Physical properties of aerosol particles at a Himalayan background site in India

Three years of particle number size distribution measurements were conducted in a remote Himalayan site in Mukteshwar, India. The 1‐h average total number concentrations varied from 220 to 27,300 cm−3. A clear seasonal cycle in measured aerosol properties was identified. The rainy season in July–August was found to decrease particle number concentrations to less than half of that observed in spring, prior to the rainy season. Highest concentrations were found in eastern and northern air masses. The air masses from these sectors arrived from higher altitudes suggesting the presence of long‐range‐transported aerosol. The particle size distributions were mostly unimodal, having a mean diameter larger than 100 nm during the dry seasons and around 100 nm during the rainy season. During the spring, atmospheric new particle formation events produced bimodal shapes for the size distributions. The diurnal variation in the particle number concentration showed slightly different patterns in different seasons. In spring the pattern was driven partly by new particle formation which started normally around midday during about one third of the days. In winter and autumn, particle concentrations increased from morning to evening and settled to lower levels again toward the night. This behavior is thought to be due to the combination of boundary layer evolution and daily behavior patterns/activities, such as cooking and heating, of habitants in the nearby valley. During the rainy season when the rain “cleaned” the atmosphere, particle number concentrations decreased notably, and no clear diurnal pattern could be identified.