Trends of absorption, scattering and total aerosol optical depths over India and surrounding oceanic regions from satellite observations: role of local production, transport and atmospheric dynamics.

The study examines trends of scattering, absorption and total aerosol optical depths (SAOD, AAOD and AOD) over India and surrounding oceanic regions and explores role of local production, long-range transport and atmospheric dynamics on observed trends. Long-term satellite observations are used to estimate trends and assess their statistical significance. Significant spatial and seasonal changes are observed in trends of SAOD, AAOD and AOD. AOD is observed to be increasing during post monsoon and winter over most of the land mass and surrounding oceanic regions, whereas decreasing trends over land and increasing trends over oceanic regions are observed in pre-monsoon and summer months. In general, SAOD and AAOD show similar trends (if there is any) as that of AOD over most of the regions in most of the months. Strongest positive trends over land regions are observed in November with trend of AOD greater than 0.01 year-1, especially over Indo-Gangetic Plain (IGP). Increase of AOD over IGP in post monsoon is contributed significantly by absorbing aerosols with rate of increase ~ 0.005 AAOD year-1. AAODs are observed to be increasing over Arabian Sea and Bay of Bengal (BoB) in December also, with rate ~ 0.003 AAOD year-1. Strongest positive trends over Arabian Sea and BoB are observed in June with rate of increase greater than 0.02 AOD year-1, whereas strong negative trends are observed over north-west India in the same period with rate of decrease greater than 0.02 AOD year-1. Over IGP, AOD, AAOD and SAOD show contrasting trends in winter and summer seasons. AAOD exhibits strongest decreasing trend over IGP during April-June. Positive trends of AOD over Arabian Sea and BoB are favoured significantly by changes in circulation dynamics. Atmospheric convergence is observed to be strengthening over these regions in April and June, leading to more accumulation and hence positive trends of AOD. Aerosol transport over to the Arabian Sea is observed to be enhancing and contributing significantly to AOD increase over the Arabian Sea in pre-monsoon and summer months. Enhancement in aerosol transport over to the Arabian Sea is observed in pre-monsoon at higher altitudes above 3 km, whereas it is observed in summer at lower levels. However, decreasing trends of AOD over north-west India and IGP during pre-monsoon and summer are observed to be due to decrease in aerosol transport from the continental regions at the west.