Abstract
Seasonal and inter-annual variabilities in aerosol optical depth (AOD) andaerosol size distribution are investigated using ground-based measurements (sun photometersand sun/sky radiometers), and MODIS (MODerate Imaging Spectroradiometer) and MISR(Multiangle Imaging SpectroRadiometer) satellites over Ahmedabad, Gurushikhar, Karachi,Kanpur and Gandhi College in South Asia during 2006–2008. An analysis and a review onthe comparison between aerosol optical depths measured from ground-based observationsand remote sensing over South Asia is performed. Karachi and Ahmedabad AODs aretwo times higher than Gurushikhar, a high altitude remote site. AODs over Kanpur andGandhi College in the Indo-Gangetic basin are higher than those measured over Ahmedabad,Gurushikhar and Karachi. Summer monsoon AODs are higher over Ahmedabad andKarachi, while winter AODs are higher over Kanpur and Gandhi College. AOD ratio,ratio of AODs obtained at 0.38 μm and 0.87 μm, is higher during postmonsoon andwinter than premonsoon and monsoon ratio suggesting the abundance of fine mode aerosolsduring postmonsoon and winter. Monsoon AOD ratios are lowest owing to the dominanceof coarse mode (mainly sea salt) particles. Ångström wavelength exponent (α) duringpostmonsoon and winter are higher than that of premonsoon and monsoon values. Lower α values over Gurushikhar and Karachi indicate the dominance of coarse mode aerosols(dust in Gurushikhar, and dust and sea salt in Karachi). Dominance of fine mode aerosolsdue to anthropogenic activities give rise to higher α values over urban locations (e.g.,Ahmedabad and Kanpur). Comparison between ground-based and MODIS (Terra andAqua) retrieved AODs show that aerosol optical depths do not change significantly inan hour and much of the diurnal AOD variation is captured well by the two MODISinstruments. The temporal difference (about an hour) between the ground-based and remotesensing measurements contributes negligibly to the observed differences in AODs. Thedifferences between ground-based and remote sensing (MODIS and MISR) AODs vary onspatial scales. During the study period (2006–2008) MODIS underestimates AODs overwestern India by about 0.04, while over the Indo-Gangetic Plain MODIS overestimatesAOD by 0.06 in (Kanpur) and underestimates by 0.07 (Gandhi College) with respect toAODs measured by hand held sun photometer and Microtops sun photometer (Ahmedabadand Gurushikhar), and AERONET sun/sky radiometers (Karachi, Kanpur and GandhiCollege) respectively. During the same period MISR underestimates AODs in the rangeof 0.02–0.17 over Ahmedabad, Gurushikhar, Karachi and Kanpur, while in Gandhi CollegeMISR overestimates AOD by 0.2 when compared to ground-based AODs. Results on spatial,seasonal and inter-annual variations in aerosol characteristics will be useful in improving theaerosol retrieval algorithms in remote sensing, and in regional and global estimates of aerosolradiative forcing.
Highlights
Atmospheric aerosols influence the global energy balance by scattering and absorbing both solar and terrestrial radiation, as well as by acting as cloud condensation nuclei and altering the cloud microphysical properties [1]
aerosol optical depth (AOD) decrease gradually as wavelength increases (Figure 7); the gradient in AODs is steeper in Kanpur and Gandhi College. 0.38 and 0.50 μm AODs are higher than that of 0.87 μm AODs during the entire study period (Figure 7) at all the locations except Gurushikhar
Though wet removal is most effective in reducing the near surface aerosol concentration, it has been shown that AOD increases significantly when relative humidity (RH) increases mainly due to the hygroscopic growth of water soluble aerosols [37]
Summary
Atmospheric aerosols influence the global energy balance by scattering and absorbing both solar and terrestrial radiation (direct effect), as well as by acting as cloud condensation nuclei and altering the cloud microphysical properties (indirect effect) [1]. Aerosols are produced in the atmosphere through various natural and anthropogenic processes, and they get dispersed horizontally and vertically through prevailing atmospheric circulation. Aerosols are short lived with a residence time of about a week in the lower troposphere. Due to their short lifetimes and widely distributed sources, aerosols exhibit large spatial and temporal variabilities. There has been a substantial increase in the interest in studying natural and anthropogenic aerosols because of their influence on climate through direct and indirect radiative effects [1]. Aerosols are still a major source of uncertainty in the prediction of climate change due to inadequate information on the variabilities of aerosol characteristics on regional and temporal scales [1]
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