Seasonal variability of aerosols over the Indo-Gangetic basin

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2005jd005938, 2005 Seasonal variability of aerosols over the Indo-Gangetic basin Hiren Jethva, S. K. Satheesh, and J. Srinivasan Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India Received 3 March 2005; revised 12 July 2005; accepted 10 August 2005; published 5 November [1] We examine the spatio-temporal characteristics of aerosols in the recent years ( ) over the Indian region with special emphasis on the Indo-Gangetic basin (northern India) using data from Moderate Resolution Imaging Spectroradiometer (MODIS), Aerosol Robotic Network (AERONET) and Total Ozone Mapping Spectroradiometer (TOMS). First, we have compared the MODIS-derived aerosol optical depth (AOD) and fine-mode aerosol fraction (FMAF ratio of the fine-mode AOD to the total mode AOD) with those of AERONET at Kanpur (26.45 N, E). It has been found that the MODIS captures the major part of the seasonal variation of aerosols in terms of abundance as well as aerosol type. The absolute errors in AOD were within the predicted uncertainty of Dt = ±0.05 ± 0.2t. The monthly mean regional maps of MODIS show high aerosol optical depth (AOD) over the Indo-Gangetic basin in the range at 550 nm wavelength with significant spatial and temporal variation during the summer (April to June). The associated FMAF was found to be low (<0.4). This indicates that the coarse-mode particles are dominant in the summer. The spatial distribution of absorbing aerosol index (AAI) derived from TOMS, Ångström exponent (a) and aerosol volume size distribution measured at Kanpur also indicated the presence of absorbing coarse-mode aerosols during summer. On the other hand, the entire Indo-Gangetic basin was dominated by the fine-mode particles during the winter (November to January) with AOD in the range Their spatial and temporal variations, however, were quite low compared to the summer. Results reported in this paper indicate that the Indo-Gangetic basin has the largest aerosol optical depth in India during both the seasons. The region is dominated by the large absorbing coarse-mode particles (possibly transported dust from the northwest of India) in the summer and by the probable widespread emission sources of fine-mode aerosols (primarily of anthropogenic origin) in the winter. The unique topography and weather condition of the region have impact on the observed spatial and temporal distribution of aerosols. Citation: Jethva, H., S. K. Satheesh, and J. Srinivasan (2005), Seasonal variability of aerosols over the Indo-Gangetic basin, J. Geophys. Res., 110,, doi: /2005jd Introduction [2] Aerosols play a vital role in the Earth-atmosphereocean system by means of their direct and indirect impact on climate [Schwartz et al., 1995]. Several investigations during last 2 decades have clearly demonstrated the impact of aerosols on Earth s radiation budget. Aerosol characterization experiment (ACE), Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX), Indian Ocean Experiment (INDOEX) and Smoke, Clouds, and Radiation Brazil (SCAR-B) are the examples of coordinated field campaigns carried out to assess the impact of aerosols on climate. Unfortunately, aerosols are still poorly characterized in climate models because of the lack of a comprehensive Copyright 2005 by the American Geophysical Union /05/2005JD database especially over the developing countries of the world. Aerosol radiative forcing is one of the most uncertain components of the Earth-atmosphere system [Hansen et al., 1997; Satheesh et al., 1999]. In India, there have been a number of studies on the characterization of aerosols carried out over the southern part of India. Over northern India, however, information on aerosols is very limited. During the Indian Middle Atmosphere Program (IMAP), a project was initiated to monitor aerosol characterization over Indian region by setting up multiwavelength radiometers (MWR) at a few selected sites. These measurements became operational in 1980s and have been continued as a part of the Geosphere Biosphere Program (GBP) of Indian Space Research Organisation (ISRO). The ground-based measurements of aerosols in India had been started in 1980s in Trivandrum (southwestern coastal city of India) and in a few cities in south India [Sikka, 2002] using multiwave- 1of15

2 length solar radiometer [Krishna Moorthy et al., 1999]. Later, the network was augmented by the addition of more stations to the ISRO-GBP. Two of these sites fall in the Indo-Gangetic basin, namely, Kanpur (26.45 N, E) and Nainital (29.24 N, E). However, Nainital is located on the southern edge of the Himalaya (see Figure 1), and it is at a height of about 2 km from the sea level. Hence it does not represent the aerosol climatology of the region. [3] The Indo-Gangetic basin is located in the northern part of India and surrounded by the unique topography with the Himalaya to the north and hills to the south (see Figure 1). The topography data shown in Figure 1 were obtained from the web site ftp://edcftp.cr.usgs.gov/pub/data/ gtopo30/. It is one of the highly polluted regions in India [Guttikunda et al., 2003] but less explored from an aerosol perspective. Because of growing population and economic growth, air pollution has also increased in this region. The concentration of particulate matter of diameter less than 10 mm (PM 10 ) as measured by Central Pollution Control Board (CPCB) of India was found to be above the critical level (>210 mg/m 3 ) in many cities like New Delhi, Kanpur and Kolkota lying in the Indo-Gangetic basin [Mitra and Sharma, 2002]. Moreover, recent inventories constructed by Reddy and Venketaraman [2002a, 2002b] indicated much higher emission of aerosols in the Indo-Gangetic basin. Unfortunately, the routine measurements of aerosols over northern India were nonexistent until an automated Sun and sky radiometer was deployed in Kanpur in year 2001 under the Aerosol Robotic Network (AERONET) program [Holben et al., 1998]. [4] Ground-based measurements provide information over a limited spatial region, while aerosols exhibit high spatio-temporal variability in terms of their abundance, optical properties and chemistry. Hence ground-based measurements of aerosols over only one station are not adequate to characterize aerosols over the entire Indo-Gangetic basin. Several studies [Moorthy and Satheesh, 2000; Sakerin and Kabanov, 2002; Smirnov et al., 2002] have shown that the aerosol properties show significant variations, even over reasonably small spatial scale, and these spatial changes are distinctly different in different seasons. This is due to the changes in air mass types, the impact of advection and transport and the proximity to the source regions. [5] The winds over Indian land mass depict contrasting seasonal behavior associated with Asian monsoon system. During winter (November to January), the low-level winds are mainly northwesterly and are weak (with speeds <5 m/s) over the Indo-Gangetic basin. In the summer (April to June), rainfall is low, and hence the aerosols have a long atmospheric residence time in this season. Monsoon rainfall starts by July, and during the season, winds are westerly with generally high speed. [6] In this paper, we have compared the Moderate Resolution Imaging Spectroradiometer (MODIS) derived monthly mean values of AOD and fine-mode aerosol fraction (FMAF) at 550 nm wavelength with those of AERONET at Kanpur. Fine-mode aerosol fraction is the fraction of total AOD contributed by fine-mode particles (submicron size w.r.t. particle radius). We also examine the spatio-temporal characteristics of aerosols over the Indian region with special emphasis on northern India including the Indo-Gangetic basin. Girolamo et al. [2004] have noted high AOD (0.6) over the Indo-Gangetic basin derived from multiangle imaging spectroradiometer (MISR) in the winter seasons of 2001 to However, no detailed investigations have been carried out on the optical and microphysical properties of the aerosols over the region. Singh et al. [2004] have reported spectral, seasonal and interannual variability of various aerosol optical properties at Kanpur using AERONET measurements which lack in information on the spatial and temporal variations of aerosols over larger region. Here we have used MODIS-derived regional maps of AOD in conjunction with the groundbased aerosol measurements at Kanpur (AERONET) for the detailed study of spatial and seasonal variations of aerosol characteristics over the Indo-Gangetic region during summer (April, May and June, hereinafter AMJ) and winter (November, December and January, hereinafter NDJ) seasons. While MODIS total AOD provides only a measure of column aerosol loading over a unit cross section, it has a major advantage of providing size of dominant mode and ratio between modes over ocean [Tanre et al., 1997] as well as over land. FMAF thus can be used to quantify the relative dominance of submicron-size particles over the coarse-size particles in different seasons. The validation of FMAF at eight specific sites over ocean shown by Remer et al. [2005] suggests that the MODIS values agree with AERONET measurements within 20%. Over land, MODIS provides AOD in three channels (470, 550 and 670 nm) in addition to the FMAF at 550 nm wavelength. The accuracy of FMAF retrieval depends on the assumption of aerosol model as well as the surface reflectance. Since the uncertainty in the estimation of surface reflectance is large over land, the accuracy of FMAF also degrades under such a situation. Therefore MODIS size parameters over land are not expected to be as accurate as over ocean. While the validation of MODIS-derived AOD over land has been done by Chu et al. [2002] and by Remer et al. [2005], the FMAF has not been validated over land. However, the FMAF still gives a good indication whether the aerosols are fine or coarse. Kanpur is situated in the center of the Indo-Gangetic basin (Figure 1). Aerosols measurements at Kanpur therefore play a key role in characterizing aerosols as well as acting as a ground truth for the validation of MODIS aerosol retrieval. However, it may not be representative of the entire region. In addition to the above, the regional maps Total Ozone Mapping Spectroradiometer absorbing aerosol index (hereinafter TOMS AAI) were used to identify the absorbing aerosols over the basin. The roles of meteorology and topography on the spatial distribution of aerosols have been studied. 2. Data Set 2.1. AERONET [7] Ground-based aerosol measurements at Kanpur, India, are a part of AERONET program which is a federation of ground-based remote sensing aerosol network [Holben et al., 1998]. Several aerosol parameters are being routinely measured from the direct Sun radiance measurements at eight spectral channels (340, 380, 440, 500, 670, 870, 940 and 1020 nm) and sky measurements at four spectral channels (440, 670, 870 and 1020 nm). Under cloud-free condi- 2of15

3 Figure 1. Topography (in meters) over central and northern part of India where the Indo-Gangetic basin is surrounded by the Himalaya to the north and by the hills to the south. Important cities like New Delhi, Kanpur, and Patna fall within the basin. tions, the uncertainty in calculation of AOD is <±0.01 for wavelength >440 nm and <±0.02 for shorter wavelengths [Holben et al., 1998; Eck et al., 1999]. In the present study, the data acquired for the period January 2001 to March 2004 have been used; this includes the monthly mean aerosol optical depths (total and absorption) at 500 nm wavelength, Ångström exponent (a) derived in the spectral range nm, single-scattering albedo (w) at 441 nm wavelength and aerosol volume size distribution. For the comparison of MODIS with AERONET at the same wavelength, AERONET-derived AOD (at 500 nm) was interpolated to 550 nm wavelength using power law, where t 550 t 500 a t 550 ¼ t 500 ð550=500þ a ð1þ = aerosol optical depth at 550 nm wavelength = aerosol optical depth at 500 nm wavelength = Ångström exponent in the wavelength range nm 2.2. MODIS [8] We have used MODIS-derived Level 3 daily QA_Weighted data on a spatial resolution of 1 1 to construct the monthly mean climatology of AOD and FMAF at 550 nm wavelength. MODIS was flown on NASA s EOS Terra and EOS Aqua satellites in December 1999 and May 2002, respectively. Both satellites view entire Earth s surface in 1 to 2 days, acquiring data in 36 spectral bands ranging from 0.4 mm to 14.4 mm [Kaufman et al., 2000]. The first seven wavelength bands ( nm, nm, nm, nm, nm, nm, and nm) are used to retrieve aerosol properties over ocean [Tanre et al., 1997; Remer et al., 2002] whereas over land, the retrievals are limited to three channels (470 nm, 550 nm and 670 nm) [Kaufman et al., 1997]. The monthly mean AOD and FMAF climatology was derived in accordance with the MODIS recommendation ( qa.html). In constructing the monthly mean values, we had selected only those L3_daily grid shells, which had at least six Level 2 pixel observations (10 km resolution at nadir), and we also ensure that for these Level 3 grid shells, there must be at least 10 days in a month for which the above condition was satisfied. For these grid shells, the monthly mean value of aerosol optical depth was weighted by the number of Level 2 pixel observations as well as by (1-cloud fraction) and can be written as, where AOD_avg N P cldfrac AOD avg ¼ X i¼n i¼1 X i¼n P i ð1 cldfrac i ÞAOD i i¼1 P i ð1 cldfrac i Þ = monthly mean aerosol optical depth = number of days for which above condition satisfied (minimum 10 days) = number of Level 2 pixel observations in a 1 1 grid shell = cloud fraction ð2þ 3of15

4 Figure 2. Comparison of MODIS-derived monthly mean aerosol optical depth (at 550 nm) with those of AERONET at Kanpur. (top) Time series for the period January 2001 to July 2003 and (bottom) same data on a scatter diagram along with the retrieval errors (solid lines) of Dt = ±0.05 ± 0.2t, where t is aerosol optical depth. [9] In the case of FMAF, it was additionally weighted by the aerosol optical depth values along with the pixel counts and (1-cloud fraction). FMAF avg ¼ X i¼n i¼1 X i¼n P i ð1 cldfrac i ÞAOD i FMAF i i¼1 P i ð1 cldfrac i ÞAOD i ð3þ [10] We refer to the months of November, December and January as winter season (for example, by winter 2001 we mean November and December of 2001 and January of 2002). The error in retrieved aerosol optical depth is expected to be Dt = ±0.05 ± 0.2t, where t is aerosol optical depth [Kaufman et al., 1997; Chu et al., 2002] TOMS AAI [11] Absorbing aerosol index (AAI) is a quantity derived from the Total Ozone Mapping Spectroradiometer (TOMS) measured back-scattered UV (ultraviolet) radiances at 340 nm and 380 nm wavelengths. The first TOMS instrument was flown on the Nimbus 7 from 1979 to 1992 followed by the second non-sun-synchronous instrument on the Russian Meteor 3 satellite from 1991 to A modified version of the previous two instruments, EP/TOMS, was launched on 2 July 1996 on board NASA s Earth Probe satellite into Sun-synchronous orbit. Herman et al. [1997] and Torres et al. [1998] have shown that AAI is a qualitative indicator of absorbing aerosols. For absorbing aerosols like dust, smoke and biomass burning, the values lie between 0.5 and 3.0 while nonabsorbing aerosols like water soluble and sea-salt particles yield negative or low positive values of AAI [Prospero et al., 2002]. In this study, the monthly averaged AAI data (only positive values) at a spatial resolution have been used to identify UV absorbing aerosols in the region. In addition to the above data sets, monthly mean meteorological fields from the European Center for Medium Range Weather Forecast-40 reanalysis (ERA40) [Simmons and Gibson, 2000] have been used to identify the role of meteorology in the observed spatial distribution of AOD. 3. Results and Discussion 3.1. Comparison of MODIS and AERONET [12] The comparison of MODIS-derived monthly mean AOD with those of AERONET at Kanpur is depicted in Figure 2. Figure 2 (top) shows the time series for the period January 2001 to July 2003, whereas the same data are represented in the form of a scatterplot in Figure 2 (bottom). It shows that the MODIS systematically overestimates the AOD during the summer and underestimates in the winter. However, Figure 2 (bottom) shows that these errors were within the predicted retrieval uncertainty of Dt = ±0.05 ± 0.2t, where t is aerosol optical depth [Kaufman et al., 1997; Chu et al., 2002]. In the case of FMAF (Figure 3), MODIS is found to be in a good agreement with the 4of15

5 Figure 3. Comparison of MODIS-derived monthly mean fine-mode aerosol fraction (at 550 nm) with those of AERONET at Kanpur. AERONET in the winter where both show larger contribution of fine-mode aerosols in the total AOD. In the premonsoon (summer) period, both show the larger contribution of coarsemode aerosols with MODIS biased high on coarser size. This could be due to the errors in the estimation of surface reflection which in turn can lead to the selection of inappropriate aerosol model in the retrieval. Overall, MODIS captures the major part of the seasonal variation of aerosols in terms of the AOD as well as the dominant aerosol type (FMAF). Therefore, within the given retrieval uncertainty, the MODIS-derived regional maps of AOD and FMAF can be used to characterize aerosols over the larger region of northern India AOD Monthly Climatology [13] Figure 4 shows the monthly climatology of MODISderived AOD at 550 nm wavelength for the winter (Figure 4, top) and summer (Figure 4, bottom). A striking feature in Figure 4 is that the Indo-Gangetic basin has the highest AOD in India both in winter as well as in summer. High AOD can be also seen over the northwest part of India and over Bangladesh. Note that the spatial and temporal distribution and magnitude of AOD are different in winter and summer. While AOD is in the range without much spatial variation in winter, it is in the range with large spatial variation in the summer. The spatial distribution of AOD during winter indicates the existence of widespread source of aerosol emission whereas the east-west gradient of AOD in summer indicates the transport of aerosols from northwest India to the Indo- Gangetic basin. It is to be noted that the month of June is devoid of retrievals over the most part of India. This is due to the insufficient MODIS Level 2 pixels for the computation of the monthly average AOD (2) on account of the arrival of Indian summer monsoon. [14] Figure 5 shows the time series of climatological mean AOD (Figure 5, top) and a (Figure 5, bottom) measured at Kanpur (AERONET) along with their standard deviations. A clear seasonal variation in AOD can be seen with the maximum values (0.8) in May and June and a second maximum (0.6) during November to January. The interannual variability of AOD is found to be larger during summer and lower in winter as is evident from their respective standard deviation depicted in Figure 5. Large AOD associated with low a (<0.5) during May to July indicates the dominance of coarse-mode aerosols whereas large values of a in winter indicate the dominance of finemode aerosols. [15] Figure 6 shows the regional distribution of averaged TOMS AAI for winter ( ) and summer ( ) months. The high positive values of TOMS AAI during summer clearly indicate the large abundance of absorbing aerosols over the northern part of India. A large gradient of TOMS AAI from northwest of India toward the basin, prevailing northwesterly wind pattern (discussed in section 3.4) and the lower values of alpha (AERONET) together can explain that the dust aerosols may have transported from the Thar desert (see Figure 1), which is a prominent dust source in the region, to the basin. In the winter, the values of TOMS AAI were less than 1.0 over the 5of15

6 Figure 4. Spatial distribution of MODIS-derived monthly mean climatology of aerosol optical depth at 550 nm wavelength over the Indian subcontinent. 6of15

7 Figure 5. Time series of (top) climatological aerosol optical depth at 500 nm wavelength and (bottom) Ångström exponent derived in the wavelength range nm at Kanpur (26.45 N, E) by AERONET. Monthly means are represented by dots while corresponding standard deviations are represented by bars. 7of15

8 Figure 6. Regional distribution of averaged TOMS AAI (>0.7) over India during (top) NDJ and (bottom) AMJ. entire northern India. The sensitivity of TOMS AAI to the height of aerosol layer has been previously noted by Torres et al. [1998] and Prospero et al. [2002]. They show that the higher values of TOMS AAI can be attributed to the elevated aerosol layer (possibly transported dust in the present case) in the atmosphere. Moreover, Torres et al. [1998] have shown that the absorbing aerosol index has the highest sensitivity to the dust aerosols followed by the carbonaceous aerosols (finemode aerosols). This indicates that the wintertime aerosols are confined to the boundary layer. TOMS AAI cannot detect boundary layer aerosols because of the modifying Rayleigh scattering by air molecules lying above it and this may explain why TOMS AAI is much higher in the summer than in the winter Contribution of Fine-Mode Aerosols [16] In this section, we examine the contribution of fineand coarse-mode aerosols to the composite aerosol optical properties. MODIS has an advantage of providing finemode aerosol fraction, and hence the contribution of fineand coarse-mode particles in the total AOD can be known separately. The time series of area-averaged (1 1 ) MODIS AOD and corresponding FMAF at 550 nm wavelength centered at Amritsar (31.37 N, E), Kanpur (26.45 N, E) and Kolkata (22.34 N, E), respectively (marked in Figure 1) is shown in Figure 7. The influence of dust particles can be clearly seen over Amritsar and Kanpur during summer months where higher AOD are found to be associated with the lower FMAF. AOD over Kolkata was slightly higher during January to March and comparable with the other two stations in other months. However, no definite seasonal change in aerosol size was observed over Kolkata where higher values of FMAF ( ) remained throughout the period with less variation in AOD ( ) compared to Amritsar and Kanpur. Figure 8 shows the variation of MODIS-derived climatological AOD in accordance with the FMAF for the three stations discussed above. A clear seasonal change in particle effective size and its influence on total AOD is apparent over Amritsar and Kanpur where the dominance of coarse-mode aerosols during summer resulted in higher AOD compared to the lower wintertime AOD mostly associated with the fine-mode aerosols. In fact, Amritsar is nearest to the dust source region followed by Kanpur and Kolkata, and hence the effect of dust is more pronounced over the central and western part of the Indo-Gangetic basin while Kolkata is situated just north of Bay of Bengal and far away from the dust sources. 8of15

9 Figure 7. Time series plot of MODIS-derived (top) monthly mean aerosol optical depth and (bottom) fine-mode aerosol fraction at 550 nm wavelength over Amritsar (31.37 N, E), Kanpur (26.45 N, E) and Kolkata (22.34 N, E). Therefore the influence of dust transport cannot be seen over Kolkata. During winter months, AOD over all three stations was found to be lower ( ) compared to the summer, and these were mostly contributed by the high FMAF. This indicates the dominance of fine-mode particles over the entire northern India region in the winter season. [17] The aerosol volume size distribution (dv/dlnr) retrieved at Kanpur is depicted in Figure 9 which shows a bimodal structure of aerosols during winter seasons (Figure 9, top). This was retrieved from the spectral Sun and sky radiance data using Dubovik and King s [2000] approach with initial guess: dv/dlnr = 0.001, n(l i )=1.5 and k(l i ) = 0.005, where dv/dlnr denotes the aerosol volume size distribution and n(l i ) and k(l i ) are the real and imaginary part of refractive index at wavelength l, respectively. Note that the dv/dlnr shown here for summer period (Figure 9, bottom) has been derived using nonspherical model, while winter month distribution is based on spherical model. Unlike two dominant modes at geometric mean radius 0.15 mm and 3.9 mm during winter months, only one dominant mode at radius 3.9 mm in summer has been found with higher volume concentration; this clearly indicates the dominance of coarse-mode particles during the summer. We find that the volume concentration of coarse-mode particles has increased from 0.52 in year 2001 to 0.70 mm 3 /mm 2 in 2003 during the summer season. Singh et al. [2004] have also shown that the volume concentrations of fine- and coarse-mode particles were almost similar during winter seasons at Kanpur. The seasonal variations of MODIS-derived FMAF over Kanpur in both the seasons are found to be consistent with the groundbased measurements of aerosol volume size distribution. [18] The regional maps of FMAF shown in Figure 10 show the difference in dominant aerosol effective size between the western and eastern part of India in winter. The Thar desert and western India were found to be dominated by the coarse-mode particles whereas the Indo- Gangetic basin and northeast India are totally dominated by the fine-mode particles. The spatial distribution of FMAF over the whole subcontinent remained similar in all three recent winter seasons analyzed here. On the other hand, very low values of FMAF existed over the west and central part of the Indo-Gangetic basin during summer months; this indicates the presence of coarse-mode dust particles. It may be noted that the eastern part of India was totally dominated by the fine particles in both winter as well as summer. [19] Absorption of incoming shortwave radiation as well as outgoing longwave radiation by aerosols can cause atmospheric heating which in turn can lead to changes in regional and global circulation. Figure 11 shows the monthly climatology (January 2001 to March 2004) of absorption aerosol optical depth (Figure 11, top) derived at 441 nm 9of15

10 and AERONET (see Figure 3) both indicate that the finemode aerosols are dominant in the winter. Very low values of coarse-mode w in the winter months are an unusual and striking feature in Figure 11 (bottom) since it is neither representative of highly absorbing pure soot aerosols (w 0.23) nor representative of moderate absorbing dust aerosols (0.8 < w < 0.9). However, Dubovik et al. [2000] have noted that the tendency of increasing retrieval errors with a decrease of optical thickness is more pronounced for the retrieval of the refractive index and single-scattering albedo. In the present case, the contribution of coarse-mode aerosols in the total absorption is small in the winter, and therefore the error in the retrieval of coarse-mode w may be large. Figure 8. Phase diagrams of MODIS-derived monthly mean (January 2001 to July 2003) aerosol optical depth versus the fine-mode aerosol fraction at 550 nm wavelength and averaged over 1 1 box centered at three stations, namely, Amritsar (31.37 N, E), Kanpur (26.45 N, E), and Kolkata (22.34 N, E). wavelength for the total fine- and coarse-mode particles at Kanpur (AERONET). Absorption aerosol optical depth is given by t abs =(1 w)t, where w and t denote singlescattering albedo and total optical depth, respectively. The highest aerosol absorption was found to occur in the month of May (mainly contributed by the coarse-mode particles) followed by significant absorption during October to January months (mainly contributed by the fine-mode particles). It is to be noted that the absorption by coarse particles during August to October is slightly higher than the absorption in March and April. In fact, the large positive values of TOMS AAI (Figure 6) and lower values of a measured at Kanpur both supported the dominance of dust aerosols during the summer months. The wintertime aerosols over the basin were also found to be absorbing where the fine aerosol optical depth contribute more in the total absorption. In fact, FMAF derived from MODIS 3.4. Role of Aerosol Transport [20] Dust storms are major known sources of aerosols which transport large amounts of dust aerosols from the Thar desert and Arabian region to the northern part of India during the March to June months. Sagnik et al. [2004] have studied the influence of dust storms on the optical properties of aerosols over Kanpur during the summer (March to June). The climatology of TOMS AAI shown in Figure 6 clearly indicates the presence of absorbing aerosols, mostly dust, over the northern part of India. The highest values of TOMS AAI (hence more absorption) over the basin were observed in May and decreased in June because of the arrival of monsoon rainfall. Meteorological fields at the 850 mb pressure level obtained from ERA-40 are shown in Figure 12 for year It shows that the winds are northwesterly over the northern India in both the seasons. In the presence of prevailing air mass pattern, it appears that the dust aerosols may have transported from the northwest of India (Thar desert) which is a dust source in the region. A similar explanation can be applied to the winter case where fine-mode aerosols (originated from widespread local sources) may have spread over the entire region by the virtue of the mean air flow. Note that the Indo- Gangetic region is surrounded by the Himalayan topography in the north and hills to the south (see Figure 1), and therefore aerosols distribution is confined to the basin only (see Figures 4 and 6). The meteorology and topography thus have an impact on the aerosol distribution in this region. [21] How about the aerosol optical depth over other regions of the world? Measurements as a part of TARFOX over the Atlantic Ocean in the haze plume off the east coast of United States have shown AOD (at 500 nm wavelength) often exceeding 0.5 (with mid visible single-scattering albedo of 0.9) [Russell et al., 1999; Smirnov et al., 2000]. Airborne measurements over the Aegean Sea during STAAARTE-MED experiment have shown aerosol optical depths larger than 0.4 (with single-scattering albedo of 0.89) attributed to the transport of polluted air masses from western and eastern Europe [Formenti et al., 2002]. Smirnov et al. [2002] have reported aerosol optical depth values as high as 0.7 during the dry season over the Persian Gulf. During an aerosol field campaign at an urban continental location, Bangalore (13 N, 77 E, 960 m asl), in India, simultaneous measurements were made of aerosol spectral optical depths, black carbon mass concentration, total and size segregated aerosol mass concentrations during the beginning of the winter season [Babu et al., 2002]. The 10 of 15

11 Figure 9. Seasonal-averaged aerosol volume size distribution for (top) NDJ and (bottom) AMJ retrieved using spherical model and nonspherical model, respectively, at Kanpur by AERONET. aerosol visible optical depth was in the range 0.24 to The physical and optical properties of biomass burning in south central Africa (Zambia) were measured during Zambian IBBE [Eck et al., 2001], and aerosol optical depth was in the range 0.7 to 1.7 with a single-scattering albedo as low as 0.82 to Measurements during SCAR-B have observed an average aerosol optical depth of 1.2 with single-scattering albedo of 0.86 [Christopher et al., 2000]. Takemura et al. [2002] have reported aerosol optical depth and Ångström wavelength exponent at Ispra, Italy (45.8 N, 8.36 E), on the basis of AERONET measurements and reported optical depths as high as 0.6 (a 1.6). Using 9 months of CERES and VIRS data, Loeb and Kato [2002] have studied aerosol optical depths over oceans. They have shown that areas of maximum aerosol radiative effects are clearly evident near central America and west of the Sahara desert, and in these regions aerosol optical depth was as high as 1.0. Our study shows that the Indo-Gangetic basin has the largest aerosol loading in India both in summer as well as in the winter. The high AOD ( ) during summer months can be attributed to the dust transport, and comparatively lower AOD ( ) can be attributed to the fine-mode aerosols (probably from anthropogenic emissions) during the winter period. It is to be noted that none of the regions mentioned above show a dramatic seasonal shift of aerosols type as is observed over the Indo-Gangetic region. 4. Conclusions [22] In this paper, we have compared the MODIS-derived aerosol optical depth and fine-mode aerosol fraction with those of AERONET at Kanpur, India. Kanpur is situated in the center of the Indo-Gangetic basin which is believed to be one of the highly aerosol loaded regions in India. It is found that the MODIS captures the major part of the seasonal variation of aerosols with the absolute error in aerosol optical depth within the predicted uncertainty. The regional maps of MODIS were used to examine the spatiotemporal aerosol characteristics over the larger region of northern India. The Indo-Gangetic basin, because of its unique topography and weather condition, is characterized by unusual aerosol characteristics. The region encounters large aerosol optical depth associated with either aerosol transport from other areas or widespread emission sources within the region. The major conclusions of our study are the following: [23] 1. The Indo-Gangetic basin has the largest aerosol optical depth in India both in summer as well as in winter seasons. However, it appears to be influenced by the two contrasting aerosol types depending on the sources and seasonal air mass pattern. The widespread local sources of fine-mode aerosols (probably of anthropogenic origin) are found to dominate in the winter, whereas coarse-mode absorbing aerosols (possibly dust) transported from the northwest of India are found to be responsible for large aerosol optical depth during the summer. [24] 2. The MODIS-derived aerosol optical depths were in the range during the summer with significant spatial and temporal variations. This was found to be associated with the low fine-mode aerosol fraction and high TOMS AAI that indicate the presence of coarse-mode absorbing aerosol over the region. On the other hand, aerosol optical depths were between 0.4 and 0.6 with less spatial and temporal variations in the winter season and 11 of 15

12 Figure 10. Spatial distribution of MODIS-derived monthly mean climatology of fine-mode aerosol fraction at 550 nm wavelength over the Indian subcontinent. 12 of 15

13 Figure 11. Time series of climatological (January 2001 to March 2004) (top) absorption aerosol optical depth and (bottom) single-scattering albedo derived at 441 nm wavelength at Kanpur by AERONET. 13 of 15

14 Figure 12. Wind vectors at 850 mb pressure level derived from monthly averaged zonal wind velocity and meridional wind velocity from ERA-40 data. were found to be mostly dominated by the fine-mode aerosols that indicate the probable widespread sources of anthropogenic aerosols over the entire region. [25] 3. Even though the aerosol optical depth and the dominant particle size show large variation with respect to the season, single-scattering albedo does not show significant variation but remains within the range 0.88 to 0.92 throughout the year. This indicates that the ratio of absorbing to nonabsorbing fraction is more or less constant with respect to the season. [26] Acknowledgments. The authors would like to thank Goddard DAAC and MODIS team for their online support of MODIS level 1, level 2, and level 3 data. The authors also thank B. N. Holben for his efforts in establishing and maintaining the AERONET at Kanpur, India, whose data are used in this study. The authors would like to thank anonymous reviewers for their peer review and valuable suggestions. References Babu, S. S., S. K. Satheesh, and K. K. Moorthy (2002), Aerosol radiative forcing due to enhanced black carbon at an urban site in India, Geophys. Res. Lett., 29(18), 1880, doi: /2002gl Christopher, S. A., J. Chou, J. L. Zhang, X. Li, T. A. Berendes, and R. M. Welch (2000), Short wave direct radiative forcing of biomass burning aerosols estimated using VIRS and CERES data, Geophys. Res. Lett., 27(15), Chu, D. A., Y. J. Kaufman, C. Ichoku, L. A. Remer, D. Tanre, and B. N. Holben (2002), Validation of MODIS aerosol optical depth retrieval over land, Geophys. Res. Lett., 29(12), 8007, doi: /2001gl Dubovik, O., and M. D. King (2000), A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements, J. Geophys. Res., 105, 20,673 20,696. Dubovik, O., A. Smirnov, B. N. Holben, M. D. King, Y. J. Kaufman, T. F. Eck, and I. Slutsker (2000), Accuracy assessments of aerosol optical properties retrieved from AERONET Sun and sky-radiance measurements, J. Geophys. Res., 105, Eck, T. F., B. N. Holben, J. S. Reid, O. Dubovik, S. Kinne, A. Smirnov, N. T. O Neill, and I. Slutsker (1999), The wavelength dependence of the optical depth of biomass burning, urban and desert dust aerosols, J. Geophys. Res., 104, 31,333 31,350. Eck, T. F., B. N. Holben, D. E. Ward, O. Dubovik, J. S. Reid, A. Smirnov, M. M. Mukelabai, N. C. Hsu, N. T. O Neill, and I. Slutsker (2001), Characterization of the optical properties of biomass burning aerosols in Zambia during the 1997 ZIBBEE field campaign, J. Geophys. Res., 106, Formenti, P., et al. (2002), STAAARTE-MED 1998 summer airborne measurements over the Aegean Sea: 2. Aerosol scattering and absorption, and radiative calculations, J. Geophys. Res., 107(D21), 4551, doi: / 2001JD Girolamo, L. D., T. C. Bond, D. Bramer, D. J. Diner, F. Fettinger, R. A. Kahn, J. V. Martonchik, M. V. Ramana, V. Ramanathan, and P. J. Rasch (2004), Analysis of multi-angle imaging spectroradiometer (MISR) aerosol optical depths over greater India during winter , Geophys. Res. Lett., 31, L23115, doi: /2004gl Guttikunda, S. K., G. R. Carmichael, G. Calori, C. Eck, and J.-H. Woo (2003), The contribution of megacities to regional sulfur pollution in Asia, Atmos. Environ., 37, Hansen, J., M. Sato, and R. Ruedy (1997), Radiative forcing and climate response, J. Geophys. Res., 102, Herman, J. R., P. K. Bharatia, O. Torres, C. Hsu, C. Seftor, and E. Celarier (1997), Global distribution of UV-absorbing aerosol from Nimbus 7/ TOMS data, J. Geophys. Res., 102, 16,911 16, of 15

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