The good, the bad, and the future: Systematic review identifies best use of biomass to meet air quality and climate policies in California

California has large and diverse biomass resources and provides a pertinent example of how biomass use is changing and needs to change, in the face of climate mitigation policies. As in other areas of the world, California needs to optimize its use of biomass and waste to meet environmental and socioeconomic objectives. We used a systematic review to assess biomass use pathways in California and the associated impacts on climate and air quality. Biomass uses included the production of renewable fuels, electricity, biochar, compost, and other marketable products. For those biomass use pathways recently developed, information is available on the effects—usually beneficial—on greenhouse gas (GHG) emissions, and there is some, but less, published information on the effects on criteria pollutants. Our review identifies 34 biomass use pathways with beneficial impacts on either GHG or pollutant emissions, or both—the “good.” These included combustion of forest biomass for power and conversion of livestock‐associated biomass to biogas by anaerobic digestion. The review identified 13 biomass use pathways with adverse impacts on GHG emissions, criteria pollutant emissions, or both—the “bad.” Wildfires are an example of one out of eight pathways which were found to be bad for both climate and air quality, while only two biomass use pathways reduced GHG emissions relative to an identified counterfactual but had adverse air quality impacts. Issues of high interest for the “future” included land management to reduce fire risk, future policies for the dairy industries, and full life‐cycle analysis of biomass production and use.


| INTRODUCTION
Delivering the greenhouse gas (GHG) emission reductions required to meet the Paris Agreement targets requires a shift to low-carbon energy sources across all economic sectors (IPCC, 2018(IPCC, , 2023)).Biomass will be key to this transition, owing to its versatility in substituting for fossil fuels across multiple energy vectors and the potential to generative "negative emissions," such as through biochar, or when used with carbon capture and storage (BECCS; IPCC, 2021).Conventional uses of biomass include wood use as sawn timber, pulp, and paper.More recent biomass use pathways include its combustion for power generation (including with CCS); the production of biofuels, such as for transport; feedstocks for bioproducts, such as chemicals and bioplastics; and the production of soil amendments, such as biochar.Some alternative biomass pathways, such as the generation of biofuels and electricity, can deliver reductions in GHG emissions and criteria pollutants, but are more costly than the established practice, and uptake is therefore influenced by regulations and financial incentives.As countries across the world look to decarbonize transportation, various low-carbon pathways for fuel production are gaining attention (Kim et al., 2019), with introduced standards including the California Low Carbon Fuel Standard (LCFS) and the US Renewable Fuel Standard (RFS).The LCFS has significantly affected the share of alternative fuels, mainly bioethanol, in California's regulated transportation sector, which increased by 30% between 2011 and 2015 (ARB, 2015).For the US (Yeh & Sperling, 2010), and in other countries (EIA, 2021), the use of renewable transportation fuels is predicted to continue to increase.Where incentives for biomass use are insufficient or reduced, the uptake of pathways can decline.For example, California's use of biomass in power generation peaked between 1990 and 1993 and has subsequently fallen with declining policy support (Li et al., 2023;Tittmann, 2015).Li et al. (2023) comment that "…efforts to construct new biomass-fueled electricity generation capacity at any scale over the last several decades in the state have been hampered by increasing costs without similar increases in revenue from energy sales, including competition from new wind and solar development."Similarly, Tittmann (2015) notes that the predicament of California's biomass energy sector stems from an otherwise positive development: the rapid expansion of low-cost solar photovoltaic power.This development has however created imbalance between peak demand and solar power generation-the so-called "duck curve"presenting an opportunity for the return of bioelectricity in supporting grid balancing.In contrast, the policy framework which incentivizes the diversion of organic waste away from landfills is having an ongoing impact with the percentage of municipal solid waste (MSW) diverted from landfills continuing to increase (www.calrecycle.ca.gov/ LGCen tral/Annua lRepo rt/Rates/ Graph s/Total waste/).
Although crop growth removes CO 2 from the atmosphere, agriculture management practices and the use of agricultural by-products (residues) can be sources of GHGs and criteria pollutants (IPCC, 2018;van Vuuren et al., 2011).Thus, the agricultural, forestry, and land-use sectors-which together account for almost a quarter of global GHG emissions (IPCC, 2019)-have a significant part to play in reducing GHG emissions and in enhancing sinks, while providing economic and environmental co-benefits.The disposal of municipal waste and the recycling of waste can result in GHG, toxins, and criteria pollutant emissions (Cambaliza et al., 2017).In regions where wildfires are a significant risk, there is often a critical need to reduce fuel loads in forests, with this loading liable to exacerbate the frequency and severity of catastrophic wildfires (Jang et al., 2017;North et al., 2015).Thus, in principle, GHG and criteria pollutant emissions can be reduced through burning less agricultural residues on site, reducing the emissions from animal waste, diverting organic material from landfills, and managing forest biomass.
There are good reasons to consider that biomass use will result in net environmental benefits, when deployed to produce renewable fuels, replace fossil fuels in energy production, or to create soil amendments and other useful products (e.g., as a feedstock for the bioeconomy; Baker et al., 2020aBaker et al., , 2020b;;Carreras-Sospedra et al., 2016;Stanton et al., 2018).However, this may not always be the case, and incentives or regulations may be required to achieve the optimal biomass use pathways.Moving to biomass pathways which are energy efficient and have lower GHG and pollutant emissions is likely to have a major role to play in future strategies (Liu & Rajagopal, 2019).These pathways offer a wide range of energy vectors, grid balancing, and cost-effective decarbonization of difficult-toabate sectors, such as aviation and marine fuel (Bauer et al., 2018;Reid et al., 2019).The issues described above are relevant globally, including in California, where there are challenges associated with the large agriculture sector and expansive forest ecosystems.California is at the forefront of developing policies and programs to promote uses of biomass that achieve the lowest criteria pollutant and GHG emissions (ARB, 2018a(ARB, , 2018b)).Here, we have systematically reviewed published research on the use of biomass in California to determine whether current policies are well supported by the available science and to identify current and future research needs.Our systematic review was designed to determine the extent to which the GHG and pollutant emissions associated with current and potential biomass use pathways are understood, that is, have published evidence which quantifies these emissions.We also investigated whether published research could identify any unintended cost and benefits of specific biomass use pathways and whether there are any biomass uses, introduced as GHG mitigation measures, that have negative or unknown impacts on air quality (emissions of criteria pollutants).

| MATERIALS AND METHODS
The systematic literature review was undertaken following an established protocol (Pullin & Stewart, 2006), using the Web of Science database, and with the results collated and organized in Endnote and Mendeley.Systematic literature review protocol requires that the issue under consideration be formulated as research questions and two were identified here: 1. What impact does the cultivation/production of biomass feedstocks have on air quality and GHG emissions in California? 2. What impact does the use of biomass feedstocks have on air quality and GHG emissions in California?
We define cultivation as the combination of planting, managing, and harvesting; we define use as the combustion, application, or recycling, etc. (see full list in Table 3).Search strings were formulated and entered into the Web of Science database to return publications relevant to these research questions.We designed our search strings to capture a broad range of relevant studies, which were screened (initial screening) using predetermined criteria and then critically appraised (full-text appraisal).The search terms were categorized into four main groups: biomass feedstock, biomass use, criteria pollutant, and climate pollutant.This approach produced 122 search strings (see Data S1 for further details) for the identification of papers on biomass feedstocks and 72 search strings to identify papers on biomass use.These searches were run to include publications up to October 2022.
After running the searches, an initial screening was undertaken using the paper titles and abstracts (see details in Data S2).We adopted a conservative approach because we were concerned to review papers which had even partial relevance to the review objectives.A paper passed the initial screening if it covered biomass and either criteria pollutants or GHGs and described work in California or was relevant to California.We recorded why papers were excluded from, or included in, the analysis.The Web of Science searches provided 3108 published papers, reduced to 419 papers for full-text appraisal after the initial title and abstract screening.At the full-text appraisal stage, papers were categorized according to their contribution to understanding how biomass use best meets California's air quality and climate policy goals.We identified 15 biomass categories during the full-text appraisal (Table 1).
During the full-text appraisal, for each of the 419 papers (Table 1), we identified the biomass category and use, or uses, of the categories described.This resulted in 16 specific categories of biomass (Biomass feedstocks) and 23 potential uses (Biomass uses), as shown in Tables 2  and 3. We term the combination of biomass feedstock and use a "pathway," and clearly some pathways are nonviable (e.g., the use or disposal of forest biomass in landfill), while others represent current good practice (e.g., the use of the organic portion of MSW to produce biogas in anaerobic digesters [ADs]).We then separated the conclusions from these papers based on whether adverse or beneficial effects on GHG and criteria pollutant emissions were reported for the biomass uses relative to an identified alternative use (a "counterfactual").Some papers did not present sufficient data to enable the determination of the impacts of specific biomass use on GHG balance, pollutant emissions, or both.Some papers reported a mixture of adverse and beneficial impacts of an identified biomass use and some reported no effect, neutral.Of the 419 relevant papers published between 2005 and October 2022, 208 papers provided sufficient data to quantify the impact of specific biomass use on GHG balance, pollutant Wastewater 5 Urban green clippings 3

Dedicated bioenergy crops 3
Note: The number of papers in each category is also shown.
emissions, or both, and these are presented in Figures 1  and 2. Beneficial effects are indicated in green, neutral in purple and detrimental in red, and the number of papers supporting each effect (+ or −) or which are inconclusive (in) are given in each cell.Some papers conclude that effects were neutral (N), neutral tending to positive (N+) or neutral tending to negative (N−).Shading (light, medium, or dark) indicates the confidence associated with each conclusion; this is a similar "traffic light" matrix system published and explained in more detail by Holland et al. (2015).

| RESULTS
Forest biomass is routinely categorized as California's largest biomass resource.For example, the California Biomass Consortium's (CBC) 2013 update gave a gross California forest biomass resource of 26.8 million bone dry tons per year (MBDT/year) (Williams et al., 2015).The CBC has provided estimates of both gross resource and "technically available resource": the resource remaining after subtracting existing uses.For forest biomass, the technically available resource was 14.3 MBDT/year.Thus, although there is an established conventional pathway for the use of forest biomass (sawn timber, pulp and paper, and disposal in prescribed burning), there remains a substantial available resource and uses of significant potential include: combustion for power, conversion to biogas (by AD), biodiesel or cellulosic biofuel, chip and creation of soil amendments, and use in the bioeconomy.Of the 419 papers reviewed in full here, the largest biomass category was forests and forestry residues, with 83 papers (Table 1).As with other biomass resources, especially agricultural residues, processing and conversion infrastructure varies across California, and the collection and transportation costs can be significant when it is not possible to process the resources nearby.A total 68 of these 83 papers quantified the effects of wildfires or prescribed burning on air quality.Wildfires produce smoke consisting of carbonaceous aerosols, particulate matter, gaseous pollutants and trace gases, and other polycyclic aromatic hydrocarbons (PAHs; Mühle et al., 2007).Although prescribed burning also produces GHG and criteria pollutant emissions, one paper contained data showing that T A B L E 2 Biomass feedstock terms used for analysis of the GHG and criteria pollutant impacts of specified biomass use pathways.

Biomass feedstocks Definition
Wildland Mostly forest but also other land uses vulnerable to wildfires including grassland, chaparral, bush, and brush Forest biomass Biomass coming directly from conventional forest harvesting, removal of residues and thinning operations Biomass: unspecified Often associated with burning.This biomass feedstock is likely composed of a number of biomass sources that cannot be individually identified from the paper, for example, wildland, forest biomass, agricultural residue, wastewater, and MSW

Agricultural residue
Crop waste as a result of agricultural activities, for example, corn stover Livestock associated Biomass emissions associated with livestock production, mostly including nonenteric sources, for example, manure and silage.

MSW Municipal solid waste
Urban green waste Organic waste produced in an urban environment that has the potential to be composted prescribed burning produces less GHG emissions than wildfires (Stephens et al., 2012).The arguments around the merits of using prescribed burning are complex, and a number of papers raise questions such as whether prescribed burning releases more carbon from forests than wildfires (Wiedinmyer & Hurteau, 2010), how mechanical chipping of residues prior to prescribed burning effects GHG emissions (Naeher et al., 2006) and whether forest thinning and prescribed burning help to develop wildfire resilient stands (Busse et al., 2009;Cabiyo et al., 2021).A number of the papers presented the results of regional-scale air quality monitoring associated with forest fires (e.g., Jaffe et al., 2008;May et al., 2014), while others studied the air quality impacts of specific fires, such as the 2002 McNally fire in the San Joaquin Valley (Cisneros et al., 2012), and the Rim "mega-fire" of 2000 (Navarro et al., 2016).Both wildfires and prescribed burning have impacts on emission of pollutants for which standards are set in California's NAAQS (Schweizer et al., 2017).In addition, the PAHs, for which there are no NAAQS, are emitted and are precursors for O 3 formation, which occurs down-wind of fires and increases O 3 exposure in urban areas (Gong et al., 2017;McClure & Jaffe, 2018;Singh et al., 2012).Residential wood fuel burning was found to have adverse GHG balance in two papers (Ge et al., 2012;Heo et al., 2013), and adverse criteria pollutant impacts in 15 papers.These impacts result from combustion conditions and the associated primary and secondary carbonaceous aerosol production.There was little information on the use of forest biomass as a feedstock for the bioeconomy, with one paper extolling the benefits of constructing decking from redwood biomass (Bergmant & Oneil, 2014), and a report exploring a range of innovative wood products pathways using forest resources (Sanchez et al., 2020).The 83 forest biomass papers included a number discussing woodchip-derived biochar.This biomass pathway is of interest because biochar application has been suggested to reduce GHG emissions, especially of N 2 O (Suddick & Six, 2013).Two studies monitored the effects of biochar application on pasture, vineyards, and dairy meadows (Angst et al., 2014;Verhoeven & Six, 2014).Neither study T A B L E 3 Biomass use terms adopted for analysis of the GHG and criteria pollutant impacts of specified biomass use pathways.

Biodiesel
The production of biodiesel from biomass for use in combustion engines

MSW disposal in landfill
The disposal of municipal solid waste (including organic matter) in landfill sites

Burning
The combustion of biomass in a form not necessarily specified or known to the authors recorded decreased GHG emissions as a consequence of biochar application to each of these three land uses, although three studies showed an increased carbon soil content with biochar application (Angst et al., 2014;Keith et al., 2016;Verhoeven & Six, 2014).
In a number of the final 419 papers, the consequence of various biomass uses was described without specification of what biomass resources were used (Figures 1 and 2, row 3).These papers mostly described air quality impacts arising due to emissions from a number of mixed sources.For example, Heo et al. (2013) describe biogenic emissions of PM 2.5 arising from biomass use generally.Carreras-Sospedra et al. (2016) describe the beneficial GHG balance from the use of unidentified biomass in power generation, while Stanton et al. (2018) describe the GHG benefits of using unidentified biomass to produce compressed natural gas via gasification and to produce biochar for soil application.The burning of unspecified biomass resource was described as leading to adverse results for criteria pollutant emissions in 26 papers (Figure 2, row 3).Biogenic emissions of criteria pollutants were also assessed at a regional scale (Brewer & Moore, 2009;Heo et al., 2013;Peltier et al., 2008;Yu et al., 2019).
MSW represents the second largest biomass resource in California, after forest biomass, and in 2013 the CBC estimated the gross MSW resource at 26 MBDT/year, with 9.0 MBDT/year considered technically available (Williams et al., 2015).The primary disposal and use options for MSW are diversion for reuse, potentially including in the bioeconomy (recycling and waste reduction), diversion and use in ADs, incineration, and disposal at landfill sites.A number of the studies quantified methane (CH 4 ) emissions from California's landfills, and show that the amounts emitted are determined by the presence or absence of engineered gas extraction, the physical properties of the soil cover, and the rates of methanotrophic oxidation (Cambaliza et al., 2017;De La Cruz et al., 2016;Fei & Zekkos, 2016).Both the conversion of MSW to landfill F I G U R E 1 Effects of different biomass use pathways on greenhouse gas emissions as identified in a systematic review of Californiarelated papers between 2005 and October 2022.Each cell represents a pathway or combination of biomass resource (rows) and uses (columns).Beneficial effects are identified in green, neutral effects in purple and detrimental effects in red, and the numbers of papers supporting each effect or which are inconclusive (in) are given in the cells.The confidence associated with each of these conclusions is determined by the number of supporting publications and the clarity of their conclusions; low = 1-2 papers, medium = 3-4 and high ≥5 using light, medium, or dark shades of green, orange, or purple, respectively.Where papers report inconclusive effects, a light gray color is used, while viable pathways with no published data captured are clear cells and nonviable pathways are dark gray.
gas and the production of biogas by anaerobic digestion of MSW reduce CH 4 emissions, as well as offset emissions from fossil fuel consumption (Parker et al., 2017;Stephens-Romero et al., 2010, 2011) As a result of the NOx emissions, which arise from the combustion of organic material in MSW or of the biogas derived from it, conversion plants have to meet emission standards for NOx, SO 2 , heavy metals, and dioxins.Nonthermal technologies are also available (Dabir et al., 2017;Thomas, 2017).These can be used to recover energy from MSW without the need for combustion, thus avoiding the associated pollutant emissions.
Of the 419 papers reviewed in our full-text appraisal, 40 explored the impacts of the third largest biomass source in California-agricultural activities-on air quality and GHG emissions.This resource includes livestock manure and crop residues, and the CBC 2013 estimates animal manure resources at 11.62 and 4.35 MBDT/year, gross and technical availability, respectively, and crop residues at 8.8 MBDT/year gross and 4.1 MBDT/year available (Williams et al., 2015).Significant potential uses of this resource include the production of fuels, combustion of crops, production and use of biochar, and livestock manure management.McCarty (2011) estimated GHG emissions from burning crop residues on site, using remote-sensing data, while Keshtkar and Ashbaugh (2007) estimated the release of PAHs as a result of burning biomass from almond pruning and rice straw in California.The burning of agricultural residues on site has adverse effects on criteria F I G U R E 2 Effects of different biomass use pathways on the emissions of criteria pollutants as identified in a systematic review of California-related papers between 2005 and October 2022.Beneficial (green), neutral (purple), and detrimental (red) effects and with confidence identified using the same criteria as for greenhouse gas emissions in Figure 1.
pollutant (Chow et al., 2010(Chow et al., , 2011;;Keshtkar & Ashbaugh, 2007;McCarty et al., 2009;Roy et al., 2018;Young et al., 2016).While these are both negative impacts, two Californian studies found clear GHG balance benefits from the use of agricultural residues to produce ethanol and to generate power via biomass gasification (Canter et al., 2016;Pereira et al., 2016).Alexiades et al. (2018) considered the use of sugar beet to produce ethanol as an approach to meeting California's LCFS.The proposed pathway estimates GHG reductions of up to 71% compared with gasoline use in California.The use of crop-based biofuels has led to concerns of the GHG emissions released as a result of indirect land-use change (Hertel et al., 2010), and such concerns could limit the contribution of ethanol from maize to California's LCFS (Gopal & Kammen, 2009).Four papers described neutral effects on GHG balance of biochar application produced from agricultural residues (Pereira et al., 2016;Suddick et al., 2010Suddick et al., , 2011;;Verhoeven & Six, 2014).Three recent papers present data to support the beneficial effects (positive) of biochar application on GHG emissions (Thengane, Burke, et al., 2020;Thengane, Kung, et al., 2020;Duan et al., 2022).
Our systematic review included several papers sampling emissions from livestock, mainly dairy cows (Figures 1 and 2, row 5).Shaw et al. (2007) measured volatile organic (VOC) emissions from dairy cows, including from manure.Emission measurements from dairy cows have also demonstrated temporal complexity: Ammonia (NH 3 ) emissions have been shown to fluctuate by a factor of 10 through the course of a day (Moore et al., 2014).Emission rates and cumulative emissions can be affected by the conditions of anaerobic storage (El-Mashad et al., 2011).The O 3 -forming potential of reactive organic gases from poultry was more than double that of dairy cattle, beef cattle, or swine and approximately double that released by light-duty vehicles (Howard et al., 2010).
Urban green waste is a significant component of the municipal biomass resource, and its combustion produces CO 2, NOx, and VOC, which are all O 3 precursors (Figure 1, row 7, column 4).Kumar et al. (2011) characterized over 100 VOCs from this pathway.In contrast, green waste composting (Williams et al., 2019;Zhu-Barker et al., 2017), direct incorporation on site (Zhu-Barker et al., 2016), use as a feedstock for the bioeconomy and, in particular, conversion to biogas in ADs, have overall positive benefits on GHG balance (Figure 1, row 7), and for criteria pollutants (Kumar et al., 2011;McPherson et al., 2015;Stephens-Romero et al., 2010, 2011;Zhu-Barker et al., 2016).These pathways are also beneficial compared with green waste disposal in landfill.Williams et al. (2019) identified the need to optimize airflow during composting to reduce damaging atmospheric emissions from green waste.At 4.5 MBDT/year gross and 3.6 MBDT/year technical availability in 2013 (Williams et al., 2015), food processing residues are the fourth largest category of biomass resource in California, and they include a wide range of materials: nut shells and hulls, fruit pits, cotton gin trash, meat processing residues, grape and tomato pomace, cheese whey, beverage wastes and wastewater streams containing sugars, and other degradable solids.Zhang and Ying (2010) and Zhang et al. (2014Zhang et al. ( , 2015) ) showed that food waste is a highly desirable substrate for ADs and, over recent years, anaerobic digestion of fats, oils, and grease from the grease traps of wastewater treatment facilities has become increasingly employed in California (Palacios et al., 2011).The use of food processing residues in ADs to generate biogas has a net positive GHG emission impact compared with disposal at landfill sites (Kuo & Dow, 2017).The use of food processing residues also has net beneficial effects on criteria pollutant emissions relative to disposal in landfill (Kuo & Dow, 2017), such as their conversion to, and the use of, biodiesel (Hajbabaei et al., 2012).However, the combustion of biogas for electricity is accompanied by emissions of NOx, SO 2 , CO, CO 2 , and CH 4 , regardless of the feedstock used in production (Kuo & Dow, 2017).Emissions of VOCs from rendering plants give rise to the neutral and neutral-negative impacts on criteria pollutants and, since about 50% of the weight of livestock is not consumed by humans, the rendering of the byproducts of butchering operations occurs on a significant scale (Guerra et al., 2017).An additional biomass use pathway of food residues is gasification, with the use of walnut shells in this pathway producing energy as well as sequestering carbon (Pereira et al., 2016;Suddick et al., 2011;Suddick & Six, 2013).
There are a number of other potential uses of specific bioproducts from food processing.Tomato and red grape pomace from tomato paste, and wine production, also represent abundant waste streams in California.There is potential for extraction and use of carotenoids, lycopene, anthocyanins, and other antioxidants from these materials prior to the use of the residual lignocellulose as a biofuel in ADs (Allison & Simmons, 2017, 2018).There is also an established route for using almond hulls as a component of cattle feed, but little or no published information on the GHG and criteria pollutant impacts of these pathways.
The incineration of dried sludge derived from wastewater (sewage) is now being phased out in most countries and replaced with anaerobic digestion (Kuo & Dow, 2017).In the United States, about 50% of wastewater plants now have ADs and biogas production.In our review, all five of the papers which considered wastewater confirmed the positive GHG and criteria pollutant impacts of biogas production from wastewater (Kuo & Dow, 2017;Parker et al., 2017;Stephens-Romero et al., 2011;Wong et al., 2022).Waterland et al. (2008) conducted an LCA of alternative transportation fuels (ethanol, natural gas, LPG, electricity, and hydrogen), concluding that there are potential benefits of alternative biomass fuels in comparison with gasoline: These pathways result in reduced GHG emissions, with some biofuel pathways leading to a 75% reduction compared with gasoline.The benefits to GHG balance (reduced emissions) which result from the use of biodiesel and bioethanol are confirmed by a number of recent Californian papers (13 and 10 papers, respectively-see Supporting Information).There are two recently published papers on the GHG benefits of "syngas" (a product of gasification; de Fournas & Wei, 2022; Indrawan et al., 2017).Conclusions over the impacts of liquid biofuels on criteria pollutant emissions are more variable: The use of biodiesel is mostly seen as having a beneficial impact (Figure 2, column 1, and see, e.g., Vogel et al., 2019;Zhang et al., 2014Zhang et al., , 2015)), although two papers also suggested a neutral to negative impact (Cahill & Okamoto, 2012;Hajbabaei et al., 2012), for example, the emission of acrolein and other aldehydes from biodiesel-fueled heavy-duty vehicles.
Only two biomass pathways emerge as beneficial (good) for GHG emissions but "bad" with respect to the emission of criteria pollutants.These are the use of dedicated biofuel crops and of first-generation biofuel feedstocks to produce bioethanol (Figures 1 and 2, column 8).It is interesting that the use of forest biomass and agricultural residues in combustion for power does not show this combination of impacts (good for GHG and bad for pollutants); this conclusion is based on using open burning as the more polluting counterfactual and is well supported by the published evidence (see discussion below).The apparently clear good/bad for GHGs/pollutants in relation to bioethanol use is controversial.In a recent review of LCAs, Scully et al. (2021) found that the carbon intensity of corn ethanol was 46% lower than that of fossil fuel gasoline, although there was a wide range in how the LCAs accounted for land-use change impacts, which is an area of contention in determining the environmental impacts of bioenergy.The findings of Scully et al. (2021), that corn ethanol reduces GHG emissions, are supported by Taheripour, Delgado et al. (2022) and Taheripour, Mueller, et al. (2022), who concluded that reports that corn ethanol had adverse effects on GHG emissions (e.g., Lark et al., 2022) were based on overestimates of the GHG emissions from corn ethanol use.The pollutant problem for bioethanol (Figure 2, column 8, bottom cells) is that under some conditions VOC emissions and associated O 3 production can be larger for high-blend ethanol fuels (E85) compared with conventional gasoline (e.g., see Ginnebaugh & Jacobson, 2012).However, there is recent evidence that different blends of biofuels in modern vehicles could allow higher engine efficiency and better fuel economy (Dagle et al., 2022). Fertitta-Roberts et al. (2017) summarized the substantial LCA evidence that cellulosic ethanol has GHG intensities less than half those of conventional gasoline.These authors also showed that the adverse pollutant effects of sorghum-derived E85 are strongly influenced by crop nitrogen management and that the adverse pollutant impacts could be partially mitigated by the efficient application of nitrogen fertilizers.

| DISCUSSION
Our systematic review on the GHG and criteria pollutant emissions of biomass pathways in California identifies 34 biomass use pathways with beneficial impacts on either GHG or pollutant emissions, or both-the "good," and 13 biomass use pathways with adverse impacts on GHG emissions, criteria pollutant emissions, or both-the "bad."The research identified by this review confirms that the controlled use of biomass to produce power by combustion and gasification is beneficial relative to wildfires, prescribed burning, and decomposition in the open.Similarly, the use of biomass resources to produce biogas in ADs is generally beneficial to both GHG and criteria pollutant balance.Eight of the pathways identified in the review were bad for both GHG and criteria pollutant emissions: These are wildfires involving any biomass, disposal of MSW in landfill (without the use of landfill gas), and open decomposition of livestock-associated waste.Existing biomass practices can be improved to reduce negative GHG and criteria pollutant results, although this will require policymaker support to incentivize or regulate biomass use, because beneficial biomass use pathways are not necessarily financially viable.Our review led to an overall conclusion that much of California's current policy is well supported by the science evidence.More recently developed biomass use pathways have less data to support their overall impacts on GHG or criteria pollutant emissions, although several hold particular promise, including the production and application of biochar.This systematic review identifies some specific areas where current policies are not yet aligned with emerging science although it may also be that current practice on the ground has not caught up with emerging policy.For example, GHG emissions from the burning of forest and agricultural residues on site are a concern and there is a need to adopt production and conversion pathways, which improve the GHG balance of liquid biofuels (further discussion to follow).
California's wildlands grassland, chaparral, brush, etc.) represent a significant component of the standing biomass across the state, and the management of these resources is the focus of increasing policymaker attention, owing to severe wildfire risks (California's Wildlife and Forest Resilience Action Plan, State of California, 2021).As a result of the increasing frequency and severity of wildfires, these wildlands have already transitioned from a net sink to a net source of carbon and are a major source of criteria pollutant emissions (Forest Climate Action Team, 2018; Wang et al., 2019).Reducing the density of these natural resources is now a state priority, and it presents an opportunity to support biomass use pathways that achieve beneficial GHG and air quality outcomes.The state and federal governments have committed to treating 1 million acres of forest and wildlands per year (Office of Governor Gavin Newsom, 2020), using prescribed burns and mechanized removal of biomass, or mechanical thinning.Mechanical thinning avoids the air quality impacts of controlled burns and provides small-diameter trees of little commercial value for the Californian bioenergy sector.Our review found that forest biomass use for power generation or hydrogen generation via gasification reduces GHG and criteria pollutants compared with open pile biomass burning (Springsteen et al., 2011;Stephens et al., 2012).This is consistent with the findings from LCAs that all conversion pathways for forest residues result in lower GHG emission impacts relative to burning them on site (Liu & Rajagopal, 2019).California's state government currently incentivizes this biomass use pathway through the BioMAT feed-in-tariff program, which subsidizes power station combustion of forest biomass from hazardous forest zones.However, uptake has been limited, partly because the economics remain challenging: Open pile burning is often the only economic disposal of woody biomass from hazardous forest zones (Springsteen et al., 2011).There is a risk that controlled burning will be overly relied upon, at the expense of air quality and with the generation of GHG emissions in the absence of a useful product.Up to 95.1 MBDT of dead-standing forest biomass is estimated to be feasibly harvestable as energy feedstock (Tubbesing et al., 2020), and a further 15.1 MBDT is available annually through thinning operations (Baker et al., 2020a(Baker et al., , 2020b)).This biomass use pathway could also support California's 2045 net neutrality target if bioenergy facilities also use carbon capture and storage (BECCS) to generate negative emissions (Baker et al., 2020a(Baker et al., , 2020b)).
Global CH 4 emissions continue to rise, and landfills account for an estimated 18% of global anthropogenic methane emissions (Jackson et al., 2020).Our review found that conversion of MSW to landfill gas reduces criteria pollutant emissions compared with simple decomposition in landfill (Chavero et al., 2011;Dabir et al., 2017;Kuwayama et al., 2019;Stephens-Romero et al., 2010, 2011).Breunig et al. (2018) show the spatial distribution of California's nonforestry biomass resources, including MSW.The policy framework which has driven the diversion of organic matter away from landfill sites is having an ongoing impact and, although California's volumes of MSW continue to increase, the percentage being diverted away from landfill is increasing (www.calrecycle.ca.gov/ LGCen tral/Annua lRepo rt/Rates/ Graph s/Total waste/).This reflects the success of the state's laws to accelerate organic recycling (AB 1826 andAB 1594), and California's Plan for Optimal Use of Organic Waste (Ca SB 1383, 2016).
Animal manure is California's third largest biomass resource, and the enteric digestion of cattle and the decomposition of animal waste both produce methane (CH 4 ).Breunig et al. (2018) estimate the level of California's nonforest biomass production in 2050, with dairy manure projected to increase while beef manure decreases.Methane emissions from California's dairy-responsible for nearly half of the state's CH ), including the use of ADs which capture biogas from decomposed manure, with the biogas then available as a transport fuel or to generate electricity.While concerns have been raised with how this biogas production is currently subsidized by the LCFS, our review supports the positive impact of this pathway: Leaving animal waste to decompose on site (in fields) results in significantly greater GHG emissions-mostly as CH 4 (Hopkins et al., 2016;Jeong et al., 2016Jeong et al., , 2017)).Our systematic review indicates that criteria pollutant emissions also fall under the AD pathway compared with decomposition on site (Moore et al., 2014;Stephens-Romero et al., 2011).Shih et al. (2008) explored three policy options with varying economic incentives to reduce livestock emissions of ammonia and CH 4 through better management practices.
It is clear from the available evidence (Figures 1 and  2) that burning agricultural residues on site has adverse effects for GHG and criteria pollutant emissions.The spatial and temporal extent of crop residue burning in the United States has been documented and in over 30,000 ha of crop residues are burnt per year (McCarty et al., 2009).The California Air Resources Board (CARB) GHG inventory data show that emissions from the burning of crop residues have been rising steadily since 2001 and by 2020 they had reached more than 0.1 Mt CO 2 per year (ARB, 2020).Urban green waste represents a relatively small component of California's biomass, but our results show that the various use pathways have significantly different GHG and criteria pollutant impacts (see, e.g., Kumar et al., 2011;Williams et al., 2019).Optimizing composting practice (controlling feedstock mixes and aeration) significantly reduce ammonia and VOC (mainly alcohols) emissions (see CalRecycle, 2008).Most municipalities already have programs of green waste collection and, under Senate Bill 1383, California's cities and counties are now required to provide organic recycling collection to all residents and businesses.
One of our review objectives was to identify whether there are biomass uses being introduced as GHG mitigation measures which have negative impacts on air quality (criteria pollutants).Other than the blends of bioethanol with conventional gasoline in transportation fuels, which was discussed above (Dagle et al., 2022;Ginnebaugh & Jacobson, 2012), concerns over this have focused in particular around the use of biomass combustion for electricity generation.However, there is a clear weight of evidence here that controlled combustion of biomass in power stations results in lower criteria pollutant emissions than occur in wildfires and when agricultural residues are burnt on site.Springsteen et al. (2011) make this comparison directly, and seven papers in our systematic review reported adverse effects on criteria pollutants from the burning of agricultural waste, with an additional 26 papers reporting adverse effects on criteria pollutant emissions from the open burning of biomass from unspecified sources.Thus, concerns over criteria pollutant emissions from the use of biomass in cogeneration appear unfounded.Today, there are about 30 direct-combustion biomass facilities in operation in California with a total generating capacity of 640 MW.This is less than half the number of facilities which were utilizing biomass during the industries peak (ww2.energy.ca.gov/bioma ss/bioma ss.html), and biomass combustion for power generation looks set to decrease further in California (California Energy Commission, 2022).This technology could be used as dispatchable power to support California's electricity grid, mitigating the risk of blackouts which are already experienced during periods of peak demand (Abido et al., 2021), and to provide negative-carbon electricity when coupled with carbon capture and storage (BECCS; Baker et al., 2020aBaker et al., , 2020b)).Yang et al. (2019) clearly showed that for the biomass combustion for power pathway, the incorporation of CCS would move the GHG outcome toward positive.On recent trends, however, biomass combustion will not deliver the potential it could to California's 2045 target date for 100% zero-carbon electricity and carbon neutrality.This is disappointing given the conclusions of this review, the size of California's biomass resource, and the expected co-benefits to wildfire mitigation.In the United States, the major uses of first-and second-generation dedicated biomass crops are the production of biodiesel or bioethanol and gasification to produce syngas.Growing dedicated biomass crops for the production of transportation fuels has not been a priority in California, but the State's LCFS has resulted in the significant use of biodiesel, bioethanol, and electric vehicles.
For biodiesel, the overall conclusion from the review presented here is that with the correct blend proportion and combustion technology, and when analyzed over the whole life cycle, the net effects on GHG and criteria pollutant emissions are beneficial (Figures 1 and 2, column 1).Similarly, work on combustion technology and blend proportions is likely to address concerns over criteria pollutant emissions from bioethanol for second-generation biofuels (Dagle et al., 2022;Hajbabaei et al., 2012).Criteria pollutant impacts must also be balanced against the clear GHG emission benefits (Scully et al., 2021).The logistics, economics, and other details are often complex, and projects looking at the practicalities of introducing new technologies and incentive programs are important to achieve implementation.For example, H 2 could be a very effective transportation fuel and there is the potential to produce H 2 from wastewater, MSW, livestock waste, and green waste (Chen et al., 2017;Parker et al., 2008).The published information on the GHG and criteria pollutant impacts from alternative, biomass-based, transportation fuels, and from electric vehicles, show that these developments have had, and are likely to continue to have, benefits for air quality and the climate.
The net GHG balance, including land-use change, associated with biofuel production is critical in determining whether a fuel qualifies as a biofuel or an advanced biofuel under California regulations (Broth et al., 2013;Efroymson et al., 2016;Mullins et al., 2011;Scully et al., 2021).The farmland of California's Central Valley grows highvalue produce that contributes a major proportion of the US food supply, likely explaining why there is limited interest in growing dedicated bioenergy crops in this region.Nonetheless, as decarbonization of California's energy sector continues, including areas hard to decarbonize with wind and solar energy, there is potential for the utilization of bioenergy sourced from nonfood crops (secondgeneration bioenergy crops) that can be integrated into food production landscapes.are four major trends in biomass use in California: (1) the use of forestry and crop biomass (often residues) in power generation is reducing; (2) there is increasing diversion of organic waste from landfills, largely to ADs; (3) the increased use of livestock manure in ADs, as the alternative to conventional disposal routes on fields; and (4) the growing use of biomass to manufacture liquid transportation fuels.Our review identified 34 biomass pathways beneficial in respect to GHG, criteria pollutant emissions, or both-the "good."Of these, 14 pathways are "win-wins," beneficial for both GHG and pollutant emissions.These pathways include a variety of biomass resource uses, confirming that choice of conversion pathway can be an effective strategy for California's heterogeneous and diffuse biomass resources.We show that two of these win-win pathways-combustion of forest biomass and agricultural residues for power-are declining.For forest biomass, it is likely that prescribed burning is replacing use in power generation, while agricultural resides are likely being left on site or used as mulch.Thirteen current biomass use pathways have adverse effects on GHG, pollutant emissions, or both-the "bad."The eight pathways (or outcomes) bad for both GHG and pollutant emissions were as follows: fires of wildlands, unspecified burning and prescribed burning of forest biomass, burning of agricultural residues, decomposition of livestock-associated waste, the composting of MSW, use of MSW in landfill (without the use of landfill gas), and the combustion of urban green waste.There are a range of policy measures driving these changes.The policy papers identified in our systematic review support the conclusion that meeting GHG emission targets will require a shift to low-carbon energy sources across all sectors and the use of a combination of mitigation strategies.Low-carbon fuel standards influence demand for biogas, biodiesel, and bioethanol produced from biomass.The published information on the GHG and criteria pollutant impacts from alternative, biomass-based, transportation fuels, and from electric vehicles, show that these developments, along with the "good" identified above, are likely to have benefits for air quality and the climate in California and elsewhere-the "future." . Two papers looked specifically at the fate of the organic component of the MSW going to California's landfill sites: Bogner et al. (2011) examined how to improve the effectiveness of engineered landfill covers to limit GHG emissions (CH 4 , CO 2 , and nitrous oxide-N 2 O), and Kong et al. (2012) present Californiaspecific life-cycle assessments (LCAs) of the organic compounds in MSW, mainly paper and food waste.The use of thermal and nonthermal technologies to provide energy carriers (fuels) from the organic material in MSW has the potential to reduce the volume of MSW going to landfills and thus to avoid landfill CH 4 emissions.There are NOx emissions from the combustion of biogas in engines, and Chavero et al. (2011) trialed a system for their reduction.Lam et al. (2011) trialed a system to optimize the airflow ratios in CH 4 burning engines to reduce NOx emissions.
4 emissions-increased by 25% during 2000-2020, falling somewhat during the last decade (ARB, 2022).Based on the 2012 and 2017 Census reports, ARB calculated an average annual decline of 0.5 percent in California's dairy and livestock populations from the sector between 2008 and 2017 (ARB, 2020, 2022); information which postdates the Breunig et al., 2018 prediction of increasing volumes of dairy manure by 2050.California introduced the world's first methane emission reduction target (Senate Bill 1383), which requires a 40% reduction by 2030, relative to a 2013 baseline.As part of meeting this target, manure management projects are being installed across the state (The Alternative Manure Management Program [AMMP]

T A B L E 1
Fifteen categories of relevance to Californian biomass, identified during the full-text analysis of 419 papers.

Biomass resource/relevance categories Number of papers after initial screening
Application of biocharThe application of charcoal substance produced from the pyrolysis of agricultural and forestry waste Abbreviations: AD, anaerobic digester; CNG, compressed natural gas; GHG, greenhouse gas.