Fine and ultrafine particles at a near-free tropospheric environment over the high-altitude station Hanle in the Trans-Himalaya: New particle formation and size distribution

Number‐size distributions of ultrafine, fine, and accumulation mode aerosols in the size range 5–1300 nm have been measured regularly from the pristine, high‐altitude (for 4520 m above mean sea level) station Hanle in the Trans‐Himalaya during the summer and autumn (August–November) 2009. The total number concentration ranged from 80 to 8000 cm−3 with a mean value of 1150 cm−3. Examination of the temporal variations of the size distributions indicated that formation of new ultrafine particles from the precursor gases (probably transported from the valley regions) was highly probable during the forenoon hours of the day, especially during the summer when the insolation was abundant, the process becoming increasingly less efficient as the season progressed toward winter. The time of occurrence of maximum concentration was generally during the forenoon, a few hours after sunrise, and this time shifted to later parts of the day as the season progressed toward winter, probably associated with later sunrise and low solar elevations. The number‐size distributions revealed two prominent modes: a nucleation mode with mode diameter at ∼16 nm and a consistent accumulation mode with the mode diameter ranging between 115 and 150 nm. Examining the temporal features with the air mass types, it was noticed that the number concentration increased, and the accumulation mode broadened when west Asian air mass prevailed. In summer (during August) the number concentrations tended to higher values associated with air mass from the Indian origin. The ratio of the Aitken to accumulation mode concentration indicated that the aerosol particles existing over the site are aged. Key Points Evidence of new particle formation over the Himalayas during daytime Size distribution has a nucleation and accumulation mode Advection from west Asia and Africa contribute to enhancement in aerosols