Effect of the summer monsoon on aerosols at two measurement stations in Northern India - Part 2: Physical and optical properties

Aerosol physical and optical properties were measured at two locations in Northern India during 2006-2010. The first measurement station was a background site in Mukteshwar, about 350 km northeast of New Delhi, in the foothills of the Indian Himalayas. The second measurement site was located in Gual Pahari, about 25 km south of New Delhi. At both stations, the average aerosol concentrations during the monsoon were decreased by 40-75% compared to the pre-monsoon average concentrations. The decrease varied with the total local rainfall. Also the mean aerosol size decreased during the monsoon season. The size distribution at Mukteshwar was unimodal, with a mode diameter at about 80 nm. In Gual Pahari, the ratio of Aitken and accumulation particle concentration was >1, due to wet deposition and new particle formation during the monsoon season. Aerosol concentrations during the early monsoon were found to be affected by mineral dust which in Gual Pahari was observed as an increased particle volume at around 3-4 mu m. The single scattering albedo varied from 0.73 to 0.93 during the monsoon season, being slightly lower in Gual Pahari than in Mukteshwar. The aerosol columnar properties, which were measured in Gual Pahari, showed a somewhat different seasonal behavior compared to the surface measurements, with the aerosol optical depth increasing to an annual maximum in the early monsoon season.