1Finnish Meteorological Institute, Erik Palménin aukio 1, P.O. Box 503, 00101, Helsinki, Finland
2Finnish Meteorological Institute, Yliopistonranta 1F, P.O. Box 1627, 70211 Kuopio, Finland
3Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
4Laboratory of Aerosol Chemistry and Physics, Institute of Chemical Process Fundamentals Academy of Sciences of the Czech Republic, Rozvojová 135, 165 02 Prague 6, Czech Republic
5The Energy and Resources Institute (TERI), Darbari Seth Block, IHC Complex, Lodhi Road, New Delhi 110 003, India
*currently at: School of Earth & Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332-0340, USA
Abstract. Aerosol physical and optical properties were measured at two locations in northern India. The first measurement station was a background site in Mukteshwar, about 350 km northeast of New Delhi, in the foothills of the Indian Himalayas, with data from 2006 to 2009. The second measurement site was located in Gual Pahari, about 25 km south of New Delhi, with data from 2008 to 2009. 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. In Mukteshwar, the monsoon season removed particles from all size classes, due to a combination of rain scavenging and activation to cloud and mountain fog droplets. The scavenging by rain is least effective for the size range of the accumulation mode particles. In Gual Pahari, this was the only major wet removal mechanism and, as a result, the accumulation mode particles were less effectively removed. 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 a diameter around 3–4 μm. The single scattering albedo varied from 0.73 to 0.93 during the monsoon season, being slightly lower in Gual Pahari than in Mukteshwar. This is due to the fact that Gual Pahari resided closer to high anthropogenic black carbon emissions. As the absorbing particles are typically in the accumulation mode, they were not effectively removed by rain scavenging. The aerosol columnar properties, which were measured in Gual Pahari, showed a somewhat different seasonal behaviour compared to the surface measurements, with the aerosol optical depth increasing to an annual maximum in the early monsoon season.