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Trends in aerosol optical properties over South Asia

Spatial variation of trends in MODerate resolution Imaging Spectroradiometer (MODIS) remote sensing‐derived aerosol optical properties, namely, aerosol optical depth (AOD) at 0.55 µm and Ångström exponent (α) from 2001 to 2014, is estimated on annual and seasonal basis over South Asia. Positive trends in annual mean AOD are found over 56% area of continental regions of South Asia where remote sensing data could retrieve, while ∼40% of Indian region show increasing trends in AOD. During winter and post‐monsoon, >70% of the Indian subcontinent show positive trends in AOD due to increase in biomass burning emissions and owing to shallow boundary layer height during the seasons. The increasing trends in both AOD and α over central, north and some part of south, east and northeast India are found, which corroborate that fine‐mode aerosols are increasing over the regions. The AOD exhibits negative trends over northwest, north, central, west and east region of India; and Pakistan and Sri Lanka during pre‐monsoon. The negative trends in AODs are found over maximum part of the South Asian continent during monsoon. The significant and strong positive trends in AOD (2–3% per year) and α are found in the Bay of Bengal (BoB) and the Arabian Sea, moreover post‐monsoon and monsoon seasons AODs exhibit significant and strong (>6% per annum) increasing trends in north BoB. Such positive trends reveal the enhancement in the abundant of fine‐mode aerosols in the oceanic regions. Spatial variabilities in the AOD and α trends give rise to significant differential radiative impact in the regions which affect the atmospheric dynamics and also influence the extreme events such as tropical cyclones in South Asian regions. The trend informations will be useful in improving the emission inventories and our understanding on aerosols in regional and global climate.