Collocated measurements on optical and chemical properties made at a coastal urban location Visakhapatnam on the east coast of India were used to assess the relative contribution of different chemical species to composite aerosol radiative forcing. At such a location, the dominant species that decide the atmospheric forcing are the relative mass fractions of Black Carbon (BC) and sulfate. It is observed that the composite forcing at top of the atmosphere follows the BC mass concentration during all the seasons except for some days. In such cases the hypothesis on the role of mixing state of aerosol in deciding the net aerosol radiative forcing is examined to conclude that the BC either independently or in the internal mixture state during winter months would decide the aerosol composite forcing over this coastal urban location. Though the conditions for the formation of such mixtures and their seasonal dependence however remain unclear, drier weather conditions with abundance of sulfate seem to favor the formation of well mixed aerosol.
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 Quantification of the climate impact of atmospheric aerosols requires knowledge of their physical, chemical, optical and radiative properties as well as their spatial and temporal variability [Penner et al., 1994; Huebert et al., 2003; Yoon et al., 2005]. For creating optimal control strategies, estimation of light extinction, optical depth and radiative forcing due to atmospheric aerosols with chemical apportionment is important in assessing the aerosol impact on reduction of incoming solar radiation at the surface [McInnes et al., 1998; Bush and Valero, 2003] which alters the atmospheric energetics. The direct aerosol radiative forcing for individual aerosol species remains less certain though the direct composite aerosol forcing is better quantified now [Intergovernmental Panel on Climate Change, 2007]. Besides the individual species that contribute to aerosol radiative forcing, the magnitude and direction of the direct aerosol effect depends on the aerosol absorption and up scatter fraction as well as on the albedo of the underlying surface. Tropospheric aerosols that dominate the integrated aerosol optical effects in a vertical column are thought to have substantial anthropogenic component with characteristic species like sulfate, Black Carbon (BC), Organic Carbon (OC), mineral dust and nitrate aerosol. Carbonaceous and sulfate aerosols play an important role in regulating the amount of solar radiation absorbed by the earth-atmosphere system. The atmospheric warming by BC is regulated by the ambient concentration of sulfates [Ramana et al., 2010], particularly in a coastal urban environment, where BC and sulfates are expected to be more abundant. Among the radiatively active species of anthropogenic origin in the atmosphere, BC is increasingly recognized as an important contributor to global climate change [Jacobson, 2002; Hansen and Nazarenko, 2004; Roberts and Jones, 2004; Wang, 2004]. The effect of BC on radiative forcing is important because of its high absorption of solar radiation. However, uncertainties are large due to its spatial and temporal variation and due to the lack of sufficient observational data on BC. The extent of BC induced warming is dependent on the origin of BC and on the concentration of sulfate and organic aerosol which reflect the solar radiation and cool the surface and thus the net RF is determined by relative amounts of BC and Sulfate [Ramana et al., 2010]. Haywood and Shine  calculated a global mean radiative forcing due to soot and sulfate particles between −0.1 to −0.28 W m−2. Their estimate was based on the sulfate distribution measured by Langner and Rodhe  assuming a constant soot /sulfate ratio of 0.075. Intergovernmental Panel on Climate Change  suggested a range of −0.2 to 0.8 W m−2 for sulfate, +0.33 to +0.03 W m−2 for BC and −0.07 to 0.6 W m−2 for biomass. Second the solar absorption by BC is amplified when it is internally mixed with Sulfate [Jacobson, 2001; Novakov et al., 2001; Moffet and Prathe, 2009]. Chýlek et al.  reported that though the specific absorption of BC depends on the BC refractive index, size and shape of the BC particle, the same particle will absorb different amounts of solar radiation depending on whether it is located in the free atmosphere or inside a liquid droplet or sulfate aerosol. Similarly, the amount of solar energy absorbed by a sulfate droplet depends on the size of the sulfate particle and on the amount and location of the BC within. Thus it is important to quantitatively assess the relative dominance of the BC and sulfate aerosol in controlling the composite aerosol radiative forcing. With this idea in view, we have used the optical and chemical properties of atmospheric aerosols to examine the controlling factors on aerosol radiative forcing with respect to chemical species in a coastal urban environment which is expected to be dominated by BC due to the anthropogenic nature of the location and sulfate due to both the coastal and urban proximity.
2. Data and Instrumentation
 Measurements of aerosol chemical, physical and optical properties were carried out over Visakhapatnam (17.7° N, 83.3° E; 230 m asl), an industrialized urban coastal location on the east coast of India. The observing site has an industrial area in the southwest and is very close to the sea coast (500 m away). The simultaneous measurements included 1) collection of total suspended particulates (TSP) using a high volume sampler (HVS, Envirotech Model APM 430) with a high flow rate of 1.1 to 1.7 m3 min−1. The instrument was operated by the Physical Research Laboratory (PRL, Ahmedabad, India) to collect aerosol samples at Visakhapatnam and the chemical analysis was done at PRL. The procedure suggested by Rastogi and Sarin  was adopted for obtaining the water soluble constituents (NH4+, Na+, K+, Mg2+, Ca2+, Cl−, NO3− and SO42−). The precision estimate from the standard deviation of repeat measurements of standards and samples was better than 4% for Cl−, NO3− and SO42−; 2% for Na+, K+ and NH4+; and 5% for Ca2+ and Mg2+. The samples were collected every 8th day (mostly on every Wednesday of the week). A total of 52 HVS samples collected during May 2005–November 2006 were used in the present study. 2) Black Carbon (BC) mass concentration using a seven channel Aethalometer Model NO: AE-47 developed by Magee Scientific Company. The instrument has seven wavelengths viz., 370, 470, 520, 590, 660, 880 and 950 nm and is operated with a time resolution of 5 min and a standard mass flow rate of 1.9 L per minute. The overall uncertainty in BC mass concentration is in the range 15 to 20%, with the higher percentages applicable to periods of low BC concentrations. Detailed report on the error analysis is given by Moorthy and Babu . 3) the aerosol spectral optical depth at 5 wavelengths centered about 380, 440, 500, 675, and 870 nm using a calibrated Microtops II Sun photometer (Solar Light Co, USA). Typical combined error in AOD measurement by Microtops II Sun photometer as described by Porter et al.  and Ichoku et al.  is in the range 0.009 to 0.0119 (around 1%) at different wavelengths and the absolute uncertainty in the AOD measurement is ±0.03. An error of ±0.03 is an error of ±6% for a mean optical depth of 0.5.
 Effect of aerosol on radiation budget is assessed by the parameter called “radiative forcing” which is defined as change in net radiative flux at given atmospheric level due to change in given parameter such as aerosol while holding all other parameters fixed. In the current study, OPAC (Optical Properties of Aerosols and Clouds) aerosol model developed by Hess et al.  was used in estimating the optical properties of the composite aerosols. The measured optical depth spectrum was reconstructed by constraining the number densities of soot (BC) and sulfate which were derived from measured mass concentrations and varying the number density of other components [water soluble, sea-salt (accumulation) and sea-salt (coarse)]. The AODs are reconstructed iteratively until the modeled values and measured values match within ±5% deviation. Principal optical parameters viz., the aerosol optical depth, single scattering albedo, and phase function are derived separately for composite, solely BC and solely sulfate aerosols by incorporating the LIDAR derived mean mixing layer heights and at 70% relative humidity. The optical and radiative properties derived from the above model were then fed to the Santa Barbara DISORT Atmospheric Radiative Transfer model (SBDART) developed by Ricchiazzi et al.  to derive the composite, BC and sulfate aerosol radiative forcing. As the location is a coastal one, the spectral albedo of the surface was taken as a combination of three basic surface types (ocean water, vegetation and sand) out of the available five types of SBDART. The shortwave aerosol forcing for all atmospheric layers calculated at 5° zenith interval are used to determine the diurnal averages. The diurnal averaged forcing for surface and top of the atmosphere are determined separately for composite, BC and sulfate aerosols. While, incorporation of all the components viz., sulfate, soot, water soluble, sea salt (accumulation) and sea salt (coarse) in deriving the optical parameters using OPAC which were then fed to SBDART gives the composite aerosol forcing, the BC forcing is evaluated by subtracting the forcing estimated by making soot concentration as zero from the composite forcing, the difference of which gives BC forcing. Similarly the sulfate forcing also is estimated. This procedure is described and used in many earlier studies [Satheesh, 2002; Babu et al., 2002; Vinoj and Satheesh, 2003; Sreekanth et al., 2007; Niranjan et al., 2007a]. To determine how well SBDART predicts the total Short wave (SW) surface irradiance, Ricchiazzi et al.  compared the predictions of the SW irradiance with Normal Incidence Pyranometer (NIP) and reported that SBDARTs prediction is within about 1% of the NIP observations which is considered as a validation of SBDARTs accuracy. In the current study, the OPAC reconstructed optical depths match with the measured optical depths within ±1% and considering the measurement accuracy of approximately 6% with the Microtops II sun Photometer and 1% match in SW radiance calculation, the overall uncertainty in the forcing calculation will be less than 10%. Of the 52 days of data, we selected only 12 days with extremely clear sky condition in the present study as inclusion of the other days may contaminate the forcing estimation due to intermittent presence of clouds in the visual field of view and following Coakly et al.  that inclusion of aerosol forcing over the cloudy part of the atmosphere increases the direct aerosol forcing by 20–25%.
3. Results and Discussion
 Quantifying and assessing the climate impact of atmospheric aerosols require knowledge of the chemical composition of the composite aerosol since the optical properties depend on the aerosol chemistry. With this in view, the aerosol chemical speciation of the water soluble components was examined with respect to the ambient BC aerosol mass concentration. Figure 1 shows the mass concentration of water soluble species during all the 52 days of observation during May 2005 to November 2006. The mean value of TSP during this period was 115 ± 37 μg m−3 and water soluble ionic species (NH4+, Na+, K+, Mg2+, Ca2+, Cl−, NO3− and SO42−) mean mass concentration was 30 ± 12 μg m−3. The dominant ionic specie was SO42− during whole period except during May–August 2005. The concentrations of different species and their relative abundances show significant temporal variability which could be attributed to the seasonal changes in the relative source/sink strengths and/or due to changes in prevailing meteorological conditions. Mass concentrations of SO42−, K+ and NH4+ which are mostly of continental/anthropogenic origin show high values during winter months, whereas concentrations of Na+ and Cl− which are of oceanic origin show higher values during summer and monsoon months. Chemical species like SO42−, K+, Mg2+, Ca2+ could be due to both anthropogenic as well as oceanic sources. The abundance of the sulfate aerosol throughout the year could be seen clearly in the relative mass concentrations. This is typical of a coastal urban aerosol since both marine and urban environments have a significant sulfate though the sources are different. In this context it is worthwhile to see the relative contributions of the sea salt and non sea salt sulfate to the total sulfate mass concentration.
 The sea salt (SS) and non sea salt (NSS) components are separated from the respective masses using Sodium (Na) as reference element. The mass of the NSS component in each of the above species can be estimated as [Duce et al., 1983]
where Mnss(x) is the mass of NSS component of species x to its total mass loading Mx and MNa is the mass loading due to sodium. (Mx/MNa)sw is the mass ratio of species x to Na in seawater. The mass of the NSS sulfate was estimated using equation (1) taking the ratio of (Mx/MNa)sw as 0.25 for SO42−. This value is taken from the ratios of the constituent concentrations from bulk seawater samples reported by Wilson  and Kenne et al. . International systematic surveys indicate that there is no significant spatial variations among the ratios of Cl−, Na+, SO42−, Ca2+ K+ and Mg2+ [Culkin, 1965; Wilson, 1975; Kenne et al., 1986]. Figure 2 shows the mass concentration of total, sea salt and non sea salt sulfate during different seasons. It may be noticed that more than 90% of the total sulfate is comprised of non sea salt fraction indicating the dominance of the anthropogenic aerosol compared to oceanic aerosol. Since the focus of the study is to assess using optical data, the dependence of the Top Of the Atmosphere (TOA) radiative forcing on the relative concentration of BC and sulfate aerosols we have selected 12 days (highlighted with an arrow in Figure 1) with extremely clear sky condition without the presence of clouds and plotted the BC and sulfate mass concentrations in Figure 3. Though this limits the number of observations, we prefer working only with optically clear sky conditions to reduce the error that may creep in due to the presence of clouds. The day to day variation of BC almost followed by SO42− mass concentration except on Jan 18th and Apr 26th which show higher sulfate concentrations. In an urban environment BC is assumed to be due to primary particles from incomplete combustion processes, such as fossil fuel and biomass burning and therefore much atmospheric BC is of anthropogenic origin. However, in the environment around Visakhapatnam, bio mass burning is not significant to cause any major contribution to BC. The main source of anthropogenic sulfate aerosol is via sulfur dioxide emissions from fossil fuel burning with a relatively small contribution from biomass burning. Therefore, both the species show a similar day to day variability. Figure 4 shows the mass plot of surface BC mass concentration against the non sea salt (NSS) sulfate mass concentration (which represents the anthropogenic component in total sulfate) from the present observations which shows a good correlation. The BC Mass concentration was higher in winter compared to other seasons.
 The direct cooling effect of sulfate will be reduced by the presence of BC which absorb solar radiation and decrease reflectivity [Khan et al., 2010]. Therefore, we have attempted to see the relative contribution of the BC and sulfate to the composite aerosol forcing over this typical coastal urban location. Figure 3 (top) shows the day to day variability in the Top of the Atmosphere (TOA) forcing due to composite aerosol along with the mass concentration of BC. Figure 3 (bottom) shows the Top of the Atmosphere (TOA) forcing due to sulfate and BC aerosol along with the sulfate mass concentration. It may be noticed that the variation in composite forcing follows the variability in BC mass concentration on most of the days except on two days viz., 26th April 2006 and 24th August 2006. A few points of interest to be noted are: a) on 18th January 2006, though the sulfate mass concentration doubled, the composite forcing did not show a comparable change in negative forcing, but the BC TOA forcing shows a twofold increase, b) On 26th April 2006, there is an increase in sulfate mass concentration, no significant change in BC mass, but a decrease in the sulfate TOA and composite forcing, and c) on 24th August 2006, there is an increase in composite forcing without a noticeable change in either the sulfate mass concentration, BC mass concentration and their respective TOA forcing. Though there is a reduction in the sulfate mass concentration on 13th Sept. 2006, a marginal increase in BC forcing is seen without a proportionate increase in BC mass concentration. An examination of the optical data shows that the AOD on this day was slightly higher and the coarse mode surface aerosol mass concentration was also relatively more which has contributed to the increase in BC forcing probably due to low level dust since dust also can contribute to absorption.
 On Jan 18th, ionic species NH4+ (3 μg m−3 compared to an average of 1 μg m−3), SO42− (26 μg m−3 compared to an average of 11 μg m−3) which are continental origin have highest mass concentrations while Na+ and Cl− which are oceanic origin have lowest mass concentrations. So, most probably sulfate is in the form of (NH4)2SO4. The use of HYSPLIT back trajectories (R. R. Draxler and G. D. Rolph, HYSPLIT (Hybrid single-particle Lagrangian Integrated Trajectory) model, 2003, Air Resources Laboratory, http://www.arl.noaa.gov/ready/hysplit4.html) coupled with the available physical/ optical properties were adopted and the identification of probable source regions were justified in number of studies [Niranjan et al., 2007b; Kalapureddy et al., 2009; Cherian et al., 2010; Moorthy et al., 2010; Vinoj et al., 2010]. The HYSPLIT back trajectories show (Figure 5) that the air mass at 500 m altitude originates from continental side on Jan 18th while on Apr 26th it traverses via the oceanic region and therefore the mass concentrations of Na+ (6 μg m−3) and Cl− (3 μg m−3) ions show higher values which indicates that the sulfate is in the form of Na2SO4. Second, during summer, photochemical activity increases thereby enhancing SO42− production from SO2. This may be the main reason for high SO42− concentration on Apr 26th than BC mass concentration.
 Looking at the aerosol radiative forcing as a function of the BC and sulfate mass concentrations, on 18th January, the increase in BC TOA forcing cannot be explained purely on the basis of the increase in BC mass since a proportionate enhancement in the surface BC mass concentration is not observed. Therefore, the hypothesis on the mixing state of aerosols has to be invoked in order to account for the observed changes in the aerosol radiative forcing. Myhre et al.  reported that the forcing is stronger if the effects of mixing and relative humidity are considered and the major factor influencing the radiative forcing is the degree of internal mixing. Pósfai et al.  found that internally mixed soot and sulfate appear to comprise a globally significant fraction of aerosols in the troposphere. Transmission electron microscopy (TEM) images support the theory that BC particles once emitted into the atmosphere can become coated with other components like sulfates and although BC may be internally mixed it cannot be well mixed into the particle. Thus BC may be a distinct and not a well mixed component and the coated aggregates have carbon structure similar to uncoated structures [Chýlek et al., 1984]. Lesins et al.  reported that one of the important reasons for the uncertainty in aerosol radiative forcing is the effect of the mixing state of the aerosols on their optical properties and hence the radiative forcing. During winter, drier conditions reduce aerosol wet deposition and lead to longer life times of NSS sulfate aerosol [Harder et al., 2000]. Thus drier weather condition with sulfate abundance is likely to catalyze the process of aerosol internal mixture at this coastal urban location. Haywood and Ramaswamy  and Myhre et al.  estimated slight positive forcing when sulfate and soot were well mixed internally and that the well mixed treatment over predicts BC absorption. Jacobson  reported that BC as core resulted in positive forcing that more than offset the negative forcing of all other anthropogenic components and the result reported in this article supports the treatment that BC is well internally mixed resulting in more positive forcing. In ambient atmospheric conditions, additional contribution to absorption was associated with the effects of mixing and the more is the degree of mixing, the higher is the absorption coefficient [Cheng et al., 2008; Andreae et al., 2008]. Further, Jacobson  from a simulation on the evolution of chemical composition of aerosols reported that the mixing state and direct forcing of BC component can approach those of an internal mixture largely due to the coagulation and growth of aerosol particles which implies higher forcing from BC suggesting that the warming effect from BC may nearly balance the net cooling effect of other anthropogenic components.
 On 26th April, the decrease in composite forcing and the sulfate TOA forcing is understandable, as there was a significant enhancement in sulfate aerosol. However, such a decrease in TOA forcing is possible only when the aerosol mixing is not considered. During summer season during March to June, the station is significantly affected by sea breeze which brings in humid air mass onto the observing location. Since sulfate is hygroscopic in nature, the effect of neglecting relative humidity decreases the scattering properties and the optical depth dramatically resulting in a weaker forcing in areas of high humidity and sulfate dominating over BC [Kirkevåg et al., 1999]. The mass extinction efficiency of BC aerosol is independent of relative humidity whereas that of sulfate increases with increasing RH [Khan et al., 2010]. Second, the BC mass concentration during these months is significantly lower compared to winter season as on 18th January to participate in aerosol mixing to form an internal mixture. Therefore, it is felt that the effect of sulfate negative forcing in cooling the atmosphere could not be compensated by the positive forcing by BC.
 On the 24th August neither the surface BC mass concentration nor the sulfate show any significant change but the composite forcing shows a large increase indicating a decoupling of the surface feature from the integrated atmospheric column effects. However, the backscatter intensity profiles from Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) on board CALIPSO (Figure 6, top) at locations closer to Visakhapatnam show an extended backscatter from ground to an altitude of approximately 5 km indicating aerosol loading over the location up to higher altitudes though the surface features do not show any significant increase. Second the HYSPLIT back trajectories (Figure 5) show air mass origin at both the surface level as well as up to higher altitudes from the Arabian region which indicates dust transport from the desert regions which is also confirmed by the aerosol type assessment shown in Figure 6 (bottom). Such an aerosol transport is not seen on the subsequent day of observation i.e., on 13th Sept. 2006. Niranjan et al. [2007b] from a study using the Micro Pulse LIDAR data and the ground based radiometer data that the dust transport from Arabian region is quite probable and that the aerosol optical depth spectra on such days show a distinct spectral slope with low values of the angstrom size index ‘α’. The optical depth spectra for 24th August 2006 shown in Figure 7 along with the mean AOD spectra for the month show distinct spectral features indicating lower value of ‘α’ on 24th indicating coarse particle abundance in the integrated column features. Thus the observed increase in composite forcing on this day could be due to the dust aerosol transport from the Arabian region.
 Comprehensive measurements on aerosol optical and chemical properties made at a coastal urban location in India have been used to examine the impact of the relative mass concentration of sulfate and BC on aerosol composite radiative forcing. The chemical analysis of the aerosol samples indicate that the dominant ionic species at Visakhapatnam was SO42− during whole period and more than 90% of the total sulfate was found to be non sea salt fraction indicating the dominance of anthropogenic aerosol. Composite TOA forcing evaluated using OPAC and SBDART models follows the BC mass concentration on most of the clear days considered with a few exceptions. On those days, it was observed that significant enhancement in sulfate aerosol during wet atmospheric conditions led to a net atmospheric cooling. During dry weather conditions and even with the abundance of sulfate with longer life time a net atmospheric warming was observed which is possible only with the formation of aerosol internal mixtures. Thus this study based in the aerosol radiative forcing as a function of the dominant scattering species namely sulfate and absorbing species viz., BC indicates that the aerosol radiative forcing over coastal urban India largely depends on the relative mass concentrations of BC and sulfate aerosol. Abundance of sulfate aerosol along with BC during drier weather conditions may lead to an increase in the TOA forcing and hence it is important to limit the BC and sulfate emissions particularly of anthropogenic origin which significantly influence the atmospheric energetics.
 The authors thank M. M. Sarin of Physical Research Lab, Ahmedabad, for providing the chemical analysis data. The back trajectories were produced with HYSPLIT from the NOAA ARL website http://www.arl.noaa.gov/ready/hysplit4.html. Two of the authors (T.A.D. and B.S.) thank the Council of Scientific and Industrial Research, India, for Senior Research Fellowships. The CALIOP images were obtained from the NASA Langley Research Centre atmospheric science data center. This work is supported by ISRO under the Geosphere Biosphere Programme.