is tailored to emission calculations, there is a limit to real-time responses.A global observation network will reinforce model validation and improvement and minimize observation gaps following a least-cost approach.Due to deficiencies marring responses to the urgent public environmental concern, i.e., global warming, the National Institute of Standards and Technologies (NIST) has instituted a GHG Measurements Program that employs high-tech aircraft and satellites to accurately compute the atmospheric level of GHGs. [8]lobal GHG monitoring satellites and stations are considered "top-down" methods for determining the total accumulation of GHGs in the atmosphere, although ground-based networks, established globally by different organizations, including the To-tal Carbon Column Observing Network (TCCON), and others operated by the US National Oceanographic and Atmospheric Administration (NOAA) or the Advanced Global Atmospheric Gases Experiment (AGAGE), often provide most accurate results.[9] One reason for this is the inability of satellites to pinpoint the sources of GHGs from space, the periodic residence of these satellites over landmasses, and obstructions caused by clouds or other barriers.[9] However, there are still extensive blind spots, as underdeveloped and developing regions are more likely to produce high GHG emissions, despite being undetectable by far-off groundbased stations or networks.There is still much work that must be done to provide an international, coordinated effort in order to monitor illegal (such as CFC-11 under the Montreal Protocol) and polluting GHGs, especially in counteracting or reinforcing the sometimes skewed observations obtained through "bottomup" reporting.[9] Several academic or independent researchers are studying the efficacy of potentially cost-effective GHG-detecting systems that utilize nanomaterials in combination with resistance, optoacoustic, surface acoustic wave (SAW), wireless, field effect transistor, or micromechanical measurement devices.[10] All-inclusive technologies that will monitor the GHGs, analyze their prevalence trends, and produce output measurements sensitively, efficiently, and cheaply are desirable.They must withstand the stresses exerted by the GHGs in geographical regions where their accumulation is excessive and provide accurate and rapid readouts.This is especially true in densely populated cities containing narrow spaces and artificial obstacles, where 70% of GHG emissions occur, and where the geographies are not amenable to receiving signals through functioning towers and other networks.Therefore, a low-cost sensor that is easy to install in cities and has no maintenance costs is required to be developed, and it must produce mixed gas information through the sensor.
In this regard, chemical-based, optical, metal oxide semiconductor, and electrochemical sensors, among others, have been profoundly researched. [10,11]En masse, the methods by which these remote sensors typically perform have been comprehensively elucidated in the literature, and they are predominantly founded on the use or manipulation of thermal infrared energy.In most cases, the thermal infrared energy that is absorbed by GHGs is received at a sensor system's collector, detector, or optical subsystem component and then translated into readable output by a computing device, which technically determines the type or concentration of atmospheric gas being detected. [12]HG sensors have been even more meticulously classed by sources that fully describe the modifiable analysis mechanisms using conventional instruments, including spectroscopes (of the types: Fourier transform infrared (FTIR), Raman, optical-based, and mass-based) and semiconducting materials. [13]More costeffective methods have been developed with the recognition of novel nanomaterials that approach the selectivity and sensitivity properties of these traditional devices. [11,14]Specifically, the more modern sensors that are examined in this review include those that are based on the micromechanical, SAW, optoacoustic, and ultrasound measurements; and various polymer or nanoparticulate materials integrated into these units for improving sensing versatility, function, and accuracy; as well as modern 3D bioprinting approaches, are elucidated.Experiential meth-ods for distinguishing atmospheric GHGs are explored by analyzing the different types of GHGs and highlighting the advantages/disadvantages of each technique.In particular, we outline the several different experimental strategies that are being investigated to detect GHGs efficiently, sensitively, globally, and at a low cost.

Natural and Artificial GHG Emission Sources
The greenhouse effect is inherently a temperature determinant that is perturbed by anthropogenic emissions, such as those caused by the utility of fossil fuels and fertilizers, the clearance of wastewater effluents, the cultivation of nitrogen-fixing crops, and the environmental occurrence of medical drugs or heavy metal contaminants, among others. [15]Combustion processes, such as those involved in the burning of coal, natural gases, and oils, necessarily lead to high emissions of CO 2 and also produce carbon monoxide (CO) and nitrogen oxides (NOx). [16]The production of low-level O 3 becomes consequentially possible upon the exposure of CO and NO x to sunlight (i.e., by photochemical reaction) or volatile organic compounds (VOCs), whereas CO hydrogenation generates CH 4. [16,17]   CH 4 and surface-level O 3 , respectively, are the second and third most commonly occurring GHGs, after CO 2 , and they impart a significant environmental burden. [18]Overall, the strain inflicted by the enhanced greenhouse effect is attributable to high concentrations of the natural GHGs, CO 2 , CH 4 , and NOx, as well as of the synthetic CFCs, which, although existing in minute amounts, are excessively potent due to their high global warming potentials (GWPs; ≈10 000 units). [19]The greatest atmospheric burden is caused by CO 2 , which, although being universally regarded as a reference gas with a GWP of 1, does account for over half of annual GHG emissions. [19]CH 4 (GWP ≈10 to 23) and the halocarbons (GWP >100), including the CFCs, levy most of the remaining environmental ramifications. [18,19]f the GHGs, those that are long-lived (e.g., CO 2 ) tend to exact climate forcing over extended periods of time, and they are not responsive to chemical or physical stimuli, unlike the short-lived and stimuli-responsive GHGs (e.g., H 2 O(v)) that produce continual temperature feedbacks. [20]CO 2 is a persistent GHG that is generated excessively in large volumes, and several sources produce varying yields of CO 2 , including natural events (e.g., volcanic eruptions and physiological respiratory processes), or industrial fossil fuel burning and deforestation efforts. [15]The residence time of CO 2 (>one century), however, is surpassed by those for the industrial GHGs (up to 50 000 years), especially SF 6 and the halocarbons, including CFCs, HFCs, and PFCs. [18]he fluorocarbons can be used in refrigeration units due to their easy liquid-to-gas, and vice versa, phase conversions, although CFCs, in particular, have been determined to substantially break down O 3 molecules.Although absorbing infrared light within their molecular structures and retaining extreme heat-trapping properties, HFCs are far less detrimental to the O 3 layer, causing far less O 3 depletion, than the CFCs. [3]H 4 and N 2 O are also persistent GHGs that emerge from several sources, including municipal solid waste, industrial waste, flammable gas, livestock, gas drilling, and coal mining for CH 4 ; and by fertilizers use, biomass burning, organic waste disposal, and automobile exhaust for N 2 O. [21] At least one-quarter of the anthropogenic leverage to global warming is attributed to CH 4 , which can be further oxidized to produce CO 2 by methanogenic bacteria (substantially occurring in soil-capped landfills).[22] Irrespective of this phenomenon, CH 4 traps more heat, often cited as 20 times greater, than CO 2 gas.[23] Among the landfill gases composed of several volatile organic compounds, the constitution of the VOC, CH 4 , is generally within the range of 40-60%, thereby implicating a tremendous impact of CH 4 on the production of photochemical smog.[24] Of the GHG emissions leading to climate change in the US, 3% are incurred by CH 4 emissions from municipal landfills, and, as a result, the Landfill Methane Outreach Program was created in 1994 by the US Environmental Protection Agency to curb the drastic consequence of landfill gases on the environment.[25] Despite being tremendously potent and having significant outputs, CO 2 and CH 4 are exceeded in heat-trapping ability by N 2 O.Although not present in the high quantities that CH 4 is within the atmosphere, N 2 O retains a long half-life (114 years, compared with only 12 years for CH 4 ), which, coupled with efficient heattrapping, accentuates the importance of truncating the amount of this GHG in the atmosphere.[26] Figure 2(i) figuratively illustrates the interdependence of climate, the health of the ecosystem, and human well-being; as are the components contributing to climate change, and their expected consequential influences on organisms to biomes, expanded in Figure 2(ii).[27] According to estimates produced by the United Nations Environmental Program (UNEP), values are given in Figure 2(iii) for the GHG pre-industrial and postindustrial eras in parts per million in volume (ppmv), along with the anthropogenic source and GWP.[28] Figure 2(ii) figuratively illustrates the interdependence of climate, the health of the ecosystem, and of human well-being.[27] The natural and anthropogenic contributions to climate change are further elucidated in Figure 2(iv).[27]

Radiative Forcing
From the standpoint of radiative forcing, which is a measure of the influence that alterations between incident solar radiation and outgoing IR have on the atmosphere, the halocarbons are also more potent than several naturally occurring GHGs.Generally, forcing can be positive or negative, whereby positive radiative forcing tends to heat the troposphere, whereas negative forcing tends to cool it.Halocarbon molecules can be thousands of times more efficient as absorbers of energy emitted by the earth than CO 2 molecules, for example, and small amounts of these gases can contribute notably to substantial effects of radiative forcing on climate systems.While the surface warming by the tropospheric GHGs cools the stratosphere, the radiative effects of halocarbons work to heat the upper troposphere and stratosphere.As the stratosphere is strongly stratified, substances present in the stratosphere tend to have long residence times.
In Figure 3, elements contributing to radiative forcing, or the energy transfer differences in the atmosphere, and their consequential effects, are outlined.The climate-forcing changes leading to temperature change include the GHGs, other air pollutants, aerosols, as well as natural events (i.e., volcanic eruptions, land use and agriculture, and solar irradiation) that create these and other residual environmental responses (Figure 3(i)). [29]Further, the radiative forcing incurred by anthropogenic or natural events, and the total net forcing due to anthropogenic activities, are presented in Figure 3 An increase in the discount rate from 0 to 0.015 generally correlates with an increased relative GHG GWP, especially, as the residence time of CO 2 tends to disproportionately decrease with rising discount rates.Equations for calculating the discount rate of different gases are presented in Figure 3(iv). [30]Figure 3(v) summarizes the early-2000′s GHG concentrations (in ppm, ppb, or parts per trillion (ppt)) of the troposphere and increasing fluxes in radiative forcing (in Watts per square meter (W m −2 )). [31]In Figure 3(vi-vii), annual mean radiative forcing, in W m −2 , due to aerosols (both direct and indirect), with contributions from sulfates, nitrates, black carbon (BC), and organic carbon (OC), is shown to peak in the 1980s-1990s and to gradually decrease until 2050.Figure 3(viii) further provides a plot with estimated radiative forcing due to CH 4 , the N 2 Os, CO, non-methane volatile organic compounds (NMVOCs), and others, divided into their contributions to radiative forcing produced by CH 4 (red) or O 3 (yellow). [32]

GHG Curtailment Strategies
Thus far, strategies to manage GHG emissions have been approached from a qualitative or administrative context, in that committees, like those within the United Nations Framework Convention on Climate Change (UNFCCC), have delineated methods or outcomes for industrialized countries to observe in order to limit their climactic impact. [33]For instance, stipulations by the UNFCCC have attempted to extract pledges from nations that would assist in averting a 2 °C global temperature increase.However, only 60% of this target was speculated to be attainable under the direction of those pledges that were proposed.A "green industrial revolution" will ramp up the applications of resources and international power in resolving climate failures, and this will involve, especially, technology transfers of low-emission substitutes to current methods that lead to the production of large quantities of GHGs.Technology Needs Assessments, or TNAs, were defined during the seventh session of the Conference of the Parties (COP), and two main stages are implicated within their framework. [34]Initially, TNAs must be performed, in order to identify the technologies or the actions that a country must adopt to earn the benefits of sustainable development while curtailing the atmospheric release of GHGs.In the next phase, these innovations must be inculcated as standard and customary practices that override the existing operations in a region.
Figure 3.In i), the effect of radiative forcing, as the result of GHGs and other pollutants, on the climate is outlined; ii), the radiative forcing (W m −2 ) that emerges from anthropogenic and natural events; iii), the discount rate versus the relative GWPs of several GHGs compared to CO 2 ; iv), simplified expressions and coefficients for calculating the radiative forcing produced by CO 2 (C), N 2 O (N), or CH 4 (M), where C 0 , N 0 , and M 0 are the initial concentrations, and X = 0.5(X+ X 0) ; v) the concentrations of GHGs in the troposphere (1750 vs. early 2000's), the atmospheric lifetime of the GHGs (in years), and the radiative forcing (W m −2 ); vi), direct contribution of aerosols to radiative forcing; vii), indirect contribution of aerosols to radiative forcing; and viii), the net estimated radiative forcings due to CH 4 , N 2 O, CO, NMVOCs, and others, divided into their contributions to radiative forcing GHGs cycle through stages of creation, extinction, and persistence, and since the cycle is expressed as changes in concentrations, continuous monitoring of these changes is necessary.The World Meteorological Organization (WMO) provides scientific and reliable observational data on the chemical composition of the Earth's atmosphere, information on changes in the natural and anthropogenic composition of the Earth's atmosphere, and an understanding of the interaction processes between the atmosphere, the ocean, and living organisms.WMO stipulations include the following: Measures, Integrated) INFCOM is based on the necessity to establish a system for real-time sharing and utilization of observation data between countries, such as meteorological observation and sales; near real-time and operation information (addressing the need to share near real-time data like numerical weather predictions (NWP) and expand to a proper operating system); and service (establishing a service system that can contribute to actual emission reductions through monitoring, reporting, and verification (MRV) of GHGs).
In order to relegate the threat of GHGs, as it is poised to cause damage to the environment and public health (in the form of respiratory illness, ischemic heart disease, and other non-communicable illnesses), measures are being substantially emphasized globally. [35]These include reformations in personal household energy consumption or usage; transportation; electricity generation and utilization; as well as in food and agriculture.The average global energy consumption, by sector, is shown in Figure 4(i,ii).As of yet, the individual responsibility is for households to implement restrictive policies, for example, on the use of electricity or heating; and, otherwise, to budget for the procurement of energy-efficient appliances, such as clean-burning stoves, has secondary advantages.These include the minimization of direct indoor pollutants (e.g., radon, CO, par-ticulate matter from tobacco smoke) and mold levels, although they exert a higher personal cost on the household, which may not be present without the implementation of individual-rank mitigation strategies.From the vantage point of transport, lowercarbon consumption, hybrid, or electric vehicles, and the public advocacy to replace private vehicles with public or active transit, have been employed to limit local levels of air pollution.In this sector, the addendum benefits include reductions in the number of motor vehicle accidents and improvements in general health or well-being, as facilitated by heightened physical activity (i.e., walking or cycling) by populations.Figure 4(iii) further elucidates the possibility of elevated flood risks due to global warming. [29]n a larger scale, methods are being developed, with the perspicacity of modern, global technological innovations, to introduce low-carbon and efficient fuel sources that can be used to replace their notably air-polluting counterparts, e.g., fossil fuels.Alternatives in solar, wind, nuclear, and hydro energy have been markedly examined for clean energy applications, especially in generating electricity.In the domain of agriculture, substantial efforts have been made in modifying land usage and farming techniques, which notably contribute to the production of N 2 O; and in pushing for a reduction in animal or animal product demands by consumers, which have been pivotal contributors to both CH 4 and N 2 O being produced.The reduced intake of saturated fats is a possible auxiliary positive health impact of the latter approach.Based on case studies for the year 2010, the reduction in CO 2 equivalent is plotted against the disability-adjusted life-years or DALYS, which is saved for household energy, food and agriculture, electricity generation, and transport in Figure 4(i), where the circle sizes indicate relative population densities per nation examined. [36]espite the presumptive outcomes of the approaches delineated, there are several difficulties that they pose, among them being a considerable reliance on individuals and countries to enforce the changes, at a personal cost; fuel poverty caused by heightened electricity costs; and potential health implications, if nuclear methods or carbon sequestration and storage are being used to produce low-carbon fuels.Moreover, it is challenging for individuals, industries, and governments to implement the necessary changes when there is a universal lack of GHG monitoring.Data acquisition using systemized methods in localities, combined with estimates of the global GHGs being emitted, will potentiate ubiquitous efforts to drive climate change in the right direction.At least on governmental or institutional grounds, policies can be enforced to limit the GHGs released by industries and farms when GHG sensors are commonly, prevalently, and cheaply manufactured.Individuals can more competently carry out responsible personal practices to further assist in advancing global environmental efforts.Several classes of sensors, among them, utilizing principles in infrared spectroscopy, chemisresistance, electrochemistry, and others, that can be used to quantify the GHGs, will be examined in the ensuing section.
produced by CH 4 (red) or O 3 (yellow).Table and graphic in (i) and (ii) reproduced with permission. [30]Copyright 2021, Elsevier Inc. (iii) reproduced with permissions. [18]Copyright 1990, Springer/Macmillan Magazines Ltd.Table in (iv) reproduced with permission. [31]Copyright 2016, Amer Geophysical Union.(v) reproduced with permission. [29]Copyright 2011, GRID-Arendal and SMI books.(vi) and (vii) reproduced with permission. [32]Copyright 2012, Copernicus GmbH.Chart in (viii) reproduced with permission. [33]Copyright 2013, Copernicus GmbH.In i), the reductions in the disease burden across populations due to decreasing CO 2 equivalents from household energy and food and agriculture and electricity generation and transport; in ii), the average global energy consumption is shown; and iii), the effects of global warming on flood risk.(i) and (ii) reproduced with permission. [36]Copyright 2009, Elsevier Inc. Graphics in (iii) reproduced with permission. [29]Copyright 2021, Elsevier Inc.

Experimental Approaches, Perspectives, and Expectations in GHG Detection
As described, environmental pollutants and IR-absorbing GHGs are detectable using aggregate-scale approaches, whereas local measurements monitoring the regional buildup of these substances are impractical and expensive. [35]To alleviate the implications of the enhanced greenhouse effect, governing bodies and international organizations on climate change advocate the mitigation of GHGs by comprehensive customs remodeling on energy consumption, fuel switching, renewable resources, carbon capture and sequestration, and public awareness, among other strategies. [37]The reform outcomes are inestimable without widely available tools that measure atmospheric GHG levels efficiently, accurately, and inexpensively.In conventional theory, apart from their direct propulsion on the climate's reconstruction, the GHGs pose indirect and direct human health risks, by propagating extreme weather conditions or causing air quality diminutions.Wildfires, heat-or pollutant-related (i.e., respiratory) illnesses, and food production disruptions are consequentially occurring with excessive GHG emissions.The noxious, gaseous compounds that pose human health risks include indirect GHGs, such as hydrogen (H 2 ), sulfur dioxide (SO 2 ), and NO X , and the weak direct GHG, CO. [38] Current industrial methods produce dense emissions of these and other substances.More pertinent to this review, original methods must be tailored to evaluate concentrations of the direct natural GHGs, which include CO 2 , CH 4 , N 2 O, O 3 , and H 2 O(v), as well as of the synthetic PFCs, HFCs, CFCs, and SF 6. [19] As already mentioned, due to deficiencies marring responses to an urgent public environmental concern, i.e., global warming, the NIST has instituted a GHG Measurements Program that employs high-tech aircraft and satellites to compute the atmospheric level of GHGs accurately. [8]Several other academic or independent researchers are studying the efficacy of less expensive GHG-detecting systems that utilize nanomaterials in combination with resistance, optoacoustic, SAW, wireless, field effect transistor, or micromechanical measurement devices. [10]Allinclusive technologies that will monitor the GHGs, analyze their prevalence trends and produce output measurements sensitively, efficiently, and at a low cost are desirable.They must withstand the stresses exerted by the GHGs in geographical regions where their accumulation is excessive and provide accurate and rapid readouts.
In this regard, chemical-based, optical, metal oxide semiconductor, and electrochemical sensors, among others, have been profoundly researched. [10,11]En masse, the methods by which these remote sensors typically perform have been comprehensively elucidated in the literature, and they are predominantly founded on the use or manipulation of thermal infrared energy.In most cases, the thermal infrared energy that is absorbed by GHGs is received at a sensor system's collector, detector, or optical subsystem component and then translated into readable output by a computing device, which technically determines the type or concentration of atmospheric gas being detected. [12]Some of the sensors are fitted with diffraction gratings that scatter the incident thermal infrared energy in order to create light separation according to wavelengths falling into either the long-or mid-wave infrared spectral regions. [12]Once spectral isolation occurs, the readout signals may, with greater accuracy, pinpoint the molecular composition of the GHGs that are present during detection.
GHG sensors have been even more meticulously classed by sources that fully describe the modifiable analysis mechanisms using conventional instruments, including spectroscopes (of the types: FTIR, Raman, optical-based, and mass-based) and semiconducting materials. [13]More cost-effective methods have been developed with the recognition of novel nanomaterials that approach the selectivity and sensitivity properties of these traditional devices. [11,14]They include micromechanical units with cantilever and laser beams that are capable of generating resonant frequencies that are similarly proportionate to the concentration of GHGs occurring near the active substrate material (Figure 5(i)); [11,39] SAWs (Figure 5(ii)), or wireless measuring systems, that employ molecule-sorbing materials or coatings between interdigitated electrodes placed on piezoelectric crystals to produce radio frequency outputs that are proportional with the atmospheric GHG concentrations; [40] optoacoustic sensors that utilize the energy of photons (typically in the range, 10 4 -10 −2 kcal) to produce electromagnetic radiation that is interactive with the GHGs (Figure 5(iii-iv)). [41]Interestingly, in GHG detection systems, various polymers and nanoparticles have been used to enhance signal outputs.For instance, polyester supports and upconverting nanoparticles (NaYF4:Yb,Er) have been adapted into an optical CO 2 sensor (Figure 5(v)), [42] whereas polyhexamethylene biguanide and silicon nanocylinders in a CO 2 optical sensor were used in a different approach (Figure 5(vi)). [43]ther electrical methods have been adapted to GHG sensing, including field-effect transistor and resistance measurement approaches. [44]In the former, gate, source, and drain terminals, which envelop an active GHG-absorbing substance, are hooked to a potentiostat, by which electrical fields are used to predict GHG concentrations according to the magnitude of the drain current or the voltage.The latter technique relies on resistance changes that occur when the active substance, residing on a nonmetallic surface, is saturated with GHGs under ambient or experimental conditions inside a controlled-environment chamber, whereby the resistance reading correlates to the concentration of GHGs (Figure 5(vii)). [11,45]ndeed, numerous olden and novel sensor configurations are amenable to detecting GHGs or other air-borne substances, but technicalities (i.e., bulkiness; costliness; or limited resolution, function, and/or exclusivity to a single GHG) are present that limit their versatility.These include infrared analysis and manometry, the latter of which consists of measuring the volume, temperature, and pressure of a particular amount of dry air; titrimetry; Fourier transform spectrometry; and other methods.Titrimetry was first used by a Scandinavian group at 15 different ground stations to quantify CO 2 levels, whereas CH 4 and N 2 O have been measured by other instruments.The first consists of range-resolved infrared differential absorption lidar (DIAL), which is a means of measuring CH 4 emissions from various sources, including active and closed landfill sites.
Subsequently, the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) has been used for N 2 O detection.This satellite-based mission has been probing the Earth's atmosphere via solar occultation since 2004.The instruments on board include a high-resolution Fourier transform spectrometer (FTS) and a pair of filtered imagers, which In i), a micromechanical device with an active material-coated cantilever beam; ii), the concept diagram for a wireless SAW sensor; iii), an optoacoustic sensor; iv), the optical component containing the convex lens of an optoacoustic sensor, as well as, the process representation for a photoacoustic-based spectroscope with CO 2 measuring ability, utilizing fixed wavelength quantum cascade layer (FW-QCL); v), the use of polyester support and upconverting nanoparticles (NaYF4:Yb,Er) in an optical CO 2 sensor, and the relative intensities of luminescence emissions at 657 nm (red) and 542 nm (blue) at different CO 2 concentrations; vi), the use of polyhexamethylene biguanide and silicon nanocylinders in a CO 2 optical sensor, and a representation of the wavelength interrogation method result for detecting CO 2 ; and vii), the use of carbon nanotubes in gas sensing, from an approach for detecting CO 2 using gas adsorption and resistance readings.Images in (i), (ii), and (iii) reproduced with permission. [11]Copyright 2018, MDPI.(iv) reproduced with permission. [10]Copyright 2019, MDPI.(v) reproduced with permission. [42]Copyright 2010, Elsevier, Inc. (vi) reproduced with permission. [43]Copyright 2021, MDPI.(vii) reproduced with permission. [45]Copyright 2014, Elsevier inc.
are described in more detail in the section on GHG detection approaches.In the same way, other satellites are especially recognized as profound tools to measure such gases globally.For GHG detection from space, the Orbiting Carbon Observatories (OCO, OCO-2, OCO-3), and networks of ground stations, such as the Integrated Carbon Observation System, have been used.OCOs utilize spectrometers to take 24 CO 2 concentration measurements per second in the Earth's atmosphere; therefore, they provide users with accurate and consistent data.Finally, Total TCCON data from Lauder have been used extensively to support satellite missions, such as the Greenhouse Gases Observing Satellite (GOSAT), by providing data for retrieval validation and algorithm development.The Collaborative Carbon Column Observing Network (COCCON) is intended to be the lasting framework for creating and maintaining a GHG-observing network based on common instrumental standards and data analysis procedures.Currently, ≈18 working groups operating EM27/SUN spectrometers are contributing.
Even with the time demand, material cost, and intricateness of sensor designs, the research outcomes, in many cases, are specialized tools that can detect only certain GHGs, possibly under very highly regulated or specific conditions.For example, palladium-doped single-walled carbon nanotubes (SWCNTs) have been shown through a density-functional theory approach to improve the sensitivities of sensors to NO x gases. [46]Other engineered carbon nanomaterials [47] and composites have aided in detecting the natural GHGs, including CO 2 and CH 4 .Multifunctional systems adaptable to geographical differentials are sought, as are the GHG-specific sensors, and, therefore, they are examined in the following text.In particular, sensors with electronic components and inherent materials facilitating the sensing process, such as silicon microchips, graphene oxide, polymers, nanomaterials, and photonic crystal fibers, are described.Details are also provided to elucidate current trends in low-power, solar-driven, or portable devices.Gas ionization, a metal oxide semiconductor, and FTIR-based sensors, as well as others that do not specifically lead to electrical variations in sensing, including acoustic, optic, calorimetric, or gas chromatographic devices, are highlighted in much of the following text.Gas ionization methods are, moreover, described for improving the sensitivity of GHG detection down to the ppb of gas molecules range.Interestingly, precisely depositing elements with 3D or inkjet printers, among other more traditional approaches, could achieve this improved sensitivity.

Chemiresistive Sensors
As described previously, instrumental analyses of GHGs are broadly construed through unique optical and laser devices that are costly, non-specific, or suited to a limited number of GHGs, requiring multiple expensive equipment to be used for detecting GHGs having different molecular structures. [48]Moreover, current methods to quantify GHGs coalesce economic and atmospheric measures, meaning that most data report estimated emissions only.In order to address these, and other issues, researchers have been adapting simple and more cost-effective systems for universally monitoring the GHGs.
One such class is the chemiresistive sensors, first defined in 1985 by Wohltjen et al., after they uncovered the resistive comportment of copper phthalocyanine to vaporized ammonia. [49]enceforth, chemiresistors have been researched for their sensing capabilities, ease-of-use and -fabrication, minimal parts requirements, sensitivity down to the ppm levels, and low effectual costs. [50]

Working Principles
The types of chemical interactions that confer functionality to the chemiresistors include direct molecular perception of an analyte by the sensing substrate, hydrogen bond formation, and covalent bond formation, which, upon manifestation, produce an electrical resistance change. [51]For example, one mechanism of action of metal oxide chemiresistive sensors for the detection of oxygen gas may be described by alluding to surface adsorption principles. [52]By the theory of adsorption, oxygen from the air very efficaciously traps conduction electrons from the metal oxide sensor surface, leading to charge depletion and an effectual change (i.e., increase) in the resistance.Conversely, the adsorption of an oxidizing gas on the active layer of the chemiresistive sensor will produce a decrease in the resistance and electron loss from that oxidizing gaseous compound when a reducing atmosphere, such as that created by hydrogen sulfide (H 2 S) or CH 4 , occurs. [52]hus, reasonably, efforts have focused on acclimating the chemiresistors to the perception of GHGs, whereby active materials, including carbon-based nanomaterials, conductive polymers, metal-oxide semiconductors, and metal nanoparticles, are incorporated or interchanged to regulate the perception criteria.
In the basic configuration, the ravine separating conductive electrodes (e.g., gold or platinum) that provide an ohmic interface is filled by a sensing material.In another approach, interdigitated electrodes are coated using the sensing material. [53]Electrical resistance or conductance changes are measured by fastening the sensor arrangement to a DC supply, and the output data, depending on the types of materials used and the level of sensitivity required, are displayed algorithmically and in a manner that shows the qualitative or quantitative significance of the analyte being detected. [53,54]Futuristically, chemiresistive sensors with semiconducting properties have been developed as artificial sensory organs, i.e., as noses or tongues. [55]

Materials Appropriated and Designs
In addition to carbon nanomaterials-based, polymer, and metal oxide semiconductors, the transition metal dichalcogenides have emerged evidently as frontrunners in the development of flexible chemical-or bio-sensors that emulate organ functions. [56]Several chemiresistors have been reported specifically for the detection of GHGs.In a work led by Bezdek et al., a chemisresistor selective to CH 4 monitoring was assembled by coordination of functional poly(4-vinylpyridine) (P4VP) on SWCNTs to platinumpolyoxometalate (Pt-POM), a pre-catalyzer of CH 4 oxidation, as the sensing materials. [57]The electrical resistance readouts using a handheld, portable multimeter equipped with the SWCNT-P4VP-Pt-POM material precisely indicated, in a short period (2 minutes), the ppm amount of CH 4 present in the air at room temperature.
In CO 2 detection, unique chemiresistors, including those made using emeraldine green-graphene quantum dots (EG-GQD), nickel-doped tin oxide (Ni-SnO 2 ) nanoparticles, and Cu 3 (hexaiminobenzene) 2 (Cu 3 HIB 2 ) moiety-incorporated 2D metal-organic frameworks (2D MOFs), [58] have been adopted.The latter two differ in their performance when the ambient humidity is being regarded, and with structural modifications, the humidity response (or lack thereof) can undergo modulation.The chemiresistors with Ni-SnO 2 cauliflower-like sensing substrate performed as sensitive humidity sensors when pristine SnO 2 was used, and doping by Ni gradually decreased humidity detection while increasing the sensitivity of the sensor to CO 2 gas. [58]2D MOFs with the CO 2 -interacting N-heteroatom or imino-semiquinoate functionalizations, Figure 6.In i), block representation of a chemiresistive H 2 S gas sensor containing graphite blocks; ii), current responses of MoS 2 nanosheets integrated into H 2 S gas sensors in ambient and synthetic air at 200 °C; iii), scanning electron microscope (SEM) images of bulk and exfoliated MoSe 2 , used in a molybdenum diselenide (MoSe 2 ) H 2 S detector; iv), SEM and transmission electron microscope images of bulk and exfoliated WS 2 , used in a WS 2 nanosheet-containing chemiresistive ammonia sensor; v), the current response over time of tungsten nanosheets in an ammonia sensor using assynthesized or annealed WS 2 nanosheets, respectively; vi), the linear response versus ammonia concentration plots using as-synthesized or annealed WS 2 nanosheets, respectively; vii), the recovery and response time plots, respectively, using as-synthesized or annealed WS 2 in the chemiresistive ammonia sensor; and viii), the response specificity, surface topography, sensor configuration, and current response of a sensor unit containing WSe 2 to N 2 O. (i)-(iii) reproduced with permission. [59]Copyright 2019, Elsevier, Inc. (iv)-(vii) reproduced with permission. [60]Copyright 2019, IEEE.(viii) reproduced with permission. [61]Copyright 2020, Wiley, Inc.
including the high-performance Cu 3 HIB 2 , were capable of detecting CO 2 irrespective of relative humidity (RH) values in the range of 10%<RH<80%. [58]Nanosheets of molybdenum disulfide (MoS 2 ; atomic force micrographs and gas concentrationdependent current changes shown in Figure 6(i-iii)) were integrated into H 2 S sensors.They exhibited dynamic responses in both ambient and synthetic conditions. [59]In another approach, for detecting ammonia gas, Sakhuja et al. utilized tungsten disulfide (WS 2 ) nanosheets in sensors that showed transient responses at varying ammonia concentrations (Figure 6(ivvii)). [60]As shown in Figure 6(viii), a tungsten diselenide (WSe 2 )-based sensor responded specifically to N 2 O at room temperature, and the response was even more significant with heating (to 100 °C). [61]s indicated by this discussion, nanomaterials have been adopted into chemiresistor designs for detecting gases, evaporated organic volatile solvent molecules, and other airborne substances.Similar to the chemiresistor, the organic field-effect transistor (OFET) sensor may be specified to exemplify the significance of nanostructure control in compound sensing. [62]For instance, research conducted by Shin et al. expounds on the facile nature of nanorods and nanocrystals-conjugated polymer films in an OFET device to augment the charge transport by a two-fold degree; to amplify the surface-to-volume ratio after solvent vapor annealing; and to provide excellent responsivities, as well as response or recovery times, in the detection of VOCs. [62]

Electrochemical Sensors
Sensors with an electrochemical cell have been preferred for their low-powered determination of greenhouse and other polluting gases, including NO, H 2 , SO 2 , CO, NH 3 , H 2 S, NO 2 , and CO 2 .Electrochemical sensors are versatile, and as a result, the recognition of active ingredients in medications, gene sequences, biomarkers, hormones, gases, chemicals, and even microorganisms has occurred to date.The first rendition of an electrochemical sensor was an oxygen monitor proposed by Leland C. Clark to address industrial workplace exposures to toxic gases and fuels in the 1950s. [63]Despite system instability and the perpetual requirement for frequent pre-calibration, making the oxygen sensor impractical for daily use, it set forth a model for contemporary devices that utilize the concepts of electrodes, electrolyte solutions, and in-batch gas diffusion.This diffusion typically occurs through the electrolyte solution and any incorporated molecule-selecting units or membranes.Electrochemical sensing improved substantially, in terms of reliability, accuracy, efficiency, sensitivity, and other important qualities, with the rise of nanotechnology.

Working Principles
Prior to elucidating the types of sensing materials used for the conspicuous electroanalytical determination of GHGs, the essential configuration of electrochemical-based devices is elaborated.In many systems, the electrode substrates are the locales at which electrochemical reactions occur.The methods by which readings are obtained are, most commonly, by cyclic voltammetry, as well as by differential pulse voltammetry, electrochemical impedance spectroscopy, and chronocoulometry. [63,64]rom them, information about potential, current, impedance, and charge alterations are generated upon direct, indirect, or catalytic recognition of the analyte being examined.By interfacing with software, curves can be generated of cathodic peak currents versus the concentration of an analyte (as in cyclic voltammetry); [64] of potential difference versus current difference (as in differential pulse voltammetry); [64] of real versus imaginary parts of the frequency sweep transfer function (as in, electrochemical impedance spectroscopy); [64b] and of time versus charge (as in, chronocoulometry). [64] electrochemical biosensing, the immobilization of recognition molecules on electrodes, such as antibodies and aptamers, is universally accepted as a method to oxidize or reduce the target by facilitating electron transfer between bioreceptors and analytes.The analytes' concentrations are quantified by impedance, current, conductance, or potential changes that occur after the capture of the analyte by the decorated electrode substrate.The success of a reaction and subsequent analyte detection, to a large extent, rely on the types of substrates that are used as electrodes.Conventionally, these are of noble metals, mercury, or carbon materials, and, in recent innovations, chemically modified materials that incorporate conductive polymers, self-assembling monolayers, nanomaterials, electron transfer mediators, and solgel matrices. [65]A three-electrode system, with the working, reference, and counter electrodes, commonly typifies the electrochemical cell with the working solution.

Materials Appropriated and Designs
Translationally, toward their utility in analyzing the atmospheric content of GHGs, electrochemical sensors with various configurations are described in the literature.In research work by Dosi et al., an amperometric, low-power, and ultrasensitive electrochemical system for CH 4 gas sensing was developed despite the difficulties typically attributed to discerning gas-phase analytes in the presence of liquid electrolytes (Figure 7(i-xi)). [66]To address this challenge, a concept from the work of Kubersky et al., utilizing porous, solid polymer electrolytes (SPEs), was applied, and the product was an electrochemical cell with the pseudo-solidstate electrolyte (a thermoplastic cast in an ionic liquid) deposited onto interdigitated laser-induced graphene (LIG) electrodes. [67]s electrocatalysts, palladium nanoparticles were deposited onto the LIG electrodes, which, in conjunction with the porosities of the electrolyte/electrode, increased the analyte contact surface area and improved the sensitivity and rapidity of detection (i.e., by electro-oxidation) associated with the SPE-LIG CH 4 detector.
In an alternative approach to evaluating another potent GHG, N 2 O, Damgaard et al. built a device resembling the Switchable Trace Oxygen (STOX) sensor, customarily used in oceanography to determine, in situ, the dissolved oxygen in bodies of water (Figure 7(xii-xiv)). [68]As a distinguishing feature, these sensor types contain a front guard switching cathode that can be polarized before the analyte reaches the continuously polarized measuring cathode.In the configuration used to detect N 2 O, silicone membranes are situated before the switching and measuring cathodes, all enclosed by front guards, which are glass casings with electrolytes.The system relies initially on the diffusion of the gas analyte, N 2 O, through the outer front guard and on the polarization of the switching cathode.A non-polarized switching cathode precedes the diffusion of the analyte through the inner front guard and gas polarization at the measuring cathode.In contrast, polarization of the switching cathode prevents this outcome.The difference in the current measured with or without front guard switching cathode polarization then correlates linearly with the concentration of N 2 O.
The conceptualization of solid electrolytes, especially yttriastabilized zirconia, has been adopted in CO 2 sensing using potentiometric, amperometric, and resistive systems.For being ii), the influence of air of co-existing gases on the current response of the same CH 4 sensor; iii), the analytical outcomes of Raman spectroscopy performed on LIG electrodes, and an accompanying sketch of the interdigitated electrodes; iv), the current response as the result of different potentials being applied to the same CH 4 sensor, under a constant concentration of CH 4 ; v), the current response at the optimal applied potential, 0.6 V, as influenced by air or other gas interferences; vi), the appearance of the sensor before and after the current drop at 0.6 V; vii) SEM micrographs of the LIG electrodes without and with, respectively, the solid polymer-electrolyte layer; viii) side view of the LIG electrodes; ix), schematic of the same CH 4 sensor system; x), reliability of the sensor, as determined by performance up to 30 days; xi), the effects of CH 4 saturation and air drying on the sensor response; xii), a schematic of a switchable electrochemical-based N 2 O sensing device developed by Damgaard et al., that functions using a switching cathode, in the "on" and "off" configurations; xiii), the effects of opening and closing the cathode on the signal response (the transient increase in signal detection of N 2 O) and linear increase in amplitude with gas concentration; xiv), the operation of the sensor with the front guard functioning at 180 seconds "on" and 400 seconds "off" intervals.(i-xi) reproduced with permission. [66]Copyright 2018, ACS.(xii-xiv) reproduced with permission. [68]Copyright 2020, Elsevier, Inc.
integrated into an evolved gas analyzer (EGA) fabricated to measure the gas (especially CO 2 ) that is released by a decomposing sample at high temperatures, Mason et al. designed a small potentiometric solid-electrolyte instrument. [69]A beta alumina electrolyte was situated across from a platinum heater, which allowed instrumental temperature control.It was determined that the sensor feedback was affected by temperature, gas content, overall pressure, and CO 2 partial pressure.This latter effect could be controlled by operating the sensor at 530 °C, which resulted in the sensor operation being independent of partial pressures, and the total performance of the sensor satisfied the predictions made by the Nernst equation.With modifications, the potentiometric CO 2 sensor was determined to be ideal for space flight applications.Indeed, electrochemical devices are generally lauded for their selectivity, ease-of-use, miniaturization potential, and low production costs; their utility toward sensing GHGs will benefit many environmental control efforts as a result.

Fourier Transform Infrared (FTIR) Spectroscopes and Non-Dispersive Infrared (NDIR) Sensors
A pivotal method, FTIR spectroscopy, will be elaborated further as a two-beam interferometry approach that functions based on a Michelson interferometer being the heart of the spectrometer. [70]s for the NDIR gas sensors (Figure 9), they are a class of instruments that are also valuable in GHG detection.In fact, they are prevailingly used to measure carbon oxide concentrations (as well as, of N 2 O and CH 4 ) and, as a result, provide direct or indirect evidence of GHGs in the environment or industrial settings. [71]

Working Principles
In the case of ground-based atmospheric measurements, the Sun is the light source, and the absorption of the light energy along a path through the atmosphere is measured by the FTIR unit.The Sun's radiation is captured by a solar tracker on top of the FTIR container and is reflected by two movable mirrors downward into a container.After passing by three fixed mirrors, the light is focused on the input field stop of the spectrometer, ensuring that only the light of the solar disc's center is considered.This is controlled by a camera-based system. [72]The radiation reaches the beam splitter inside the spectrometer and is divided into two beams.One of the beams is reflected by a fixed mirror, whereas a moving mirror reflects the other beam.Both beams recombine at the beam splitter, where the optical path difference (OPD), , is determined by the position of the movable mirror.The beam passes several other optical components and is finally focused on the detector, which determines the intensity as a function of the optical path difference.The resulting AC part of the signal is called an interferogram.
For an arbitrarily large , the resulting interferogram would contain all the spectral information.As this is not typically the case, the information content is limited, causing a finite resolution of the FTIR.A single-mode helium-neon (HeNe) laser is additionally used to exactly determine the OPD.The laser beam has the same light path as the sunlight, and each zero crossing of the laser's interferogram represents a sampling point.To improve the signal-to-noise ratio, two or more scans are combined for each measurement.Compared to a scanning monochromator, for example, one of the most important advantages of FTIR spectroscopy is the saved time due to the simultaneous detection of all wavelengths.An FTIR spectrometer system is depicted in Figure 8.
In order to provide some degree of discretion on the perturbance of the natural carbon cycle caused by human activities, the TCCON observation network was developed in 2004. [73]Despite the maturation of an in situ network (or "Gold Standard") that provides data on the CO 2 and CH 4 levels in the atmosphere, a few limitations, especially relating to site access and maintenance, are prevalent, [73] and alternatives have been sought as a result.By integrating the principles that necessitate in situ and space-based tools (and, especially, Fourier-transform spectroscopy) to evaluate GHG concentrations using an all-encompassing approach, researchers have created an international network of ≥ 28 global, ground-positioned bases, TCCON, for the consistent, precise, and facile reporting of total column measurements for several common and trace gases, including CO 2 , CH 4 , N 2 O, fluorinated gases, CO, semi-heavy water, H 2 O(v), propane, carbonyl sulfide, and acetylene, among others. [73]riefly, to obtain the column-averaged dry-air mole fraction value of a trace gas (of interest) or X trace gas , TCCON equipment uses a GGG2014 code to concomitantly measure the oxygen column and the concentration of trace gas (of interest), then multiplies the ratio of trace gas to oxygen columns by the total oxygen column, ≈21%. [73,74]This is advantageous, as the ratio of an unknown to highly monitored (in this case, oxygen) element in the air minimizes mechanical or human errors and enables the calculation of a dry-, rather than wet-air, mole fraction.Moreover, the vertical distribution of trace gases, including any advection, photosynthetic, or respiratory behaviors near the ground, are accounted for with the column measurements employed in the formula for determining X trace gas .TCCON stations maintain coordinated operations that do not deviate from this standard measuring model, as well as conformant equipment fitted with similar optical components and universal software packages.
As for the general configuration of a gas NDIR sensor, [71] it is that of a chamber containing the infrared light source or lamp, optical filters, a detector element, and, typically, a reference cell. [75]Optical filters are situated immediately anterior to the detectors, thus allowing the transmission of light wavelengths corresponding to the gas of interest (and hence electro-optical detection), in accordance with the principles of infrared absorption by GHGs.NDIR setups utilize principles of infrared spectroscopy, whereby, a spectrum is collected when the incident radiation on a sample is resolved into components, of which the fraction of the absorbed radiation at a specific energy determines the spectral distribution.In measuring gases, the absorption wavelength of the infrared can help signify the precise molecular structure of the gas being detected within range.Commonly, the Beer-Lambert law, or a modification of it, is fundamentally followed in NDIR sensor technology, meaning that the absorbed energy or incident beam intensity (after light transmission through the gasrich chamber) is proportional to the concentration of the gas. [76]herefore, an expression containing the exponential of unknown gas concentration multiplied by a known absorption coefficient and the chamber path length, correlated to the final overall light iii) It shows the equipment image of FTIR (FTS) and the movement path of the internal light source.Reproduced under the terms of CC BY license. [77]Copyright 2018, The Authors, published by Copernicus Publication.iv) Describe the actual equipment placement and semi-automated monitoring of the observation laboratory.v) The value of Modulation Efficiency (ME) according to OPD is maintained at 99.2%, which means that the mechanical error of the observation equipment is very small.intensity divided by the initial intensity, can be used to efficiently calculate gas concentration. [77]

Materials Appropriated and Designs
Ground-based FTIR measurements are available at various sites worldwide for determining atmospheric trace gas concentrations and investigating climate change.Most of these sites take part in FTIR networks to ensure a network-wide quality standard concerning both measurements and data analysis.The retrieved results are stored on a network database available for public access.The two most important FTIR networks are the Infrared Working Group (IRWG) of the Network for the Detection of Atmospheric Composition Change (NDACC) and the TCCON. [78]CCON has rigorously taken on the role of a transfer standard for unifying satellite gas measurement data and in situ, ground-based network data.By a continuous process, researchers consolidate the atmospheric profiles above TCCON stations with those obtained using other methodologies with column averaging, yielding similar results.Multiple profiles of CO 2 , CH 4 , N 2 O, CO, and H 2 O(v) have been collected by this approach at >10 TCCON locations with accuracy and with the assistance of aircraft and Aircore balloons to monitor the TCCON-derived data's quality. [73]he Network for the Detection of Atmospheric Composition Change (NDACC) constitutes another ground-based network of FTSs for measuring GHG concentrations, although differences have been documented in the data outputs, the acquisi-tion mode (which for NDACC utilizes CO columns) and in the code used to obtain the NDACC readings, being either SFIT4 or PROFFIT9. [73,74]Moreover, small-scale research is underway by environmental control labs internationally in order to determine low-cost, regional, and potentially wide-ranging GHG detection methods.Recently, for example, and with these principles in tow, Oh et al. fabricated 3D laminated graphene-containing constructs for the highly sensitive and responsive photodetection of CO 2 , CO, and CO 2 /CH 4 ; and Zhou et al. developed a strategy whereby a fiber Bragg-grating sensor coated by polyether sulfone (PES) could selectively monitor CO 2 . [79]herefore, ground-based networks such as TCCON are being utilized to help revamp space-obtained data, whereas new technologies are expected to improve the operations of ground-based stations, thus indicating the reliant growth and future evolution of these networks to provide facile, accurate, and worldwide detection of several types of accumulating atmospheric gases.
From an optimistic standpoint, NDIR sensors can be minimally power consumptive and are operational at relatively low temperatures compared with other sources, although their usefulness is diminished by high limits of detection, spectral interference, and potentially high costs.Nevertheless, numerous commercial products and experimental renditions utilizing NDIR technology to measure GHGs exist, and a few of them will be elucidated.For instance, Xu et al. developed a multi-gas detection system containing a four-channel pyroelectric element for determining simultaneously the amounts of CO 2 , CO, and propane (i.e., automobile exhaust) in the air (Figure 9(i)). [80]The conventional NDIR consists of seven basic units, including a gas chamber with inlets and outlets, an infrared source, a window, a filter, and a detector; several different configurations have been discerned from them (Figure 9(ii-vi)). [71]In a report that Gibson and MacGregor released, an NDIR sensor for measuring CO 2 was similarly investigated (Figure 9(vii-ix)). [81]heir findings were comprehensive and showed that a light emitting diode (LED) and photodiode (PD) assembly utilizing pentanary alloys, functioning in the mid-infrared, could be used to make a completely solid state NDIR gas sensor that was wireless, portable, and inexpensive, and which also stabilized rapidly without consuming a large amount of power.From the LED and PD model, the enhanced measurement of CO 2 levels in realtime could be achieved using high-frequency power cycling, and this technology is being utilized in sensors with the trade name of SprintIR and translated to medical usage or to CO and N 2 O measuring. [83]In work published by Martin et al., a multivariate model of linear regression was used to calculate correction coefficients that would improve the precision of a commercial device, the K30 CO 2 monitor that is manufactured by SenseAir (Figure 9(x,xi)). [82]ther techniques have utilized the sensor-on-a-chip configuration, nanoantennas, and enrichment layers with NDIR in order to achieve enhanced detection combined with power efficiency and sensitivity for detecting CO 2 and other GHGs. [84]Lastly, in addressing the limitations attributable to traditional NDIR systems, i.e., spectral perturbations and high detection limits, modifications can be made systematically. [71,85]These include changing the types of detectors and light sources, regulating the inlet gas concentrations, and re-arranging the optical designs.

Optical Sensors
Despite the prevalence of chemical-and temperature-based gas detection systems, other types of optical sensors in production, as well as acoustic, calorimetric, and gas chromatographic instruments for measuring GHGs, are also reported.Considering optics and the optic measurement of GHGs, conventional techniques that are well-characterized for determining the GHGs in the air/ on land, include infrared, differential absorption, and spectrometry devices; whereas certain classes of optodes have been defined in perceiving GHGs from anthropogenic sources within oceanic environments, where the carbonate system has become imbalanced due to the marine uptake of CO 2 .

Working Principles
In optics, detection instruments rely on distinct light absorption by distinct gases in order to provide information on the local gaseous molecules being fingerprinted by an instrument.Infrared, differential absorption, and spectrometer-based devices have been developed and evaluated for their efficacy in providing data on the concentrations of certain gases.
The absorption-based methods for quantifying gases include FTIR and NDIR (Figure 9), as well as differential optical absorption spectroscopy (DOAS), tuneable diode laser absorption spectroscopy (TDLAS), cavity ring-down spectroscopy (CRDS), photoacoustic spectroscopy (PAS); whereas, the non-absorptionbased methods include detection by gas chemiluminescence (CL), UV fluorescence (UVF), and with the use of photoionization detectors (PIDs). [71]In the absorption-based methods, light is supplied by an emitter, such as a high-powered xylene lamp (DOAS), a laser (TDLAS and CRDS), or any type of infrared source, such as a laser, LED, or blackbody emitter (PAS).Generally, in many of these techniques, light passes through an empty or void space that is supplied by gas(es) that make contact with an intrinsic measuring cell or similar device, where the amount absorption, with a specific wavelength of the supplied light, correlates with the gas concentration (typically, by application of the Beer-Lambert law).
In contrast, the non-absorption-based methods for gas detection vary and often rely on intra-instrumental reactions or gas decay and the breakage of airborne molecules to produce a quantifiable signal.For instance, CL devices produce nitrogen dioxide (NO 2 ) and oxygen from O 3 (within the instrument) reacting with supplied or ambient N 2 O. NO 2 consequently returns to the ground state and produces light at a peak intensity of 1200 nm, which is recognized by a photomultiplier tube (PMT).UVF relies on the absorption of light at wavelengths that are unique to a gas (such as 190-230 nm for SO 2 ), followed by the decay of excited molecules to a lower energy state and emission of higher wavelength UV light, which is detectable and correlates with the concentration of the supplied gas.In the case of PIDs, a deep-UV lamp is used to break gases and vapors into positive ions that flow between electrodes inherent to the system, whereby the flux is quantifiable, although this is the least selective of the optical systems.
One advancement attributed to optics is the use of optical fiber gas sensors with various configurations and transducer assemblies or coating materials (Figure 10(i-iii)). [86]The most basic optical fibers are thin, translucent filaments of drawn-out silica glass or plastic and, with complexity, can include other doping, coating, or fiber materials. [87]n area of significance that may benefit particularly from optic signaling is the determination of GHGs in oceanic sinks.The work of Clarke et al. examines biogeochemical sensors and their deployability in the in situ observation of oceanic CO 2 , where optode sensors are particularly aligned to and distinguished for their high-resolution computation of pCO 2 within ocean waters, especially as compared to gas chromatographic, NDIR, spectroscopic/spectrophotometric, or electrochemical sensors, which may be incompatible (or short-lived) to the procurement of accurate seawater pCO 2 measurements, predominantly, due to their mechanical components or reference gas requirements that may confine their operability in aqueous atmospheres. [92]The optodes function primarily by the photodetection of the photons and xi), a calibration curve for correcting the measurements obtained using an LGR-24A-FGGA fast GHG analyzer versus the SenseAir K30 CO 2 device.(i) reproduced with permission. [80]Copyright 2022, MDPI.(ii)-(vi) reproduced with permission. [71]Copyright 2016, Elsevier, Inc. (vii)-(ix) reproduced with permission. [81]Copyright 2013, MDPI.(x) and (xi) reproduced under the terms of CC BY license. [82]Copyright 2017, The Authors published by Copernicus Publication.
Figure 10.In i), a representation of the intrinsic features that enable optic fiber functions; ii), light transmission occurring in optical fiber sensors; iii), the core, cladding, and protective jacket constituting optical fibers; iv) microscopic images of the internal hollow structures assumed by spectroscopic optical fibers; v) a MOF CH 4 nanosensor, with SEM cross-section, CH 4 response, power change ratio plot, and multiple test cycle CH 4 responses shown, moving in the clockwise direction from the upper left panel; vi) the setup for an optical fiber CO 2 sensor composed of a sol-gel matrix that is doped by 1-hydroxy-3,6,8-pyrenetrisulfonic acid trisodium salt (HPTS, PTS-); and vii) the emission spectra at various CO 2 concentrations, evaluated using the HPTS doped and optical fiber sensor unit.(i)-(iii) reproduced with permission. [88]Copyright 2005, Elsevier, Inc. (iv) reproduced with permission. [89]opyright 2021, MDPI.(v) reproduced with permission. [90]Copyright 2020, Elsevier, Inc. (vi) and (vii) reproduced with permission. [91]Copyright 2005, Elsevier, Inc.
that are emitted by LED activation from a sensor spot consisting of an analyte-responsive indicator affixed to a gas-permeable membrane.Their functionality is capitally regarded in water column assessments of GHGs, especially of CO 2 , as such optodes, although being miniaturized, simple devices, have successfully been deployed to obtain accurate pCO 2 readings at sediment-water interfaces within minutes, as actuated by the GHG diffusion rate and the depth of the corresponding boundary layer.
Optodes deployed in aqueous environments for evaluating the pCO 2 gradients within ocean sinks consist of simple electronics (waveguide, light source, and photodetector) and a filter component. [92]Of the hardware elements, laser diodes and LEDs are small, stable, and, thus, favored, as compared to other light sources, although LEDs do surpass laser diodes in terms of their powering stability and lower power specifications in situ.Consisting of the photodetector unit are commonly photodiodes or PMTs, which are selected based on the anticipated application and, consequentially, precise sensitivity requirements.Cost and sensitivity share an inverse relation, whereby PMTs, being the more costly among the line of photodetectors, present the greater sensitivity.Notwithstanding, photodiodes have been contemporarily, continuously optimized into highly complex, highperformance devices by manufacturers, e.g., Avalanche Photodiodes, that are preferred for in situ measuring in aqueous bodies, as galvanized by their relative simplicity (i.e., to PMTs) and an admissible signal-to-noise ratio at low light/photon densities.In a conventional design, the photodetector may be situated adjacent to the LED, laser diode, or other light source; or, placed opposite to the sensor spot.
The sensor or optode spot with support, membrane, and indicator molecules is a complex and essential feature of in situ devices for water column GHG measurements.Supports, membranes, and indicators, respectively, do include robust, inert compounds, such as polyethylene terephthalate; hydrogels, sol-gels, or cellulose-based substances; and substances that are responsive, with an effect on fluorescence excitations, to variations in CO 2 sinks, such as to changing pH levels, as 1-hydroxy-3,6,8-pyrenetrisufonic acid or diketo-pyrrolo-pyrolle (DPP).Often with the indicator, an anion stabilizer is beneficial, in order to avert indicator leaching and to function as an immobilized buffer; these stabilizers typically consist of quaternary ammonium hydroxides, which positively drive the quantum yield and instill both high sensitivity and photostability to the optode systems with pH-sensitive indicators in the sensor spot.After the indicator is selected (based on cost-analyses, perspicacity of the intended application, and anticipated performance), a compatible membrane may be accepted for applicability in an optode, that will be placed adjacent to the internal support providing hardware-to-detection unit meshing.Mainly, polymers (esp., cellulose-based, for example, ethyl cellulose) and hydrogels are applied as the membrane material, and they utilize ionic or co-valent coupling, respectively, with an indicator in order to maintain membrane-indicator coalescence.In the selection of a hydrogel or sol-gel as the membrane material, careful consideration must be taken of the base polymer or silica, and of the curing time that is applied, as the membrane/indicator interactions and pore sizes are influenced by these organizational decisions.Moreover, silicone layers may be added to ethyl cellulose membranes in order to mitigate unwanted ingress of ions.Materials that may be appropriated, in order to inhibit interferences caused by sunlight or the surrounding media on photodetection, include Teflon and silicone rubber, which, provide high gas permeability.

Working Principles
Highlighting the recent literature on acoustic-based sensors, numerous pieces describe the application of SAWs and similarly functioning instruments to obtain local or remote data on GHGs.As mentioned beforehand, SAWs operate by the conversion of radio frequency or mechanical waves into electrical signals that are readable by a computer (Figure 11(i-iii)). [95]Acoustic waves that are densely occurring near the propagation surface, or SAWs, typically include the Lamb, Love, Rayleigh, and surface transverse waves. [96]coustic sensing is extendable to the utilization of ultrasound or ultrasonic waves to detect gas concentrations on propagation paths.In severe or containing obstructive interfaces, ultrasound is an exceptional sensing mode that can permeate surfaces and tissues non-invasively.From the vantage point of GHGs, ultrasonic detection would be considered a beneficial approach in measuring the gas concentrations occurring within plumes of steam, for example, that are produced via chemical processes.
In accordance with the physics of sound transmission, the physical, back-to-forth agitation properties of particles are equivalent in audible and non-audible sounds.Therefore, ultrasound differs from regular sounds only in that the frequency of the waves is >20 kHz, which is the upper limit of human perception, and up to ≈1 GHz.Like other sources of non-ionizing radiation, including those that produce electromagnetic waves (i.e., light-, micro-, and radio-waves), ultrasound can be safely applied in equipment for positioning among humans or ecological communities.This result is rendered as the appropriation of ultrasonics within sensors for detecting distance or presence (i.e., of objects); pressure or temperature changes; and gas concentrations.In GHG detection, the class of propagation-path ultrasonic sensors is applicable, whereby a wavefield of a transmitting transducer traverses a medium, such as air, before reaching the receiving transducer.
Process parameters that are being analyzed, such as concentration, flow, level, and conversion, are related to the acoustic wave parameters: c, the speed of sound (time measurement); Df, which is the frequency difference occurring between the incident and reflected waves (time measurement); a, the attenuation of sounds (amplitude measurement); and Z, the acoustic impedance (amplitude measurement).Otherwise, when a system is impervious Figure 11.In i), a schematic of a pulse radar-type SAW sensor with polyethyleneimine (PEI)-starch conjugated carbon nanotubes for detecting CO 2 ; ii), the reflection amplitude and humidity effect of the same sensor with the PEI-starch carbon nanotubes; iii), the assembly of SAW sensors with piezoelectric substrates, and the evanescent electrical field for the SAWs that are propagating on the piezoelectric surface; iv), the makeup of a CO 2 SAW sensor with a carbon nanotube film between inter-digital transducers; v), a depiction of a carbon nanotube-coated SAW sensor for CO 2 detection; vi), an image of a SAW sensor containing tin (IV) oxide and silicon oxynitride (all sensing layers annealed at 700 °C; vii), a dual track SAW device for measuring SF 6 , including a schematic, photo of the internal architecture, and an SEM micrograph of the incorporated multi-wall carbon nanotubes, moving in the counter-clockwise direction from the upper left panel; and viii), plots demonstrating the association between humidity and frequency shift, the gas concentration and frequency shift, selectivity to SF 6 , and the reproducibility of the sensor.(i,ii) and (iv,v) reproduced with permission. [95]Copyright 2008, Elsevier, Inc. (iii) reproduced with permission. [94]Copyright 2021, Elsevier, Inc. (vi) reproduced with permission. [97]Copyright 2013, MDPI.(vii) and (viii) reproduced with permission. [95]Copyright 2011, IOP.
to time or amplitude measurements, interferometry, in which c is correlated to the wavelengths of interfering ultrasound waves, can elucidate wave parameters.

Materials Appropriated and Designs
In acoustic sensors, patterned, piezoelectric materials are necessarily incorporated as inter-digital transducers (IDTs) that alter the mechanical energy into an electrical output; however, overlaying the perturbation surface, several materials have been shown to aid in the detection of GHGs.These can amplify the sensitivity, selectivity, resolution, stability, efficiency, or other performance features of the GHG-sensing SAWs.
In CO 2 detection, for example, SAWs incorporating films of graphene-nickel-L-alanine; carbon nanotubes or polymer-amino carbon nanotubes; and gold, titanium dioxide, poly(3,4-ethylenedioxythiophene) have been fabricated (Figure 11(iv-vii)). [91,95,97]Although several commercial devices are available for measuring CO 2 and CH 4 , SAWs with unique and complex properties have also been construed in quantifying SF 6 gas, a distinctively potent, fluorinated, and synthetic GHG.For example, Wen et al. used an acid-treated, multi-wall carbon nanotube coating on a dual track SAW in order to sense SF 6 in a reproducible and repeatable manner (Figure 11(iv-viii)).In a different approach, partial charges produced by intrinsic defects in gas-insulated switchgear caused the decomposition of SF 6 , of which the products, H 2 S, and thionyl fluoride, could be discerned by photoacoustic or other types of spectroscopies. [98]n this work, a single-step hydrothermal process was used to synthesize a sensor with reduced graphene oxide modified by tin oxide on the surface.
The primary electronics and transducer elements are integral to conditioning the functioning of ultrasonic sensors.The transducer must satisfy multiple criteria, namely, large bandwidth and high resolution, directional specificity, streamlined wave transmission, sensitivity to the analyte (without producing a skewed response as the result of contamination or moisture), and efficiency.Achieving such a device has been proven prohibitive, as strength in one attribute (e.g., acoustic properties) is burdened by a reciprocal devolution in another trait (e.g., sensitivity to contaminating substances or moisture and humidity).To overcome these challenges in the conventional technology of transducers, new materials, such as aerogels, which are synthetic, low-density, porous, and ultralight substances, have been shown to improve the ratio of efficiency to bandwidth while instilling good acoustic matching.Low Q-factor and low Z, or L2QZ transducers, that are separated by stacked piezoceramics or plastics have also been used to achieve good resolution due to high bandwidths.
Ultrasonic transducers have also been fabricated and reformed to detect gases specifically.Barauskas et al. used polyethylenimine or other gas adsorbing materials, and a capacitive, micromachined ultrasound transducer, referred to as CMUT, in order to bind and selectively detect up to five gases, including CO 2 , in the environment. [99]Assessed against the CO 2 gas concentration, a frequency response shift was detected, distinguishing the PEI-coated and uncoated CMUT in terms of their feedback to the amount of CO 2 .The PEI-modified transducer demonstrated an elevated signal output due to the CO 2 adsorption.Kim et al. constructed a standard ultrasonic sensor made up of electrical components, an acoustic matching layer, and a piezoelectric ceramic, including SP 170 and ethylene vinyl acetate. [100]Altering several parameters, such as the dimensional and material properties of the transmission and receiving elements, led the authors to conclude that fine-tuning can assist in determining a high-sensitivity ultrasonic sensor for measuring CH 4 in a constant volume chamber.
To model the detection of undesired gases in a room, Sudiana et al. adopted the conceptualization of ultrasonic measurements with a microcontroller and artificial neural networks.Lastly, Basiri-Esfahani et al. used "ultrasound sensing-on-a-chip" to produce miniaturized optomechanical systems with a high spatial resolution that could be adapted to trace gas sensing. [101]verall, ultrasonic sensing technology advancements provide a low-cost, safe, high-resolution, and efficient approach for analyzing GHGs in the air.

Working Principles
Briefly and less prevalently, calorimetric and gas chromatographic sensors have been applied to measure the GHGs.In calorimetric detection, combustible gases on the sensor surface, typically coated by catalysts, generate heat that is used to quantify the concentration of the gases.Methods in gas chromatography utilize the separation of volatile, thermally stable, and moderate to low molecular weight gases or compounds in order to achieve molecular analysis.

Materials Appropriated and Designs
In the works of Vurek and Mills et als., calorimetric devices were employed to measure, respectively, picomole amounts of CO 2 using a standard system (for biological applications) or the dissolved CO 2 with a calorimeter that was modified using plasticized or unplasticized polymer films. [102]For cost-effectivity, research that utilizes gas chromatographs to observe climate-related gases, including CO 2 , CH 4 , and N 2 O, have included the gas chromatograph integrated with an electron capture detection system which produces radiation. [103]Due to this radioactive effect, these devices cannot be operated under field conditions but require environmental sampling (collection), transport, and product investigation in a laboratory.While calorimetric and gas chromatographic sensors are inexpensive and functional, the outcomes of their usage are often associated with poor selectivity, adaptability, or sensitivity. [103,104]

Inkjet and 3D Printing in Sensor Fabrication
As described, many of the sensors, regardless of working principle, employ coatings, layers, or films in order to improve their performance.Printing technologies have emerged to abet sensor sensitivity ratings by precisely controlling the deposition of these substances, producing surface homogeneity, and regulating the deposition layer thicknesses.In one approach to inkjet printing graphene in order to detect NO 2 and ammonia gas, Travan et al. used low-oxygen content ink with graphene flakes and other components that were dissolved in terpineol (Figure 12(i)). [105]The sensors performed stably under different humidity conditions compared to metal oxide-based sensors.Moreover, homogenous layers overlaying the entire area of the electrode were achieved by inkjet printing, thus increasing the number of adsorption sites.This method also prevented variability in sensitivity across the graphene surface layers, as signals obtained at thin regions could be perturbed by high thermal noise, whereas the thicker parts had obstructed sensitivity due to the parallel migration of independent current paths.
In a different ammonia sensor that was fabricated by Zhou et al., 3D, rather than inkjet, printing technology was employed to create refractory mullite ceramic lattice substrates that were modified with polyalanine and silver nanoparticles, which provided active sites for the improved adsorption of NH 3 gas molecules (Figure 12(ii,iii)). [106]The lattice structure designs were printed Figure 12.In i), the printing process that is applied to produce NO 2 and ammonia gas sensor parts; ii) 3D printing of a lattice structure containing ceramic lattice substrates (CLSs), polyaniline (PANI) networks, and silver (Ag) nanoparticles, for detecting ammonia gas; and iii) representation of the ammonia sensor, containing 3D printed materials, showing the mechanism by which gases are adsorbed and produce a readable change.(i) reproduced with permission. [105]Copyright 2019, MDPI.(ii) and (iii) reproduced with permission. [106]Copyright 2020, Elsevier, Inc. so that a body-centered periodical and microporous structure was obtained, providing excellent gas diffusion and resistance readouts by aerodynamic and damping factor controls.
3D printing employing direct laser writing was employed by Delaney et al. in order to fabricate hydrogel arrays that were intrinsically reactive, by optical stimulation and sensory reception, to solvent vapors, which propagated a change in the swelling behavior of hydrogels and a sequential, reversible color change. [107]his effect of the solvent vapors on the wavelength being absorbed by the materials is garnered by the differing polarities of organic solvents affecting the swelling and subsequent dimensions of the constitutive hydrogels.

A Succinct Dictation of Novel Materials in GHG Sensing
Highlighted in the sections above, a vast array of traditional, novel, and hybrid complex materials have been uncovered and adopted within the frameworks of sensors dedicated toward detecting GHGs.These include the carbon nanomaterials, polymers, metal oxide semiconductors, and transition metal dichalcogenides, in calorimetric sensors; [102,108,117,122]] the solid polymer electrolytes, pseudo-solid-state electrolytes, and carbon materials, in electrochemical sensors; [67][68][69]109,121,133,137]] pyroelectric elements, LEDs, and PDs, in IR/FTIR/NDIR sensors; [72][73][74][75][76]78,84,85,109,131]] metal oxides, carbon materials, polymers, and single-or multi-mode optical fibers, in opticalbased sensors; [86][87][88][89][90][92][93][94]109,134]] piezoelectric materials, piezoceramics, carbon materials, and polymers, in acoustic/ultrasonic sensors; [91,[95][96][97][98][99]100,110,111] as well as polymer films in calorimetric or gas chromatographic sensors. [103,104,109] However, without materials developments, GHG-oriented and other sensor classes would be inadequate or even futile at producing fine data outputs, this section will provide further information on the novel materials that have recently received attention due to their applicability and usability in sensor designs.These include 2D materials, such as (aforementioned) graphene or transition metal dichalcogenides, hexagonal boron nitride, metal oxide nanosheets (MONs), black phosphorus and phosphorene, and many Xene materials (including, MXenes); covalent organic frameworks (COFs); and MOFs.[112] In the context of material selections, considerations of novel chemical structures or compositions, i.e., novel materials, will be delved into, which harness the potential to discriminate "gases-of-interest" with high sensitivity, specificity, and selectivity, and at efficient response and restoration rates.This must be instinctively (i.e., by the same sensor) accomplished at room temperature, and at the temperature extremes that varied seasons or geological areas constitute.
Therefore, in this brief discussion, the more common 2D materials in sensor designs, especially, their novel adaptations in recent literature, will first be highlighted, as precursors to the consideration of the crystalline entities that constitute the 2D or 3D frameworks of COFs or MOFs.In retrospect, the identification and recognition of graphene in 2004 as a 2D material with implications in sensing technologies shed light, generally, on the great utility of such ordered geometric architectures in sensors, due to their penchant for molecular adsorption.This effect is the consequence of the high specific surface areas, carrier mobility properties, and the plethora of adsorption sites that many these materials possess. [113]2D materials have further been uncovered to produce very low levels of Johnson-Nyquist noise, thus making them ideal for distinguishing nominal quantities of gases. [114]The operability of sensors incorporating these materials is, furthermore, facilitated by their capacities to directly take up electrons from the gas molecules being identified.This stably maintains the application of these sensor units at most standard ambient temperatures and diminishes the power consumption of the sensor (as opposed to sensors that may necessitate the influx of oxygen ions or other auxiliary inputs for standard operations).
Following from this cursory description of the 2D materials, the recognition of their many varieties, and with high regard to their immense pertinence to sensing technologies, stemming from the 2004 breakthrough with graphene (and spanning two decades), the following sections will briefly describe the more sought-after materials, in GHG sensors.Details on molecular compositions, geometrical specificities, and recent inventions incorporating 2D materials (and otherwise, materials that can also be 3D) will be elucidated.
Graphene.Perhaps one of the most widely explored materials in research on contemporary gas sensors, graphene is a singlelayer structure comprised of carbon forming a hexagonal, honeycomb lattice at the atomic scale. [115]The adsorption ability of graphene may be improved following surface treatment by oxidizers to form graphene oxide containing carboxylic (-COOH), hydroxyl (─OH), carbonyl (C═ O), or epoxide (C─O─C) functional groups; and, contrarily, reduced graphene oxide, which possesses improved electrical conductivity, may be derivatized following chemical, thermal, or electrochemical reduction processes on graphene oxide that are often complex (esp., when high-quality product is desired). [115]The abundant functional groups of graphene oxide promote the binding of gas molecules, whereas the number of adsorption sites in reduced graphene oxide is relatively higher due to defects and vacancies created during the reduction.
To complement these features of graphene and improve sensitivity or selectivity of graphene-based materials toward gases, researchers have assembled polymer (e.g., polypyrrole)functionalized, heteroatom-doped, and noble metal-composited graphene (as well as their nanoparticles). [116]As one of a few specific examples in gas sensing, Shaik et al. reported nitrogendoped graphene nanosheets (NGS) coating interdigitated electrodes, which, within the framework of the chemiresistor gas detection setup, could identify low concentrations of N 2 O gas at room temperature. [117]To resolve sub-ppm levels of NH 3 gas and conceptualize an electronic nose with pollutant gas sensing capabilities, Freddi et al. utilized the diazonium (aryl ring) functionalization of graphene layers. [118]In the work of Zhang et al., highly conductive reduced graphene oxide composited with alpha-phase iron (III) oxide nanoparticles demonstrated high selectivity toward NO 2 and showed detection limits as low as 0.1 ppm. [119]More pertinent to the GHGs, Zheng et al. carried out a density functional theory (DFT) assessment that would predict the superiority of aluminum (Al)-doped (compared to boron-, phosphorous-, or nitrogen-doped) mono-vacancy graphene in detecting CO 2 . [120]Improved charge transfers, adsorption, and a discernible band gap were attributable to the use of Al doping.
Transition Metal Dichalcogenides.In the 2D state, transition metal dichalcogenides, so-called due to the chemical composition of repeating units (MX2, where M and X denote transition metals and chalcogen atoms, respectively), generally assume one of three phases which are, geometrically, the trigonal prism (2H), the octahedron (1T), or the distorted octahedron (1T').In gas sensing applications, at room temperature, tungsten disulfide (WS 2 ), MoSe 2 , and MoS 2 are largely researched, which, depending on their physical form (bulk, layered, or otherwise), are direct or indirect bandgap semiconductors that provide excellent ratios of surface area-to-volume in the gas adsorbing region.
From the recent literature on the application of transition metal dichalcogenides as sensors of GHGs or other pollutant gases, Aghamalyan identified WSe 2 , in its 2H or 1T' phase, as an excellent material for the physisorption of CO 2 , which, due to the complementary low adsorption energies, could be easily integrated into an impedance gas sensor. [121]In another study, Rathi et al. combined exfoliated WS 2 nanoflakes and ruthenium in a mixed solvent reaction medium to fabricate ruthenium-WS 2 quantum dots, which, once deposited on gold interdigitated electrodes, could simply amplify CO 2 gas sensing using the chemiresistor approach, whereby CO 2 concentrations from 500 to 5000 ppm produced a discernible resistance change. [122]enes.In their most basic forms, xenes are monoelement 2D honeycomb materials that are highly adaptable to possess desirable functionalities with stellar and modifiable physicochemical traits, conformable band gaps, and significant surface electronic properties. [123]Integral morphologies, sizes, surface ratios, and chemical modifications are the types of physicochemical features that are tractable and may be engineered to accommodate specific gas sensing or other desired operations.Xenes are often deliberately sub-categorized into the classes: 2D monoelemental (2D Xenes) or 2D multi-elemental (i.e., MXenes). [123]Structurally, they are quite similar to graphene, and oftentimes, are applied as composites with graphene and its derivatives, transition metal dichalcogenides, MOFs, COFs, or other 2D (and 3D) materials, sometimes within a nanosheet or nanoparticle framework.
Delving deeper into the topic of MXenes, they are constituted by the assemblage of layered ternary transition metal nitrides, carbides, or carbonitrides, whereby, carbon and/or nitrogen atoms and sp elements are linked to a d-block transition metal, such as titanium or vanadium, to generate the final, repeating structure.From a materials science perspective, MXenes are easily adaptable, and their constituents may be facilely exchanged to produce many configurations that are unique or distinct in their features and functions.Therefore, grounded on their intended usage or application, MXenes must be engineered to have optimal electrical properties and gas adsorption capabilities in order to achieve strong signal intensities in tandem with high ratios of signal-to-noise.The study by Khakbaz et al., which utilized the MXene titanium carbide, or Ti 3 C 2 T x , showed that, by tuning the ratio of termination groups on the MXene substrate surface, the sensitivity of a theoretical sensor incorporating the MXene could be substantially improved to specifically detect a singular gas, in this study, NH 3 , among many others. [124]In the discernment of CH 4 gas at room temperature, Wang et al. utilized a 2D carbide-based material, Ti 2 CT x , in order to achieve a seven times improvement in the CH 4 gas detection occurring under visible light irradiation. [125]hosphorene is also often grouped the xenes class of materials, and as such, will be briefly examined here.Black phosphorus, which is an allotrope of phosphorus, in a single layer constitutes the repeating hexagonal lattice of phosphorene, which, unlike graphene displays semiconducting and metallic properties. [126]In the work of Wang et al., a vanadiumdoped monolayer of phosphorene by theoretical transport and first-principle calculations was demonstrated to optimally adsorb paramagnetic NO and NO 2 molecules, much more so than nonmagnetic gaseous H 2 O, thus the feasibility of the phosphorene-based sensor in selective gas detection within a humid environment. [127]Ghashghaee et al. further demonstrated the importance of surface modification, especially of defect engineering, in altering the electrical properties of single-layered black phosphorus and thus improving the selectivity toward CO 2 . [128]In this case, the defect-containing material showed a higher function sensitivity to CO 2 than the non-vacancydoped material.
MOFs.Among the materials that are more often used in their 2D as well as in their 3D forms for sensing or other applications, MOFs are one class of the most commonly recognized crystalline elements that assume distinct configurations at the atomic scale, subsidiary to their macroscale porous structure.In essence, MOFs are comprised of metal ions centrally coordinated to organic ligands in space groups that are often distinct, and dependent on coordination ratios, even when the same metal ions and ligands are used to assemble them.Compared with the mean specific surface areas (SAs) of other well-known materials such as graphene (SA≈2630 m 2 g −1 ), MOFs can be engineered to have SAs that are exceptional, spanning 1000-7000 m 2 g −1 .They are also flexible materials with controllable pore sizes that can beneficially dictate the capture of certain gases and guide their separation and detection.For instance, in a study by Avci et al. that screened multiple MOF materials, those with smaller pore sizes preferentially adsorbed and captured CO 2 , more so than H 2 , in a mixed gas environment, while, contrariwise, larger MOFs pore sizes led to the capture of H 2 over CO 2 ; the MOF with the greatest selectivity toward these gases was Mg-MOF-74. [129]hile MOFs typically are assembled into 3D coordination polymers or metal clusters, the highest conductivity is achieved in 2D MOFs, which undergo charge delocalization along their plane and sustain much-improved p-conjugations in their chemical networks.It is suspected that a great influence on gas sensing is imposed by the nature of metallic ions in MOFs, which affect the intrinsic charge transfer, and hydrogen bond formation.
In the literature, there have been many instances of researchers using new MOFs in order to detect GHG-type gases, among them being a study by Guimaraes et al, where it was shown by DFT that hexafluorosilicic acid (SiF 6 ) and Cu MOFs preferentially adsorbed and electrostatically interacted with CO 2 compared to other gases (including other GHGs); the high-est consistent adsorption energy of CO 2 on the MOFs was achieved in SiF 6 -3-Cu, which had smaller pores than the other substrates investigated, including both SiF 6 -2-Cu and SiF 6 -2-Cui, and the greatest in-network interactions occurred between CO 2 molecules and the inorganic SiF 6 chains, which polarized CO 2 . [130]Zhou et al. further developed a strategy to sense multiple GHGs, including CO 2 and CH 4 , by integrating MOFs and a multiresonant surface-enhanced infrared absorption, or SEIRA, technique; research outcomes demonstrated that, on a single chip, CO 2 and CH 4 could be concurrently and quickly detected (<1 min) with excellent accuracy (1.1% and 0.4% errors in CO 2 and CH 4 monitoring, respectively) and linearity up to 25 000 ppm. [131] In a paper by Day et al., a novel MOFs screening methodology was utilized in which gas sensing arrays were assembled by simulation data (Monte Carlo) to develop an electronic nose for deciphering CO 2 gas. [132]n NO 2 sensing, Zhan et al. utilized a polyhedral, solvothermal-obtained MOF containing zeolite imidazolate (ZIF-MOF), ZIF-8-8, which demonstrated a strong selectivity to NO 2 , an elevated sensor response at 100 ppm of gas exposure (resistance variation ratio of 118.5), and substantial response (113.5 s) and recovery durations (111.5 s) at high temperature (350°C). [133]Moscoso et al. synthesized a luminescent, lanthanide metal-containing MOF of Terbium and benzene-1,3,5-tricarboxylate (BTC), or Tb(BTC), deposited on transparent polymer films, and showed a photoluminescence dependence of emission spectra on the NO 2 gas concentration. [134]OFs.As the second major group of materials reviewed in this work that can be applied as a 2D or 3D material in sensing applications, COFs are porous and comprised of mainly light elements, such as carbon, nitrogen, oxygen, hydrogen, silicon, and others, forming robust organic bonds. [135]The ordered COF structures may possess unique geometries and topologies containing rhombic, tetragonal, hexagonal, octagonal, or trigonal (including honeycombs) networks and knots of varied configurations.Layered 2D COFs are fabricated by the stacking of COF sheets, whereas 3D COFs incorporate building blocks that encompass carbon or sp 3 silane atoms. [135,136]Like most other materials, COFs may be modified to have certain surface functional groups, while also featuring high specific surface areas (1000-5000 m 2 g −1 ); low densities; highly porous networks; water, thermal, and chemical stabilities; supercapacitor functions; as well as, luminescent properties.
As they pertain to GHG detection, COFs have been explored rather recently, and in works reported by Zhang et al., COFs with ionic liquid-moieties, or ILCOFs, were determined to facilitate the capture and conversion (e.g., cycloaddition and reduction) of CO 2 . [137]The COFs applied in works integrating them with ionic liquids were prepared using a series of elements (including, metalloporphyrins) with pyridinium, imidazolium, guanidinium, spiroborate, squaraine linkers coordinating the ILCOFs structures.In records prepared by Zeng et al., COFs containing boron and oxygen bonds (B-O), namely, B 3 O 3 (boroxine) and C 2 O 2 B (boronate), as well as triazine-based, imine-based, and boron/imine-based COFs were all reported to be useful in CO 2 capture. [138]Considering most of the studies analyzed, the imine-and triazine-based COFs showed high CO 2 adsorption and CO 2 /N 2 selectivity, even with smaller surface areas and lower pore volumes than boron-containing counterparts.In the work of Ease-of-use; rapidity; selectivity (depending on the gas being analyzed); practicability; visual quantification; easy functionalization; short response time; limited interference; ease-of-manufacture; operability under harsh conditions.

Conclusions and Future Perspectives
The National Aeronautics and Space Administration (NASA) has reported on the "vital signs of the planet" by publishing figures indicating the severity of climate change caused by recent human industrial activities.These include a 48% increase in CO 2 (1850present, with current emissions determined as 417 ppm, a 2.1°F average temperature increase (1870-present), a 13.1% reduction in arctic ice per decade, a 151 billion metric tons/year loss in ice mass, a 3.4-millimeter rise in the annual sea level, and an additional 326 zetajoules of ocean heat (1955-present).Global warming is a topic that is approached by many governments and agencies across the world, and, most notably, by the international Paris Agreement treaty on climate change, which includes 197 countries.Efforts are being expended to produce guidelines on the responsible consumption of energy and to encourage research on alternative energy sources since progress in other areas, including transportation, electricity, and heating, should not be hampered by the reduction in GHG emissions.An efficient, universal, and cost-effective system is needed to ensure that such GHG mitigation efforts are successful, and this ties directly into proposals to develop versatile sensors for detecting local concentrations of GHGs.The outcome would, thus, implicate standard devices that will aid environmental engineering researchers in developing clean energy alternatives and assist local governments in promoting prudent energy consumption, depending on the GHG levels in a given region.As approaches to assuage the repercussions caused by atmospheric heating are being devised, several of them that advocate GHG curtailment through societal shifts and reciprocal atmospheric compositional measuring devices have been proffered by academic institutions and governing bodies.GHGs that occupy various layers of the atmosphere have surged since the onset of industrialization, imparting a critical role for individuals, households, institutes, and governments to make strides in their regulation.Local or broad methods involving a diverse array of instruments have been developed to measure the GHGs, although an ideal (i.e., a sensitive, accurate, and cost-effective) device that can distinguish among the GHGs is only procurable with additional investigation.Industrialization has also prompted an ancillary exigent condition by evoking the production of synthetic GHGs.
This work emphasizes the types, quantities, and release mechanisms of natural (e.g., CO 2 , CH 4 , and N 2 O) and synthetic (e.g., HFCs, PFCs, SF 6 , and NF 3 ) GHGs into the atmosphere.Their climactic and environmental impacts are underscored, and the basic actions undertaken to stabilize and reverse GHG levels are elucidated in detail.Current experimental methods for calculating the accumulation of GHGs in different ambient conditions are presented, and they rely predominantly, on eight different approaches, by the utilization of chemiresistive sensors; electrochemical sensors; FTIR spectroscopes; NDIR sensors; as well as, optical, acoustic, calorimetric, and gas chromatographic sensors.Inkjet and 3D printing are propounded as state-of-the-art tools for optimizing the production of gas sensor components.Despite the numerous advantages of the primary sensor types that have been delved into throughout this article, including good sensitivity and selectivity of the chemiresistive, FTIR, NDIR, and other optical sensors, many of the subsequential technologies suffer from cost ineffectiveness and limited regional detection of GHGs.Furthermore, novel 2D or 3D materials that have been deemed applicable to capture or adsorb certain GHGs at ambient (including extreme) temperatures, are outlined, among them being, graphene, the transition metal dichalcogenides, the Xenes, MOFs, and COFs, which may pave the way to improved sensor materials.][111]140] Irrespective of this feasibility afforded to monitor the germinating GHGs with operational, research-grade instrumentation, still, quantification requirements must be carefully construed.For instance, data records should be exceptionally precise, durable, and span multiple decades for the long-term resolution of annual mean GHG gradients.The intrinsic circumstances leading to oceanic and biological fluxes, contributing to the release of GHGs, are the result of processes that transcend years of data collection.Therefore, the contents of extensive databases thus contrived to span several decades, should be used to inform future policies and verify emissions.Moreover, the overall objectives of the proposed systems are to provide simple apparatuses that environmental agencies can universally deploy to detect GHGs and enable a means for facilely determining the GHGs that are especially problematic in a given region or that pose a significant atmospheric threat.Prospectively, biologic-electronic interfaces are envisioned to advance research in medicine, engineering, and the environment, powered by underlying principles in sensor development, genetic engineering, electronics, materials science, and nanotechnology.

Figure 1 .
Figure 1.Interactions between GHGs and the climate.Reproduced with permission.[6]Copyright 1994, The Royal Swedish Academy of Sciences.
(ii), for the years spanning 1750 to 2005.The relative GWPs of several of the GHGs relative to CO 2 , including CO, CH 4 , N 2 O, HFC-22, and CFC-11 and 12, are displayed in Figure 3(iii), as a function of a discount rate, r, which alters the residence time of atmospheric GHGs according to the generalized expression,  / (1 + r).

Figure 4 .
Figure 4.In i), the reductions in the disease burden across populations due to decreasing CO 2 equivalents from household energy and food and agriculture and electricity generation and transport; in ii), the average global energy consumption is shown; and iii), the effects of global warming on flood risk.(i) and (ii) reproduced with permission.[36]Copyright 2009, Elsevier Inc. Graphics in (iii) reproduced with permission.[29]Copyright 2021, Elsevier Inc.

Figure 5 .
Figure 5.In i), a micromechanical device with an active material-coated cantilever beam; ii), the concept diagram for a wireless SAW sensor; iii), an optoacoustic sensor; iv), the optical component containing the convex lens of an optoacoustic sensor, as well as, the process representation for a photoacoustic-based spectroscope with CO 2 measuring ability, utilizing fixed wavelength quantum cascade layer (FW-QCL); v), the use of polyester support and upconverting nanoparticles (NaYF4:Yb,Er) in an optical CO 2 sensor, and the relative intensities of luminescence emissions at 657 nm (red) and 542 nm (blue) at different CO 2 concentrations; vi), the use of polyhexamethylene biguanide and silicon nanocylinders in a CO 2 optical sensor, and a representation of the wavelength interrogation method result for detecting CO 2 ; and vii), the use of carbon nanotubes in gas sensing, from an approach for detecting CO 2 using gas adsorption and resistance readings.Images in (i), (ii), and (iii) reproduced with permission.[11]Copyright 2018, MDPI.(iv) reproduced with permission.[10]Copyright 2019, MDPI.(v) reproduced with permission.[42]Copyright 2010, Elsevier, Inc. (vi) reproduced with permission.[43]Copyright 2021, MDPI.(vii) reproduced with permission.[45]Copyright 2014, Elsevier inc.

Figure 7 .
Figure 7.In i), the current response of an electrochemical-based sensor of LIG and palladium nanoparticles to CH 4 concentrations as a function of time; ii), the influence of air of co-existing gases on the current response of the same CH 4 sensor; iii), the analytical outcomes of Raman spectroscopy performed on LIG electrodes, and an accompanying sketch of the interdigitated electrodes; iv), the current response as the result of different potentials being applied to the same CH 4 sensor, under a constant concentration of CH 4 ; v), the current response at the optimal applied potential, 0.6 V, as influenced by air or other gas interferences; vi), the appearance of the sensor before and after the current drop at 0.6 V; vii) SEM micrographs of the LIG electrodes without and with, respectively, the solid polymer-electrolyte layer; viii) side view of the LIG electrodes; ix), schematic of the same CH 4 sensor system; x), reliability of the sensor, as determined by performance up to 30 days; xi), the effects of CH 4 saturation and air drying on the sensor response; xii), a schematic of a switchable electrochemical-based N 2 O sensing device developed by Damgaard et al., that functions using a switching cathode, in the "on" and "off" configurations; xiii), the effects of opening and closing the cathode on the signal response (the transient increase in signal detection of N 2 O) and linear increase in amplitude with gas concentration; xiv), the operation of the sensor with the front guard functioning at 180 seconds "on" and 400 seconds "off" intervals.(i-xi) reproduced with permission.[66]Copyright 2018, ACS.(xii-xiv) reproduced with permission.[68]Copyright 2020, Elsevier, Inc.

Figure 8 .
Figure 8. Description of the FTIR (FTS) observation system.i) The principle of spectroscopic observation using solar radiation absorption lines of GHGs in the atmosphere.ii) Photographs of the automated FTIR (FTS) laboratory.iii) It shows the equipment image of FTIR (FTS) and the movement path of the internal light source.Reproduced under the terms of CC BY license.[77]Copyright 2018, The Authors, published by Copernicus Publication.iv) Describe the actual equipment placement and semi-automated monitoring of the observation laboratory.v) The value of Modulation Efficiency (ME) according to OPD is maintained at 99.2%, which means that the mechanical error of the observation equipment is very small.

Table 1 .
Highlighted are the general materials per sensor type for enabling or enhancing GHG detection, as well as the advantages/disadvantages of the various sensor configurations for detecting GHGs.