[1] The first regional synthesis of long-term (back to~25 years at some stations) primary data (from direct measurement) on aerosol optical depth from the ARFINET (network of aerosol observatories established under the Aerosol Radiative Forcing over India (ARFI) project of Indian Space Research Organization over Indian subcontinent) have revealed a statistically significant increasing trend with a significant seasonal variability. Examining the current values of turbidity coefficients with those reported~50 years ago reveals the phenomenal nature of the increase in aerosol loading. Seasonally, the rate of increase is consistently high during the dry months (December to March) over the entire region whereas the trends are rather inconsistent and weak during the premonsoon (April to May) and summer monsoon period (June to September). The trends in the spectral variation of aerosol optical depth (AOD) reveal the significance of anthropogenic activities on the increasing trend in AOD. Examining these with climate variables such as seasonal and regional rainfall, it is seen that the dry season depicts a decreasing trend in the total number of rainy days over the Indian region. The insignificant trend in AOD observed over the Indo-Gangetic Plain, a regional hot spot of aerosols, during the premonsoon and summer monsoon season is mainly attributed to the competing effects of dust transport and wet removal of aerosols by the monsoon rain. Contributions of different aerosol chemical species to the total dust, simulated using Goddard Chemistry Aerosol Radiation and Transport model over the ARFINET stations, showed an increasing trend for all the anthropogenic components and a decreasing trend for dust, consistent with the inference deduced from trend in Angstrom exponent.

Climate change has great significance globally in general and South Asia in particular. Here we have used data from a network of 35 aerosol observatories over the Indian region to generate the first time regional synthesis using primary data and estimate the aerosol trends. On an average, aerosol optical depth (AOD) was found increasing at a rate of 2.3% (of its value in 1985) per year and more rapidly (~4%) during the last decade. If the trends continue so, AOD at several locations would nearly double and approach unity in the next few decades leading to an enhancement in aerosol‐induced lower atmospheric warming by a factor of two. However, a regionally averaged scenario can be ascertained only in the coming years, when longer and denser data would become available. The regional and global climate implications of such trends in the forcing elements need to be better assessed using GCMs.

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We examine long‐term trends in the near‐surface black carbon mass concentration, using multiyear primary data obtained from a dense network (ARFINET) of observatories over the Indian region. We report for the first time the statistically significant decreasing trend in black carbon mass concentration, based on primary data from this region, at an average rate of ~242 ± 53 ng · m−3 · year−1 during the period 2007–2016. This finding contrasts with the generally increasing trend in the columnar aerosol optical depth, reported earlier, and the steadily increasing trend in anthropogenic activities over this region. The roles of different possible mechanisms, including possible changes in the vertical redistribution of aerosols, are discussed. Over the period 2007–2015, a significant though weak, increasing trend is seen in the contribution from aerosols above 1 km to the columnar aerosol optical depth. These observations imply possible long‐term climate consequences.