Seasonal variability of aerosols over the Indo-Gangetic basin

We examine the spatio-temporal characteristics of aerosols in the recent years (2000-2003) 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 degrees N, 80.346 degrees 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 Delta tau = +/-0.05 +/- 0.2 tau. The monthly mean regional maps of MODIS show high aerosol optical depth (AOD) over the Indo-Gangetic basin in the range 0.6-1.2 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, Angstrom exponent (alpha) 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 0.4-0.6. 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.