Long term trends in aerosol optical properties and their relationship with cloud properties over Southern India and Sri Lanka

Asia is one of the continents where aerosol levels are comparatively higher across the world. India and Sri Lanka are some of the regions in the South Asian continent where pollution is increasing rapidly due to the rise in industrialization. The present study investigated the interactions between atmospheric aerosol and cloud microphysical properties and their spatial, temporal, and seasonal variation at local and regional scales during 2000-2020 using remotely sensed data sets in south India and Sri Lanka. High values (>0.5) of annual mean aerosol optical depth (AOD) were detected over southern India. On the other hand, lower values (0.2) were detected over Sri Lanka. In terms of seasonality, a high level of AOD was registered in both southern India and Sri Lanka during the premonsoon and monsoon periods. The angstrom exponent (AE(412-470)) revealed the dominance of fine-mode particles during winter and the postmonsoon, generally from biomass burning and industrial activities. The long-term analysis exhibited an increasing trend of atmospheric aerosol concentration over southern India and Sri Lanka. Interestingly, there was a decrease in AOD during the year 2020; the reduction in anthropogenic activities in the region was attributed to the COVID-19 lockdown, hence less accumulation of pollutants in the atmosphere. AOD showed a positive correlation with cloud effective radius over the western areas along with the Indian Ocean, north, southeast, and southern end of India, while negatively correlated with high AOD areas such as northeast of the study domain. The AOD and cloud optical depth were positively correlated over continental areas, while negative correlations were notable over the Indian Ocean around Sri Lanka, implying heterogeneities of aerosol's effect on cloud microphysical properties over the study area. Finally, the results from wind circulation and backward air mass trajectories reveal higher concentrations of fine-mode particles associated with the continent, whereas coarse-mode particles originate from the oceans.