Resumen
Seasonal and inter-annual variabilities in aerosol optical depth (AOD) and aerosol size distribution are investigated using ground-based measurements (sun photometers and 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 on the comparison between aerosol optical depths measured from ground-based observations and remote sensing over South Asia is performed. Karachi and Ahmedabad AODs are two times higher than Gurushikhar, a high altitude remote site. AODs over Kanpur and Gandhi College in the Indo-Gangetic basin are higher than those measured over Ahmedabad, Gurushikhar and Karachi. Summer monsoon AODs are higher over Ahmedabad and Karachi, 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 and winter than premonsoon and monsoon suggesting the abundance of fine mode aerosols during postmonsoon and winter. Monsoon AOD ratios are lowest owing to the dominance of coarse mode (mainly sea salt) particles. Ångström wavelength exponent (a) during postmonsoon and winter are higher than that of premonsoon and monsoon values. Lower a 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 aerosols due to anthropogenic activities give rise to higher a values over urban locations (e.g., Ahmedabad and Kanpur). Comparison between ground-based and MODIS (Terra and Aqua) retrieved AODs show that aerosol optical depths do not change significantly in an hour and much of the diurnal AOD variation is captured well by the two MODIS instruments. The temporal difference (about an hour) between the ground-based and remote sensing measurements contributes negligibly to the observed differences in AODs. The differences between ground-based and remote sensing (MODIS and MISR) AODs vary on spatial scales. During the study period (2006?2008) MODIS underestimates AODs over western India by about 0.04, while over the Indo-Gangetic Plain MODIS overestimates AOD by 0.06 in (Kanpur) and underestimates by 0.07 (Gandhi College) with respect to AODs measured by hand held sun photometer and Microtops sun photometer (Ahmedabad and Gurushikhar), and AERONET sun/sky radiometers (Karachi, Kanpur and Gandhi College) respectively. During the same period MISR underestimates AODs in the range of 0.02?0.17 over Ahmedabad, Gurushikhar, Karachi and Kanpur, while in Gandhi College MISR 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 the aerosol retrieval algorithms in remote sensing, and in regional and global estimates of aerosol radiative forcing.