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Tiny airborne particles called atmospheric aerosols tend to reduce summer monsoon rainfall over most of South Asia.
Dilip Ganguly and his colleagues at the Pacific Northwest National Laboratory in Richland, Washington, used a simplified atmosphere–ocean model to simulate the effects of changes in the levels and composition of atmospheric aerosols — from local and distant sources — on South Asia's mean monsoon rainfall. Increased local emissions of aerosols such as black carbon — which absorbs sunlight and produces a warming effect that tends to reduce cloud cover — weakened the monsoon rains in most of South Asia. Aerosols from outside Asia also contributed to the overall reduction in rainfall.
Only over northwest India, where aerosol emissions from local forest and grass fires are thought to be decreasing, did the mean summer monsoon rainfall increase.
J. Geophys. Res. http://dx.doi.org/10.1029/2012JD017508 (2012)