Aerosol optical properties and composition over a table top complex mining area in the Monsoon trough region

Aerosol physiochemical properties over a varied mining plateau region at the eastern end of a monsoon trough are reported for the first time and analyzed at different time scales. Aerosol optical depth (single scattering albedo, SSA) is found to be 0.49 (0.9) in pre-monsoon, 0.4 (0.94) in monsoon, 0.46 (0.92) in post-monsoon, and 0.36 (0.89) in winter, with an annual mean of 0.43 (0.91). The volume-size distribution is tri-modal, with 0.02 (ultra-fine), 0.2 (accumulation) and 7 (coarse) µm, but with seasonal signatures. The angstrom exponent (AE) varies along with the AOD, especially in winter, although they are inversely related to each other during monsoons; the increase in size may be due to the effect of humidity. AODbc varies between 13.4%–4.7% of the total aerosols, with the highest contribution in March, when forest burning in the north east is at its peak. BC is the lowest in July, the mid monsoon month with the minimum biomass burning and brick-kiln activities. It is likely that the interactions of various minerals and intermittent rains help keep the aerosol size in a mixed state with regard to the relation between AE and AOD, although more work is needed to confirm this. The chemical composition of aerosols is derived from an aerosol chemical model based on the measured amount of black carbon and the assumed components. These components are selected based on back trajectories and earlier reports from the region. Their concentrations are adjusted by constraining the model output AOD and SSA to match (±2% @ 500 nm) that observed by a sun-sky radiometer. The chemical compositions of the winter and post-monsoon months are similar, while pre-monsoon period has more coarse mode minerals, and the monsoon period has more sea-salt (accu.). The component mass concentrations were grouped into various size bins based on their modal radii, and the results indicate that PM1 is at its maximum in winter whereas PM2.5 is highest in the post-monsoon period. Monsoons leads to the effective washout of 2.5–10 µm sized particles