New particle formation infrequently observed in Himalayan foothills – why?
1Finnish meteorological Institute, P.O. Box 503, 00101, Helsinki, Finland
2Finnish meteorological Institute, P.O. Box 1627, 70211, Kuopio, Finland
3Georgia Institute of Technology, School of Earth & Atmospheric Sciences, 311 Ferst Drive, GA 30332-0340 Atlanta, USA
4Energy and Resource Institute, Darbari Seth Block, IHC Complex, Lodhi Road, New Delhi – 110 003, India
Abstract. A fraction of the Himalayan aerosols originate from secondary sources, which are currently poorly quantified. To clarify the climatic importance of regional secondary particle formation at Himalayas, data from 2005 to 2010 of continuous aerosol measurements at a high-altitude (2180 m) Indian Himalayan site, Mukteshwar, were analyzed. For this period, the days were classified, and the particle formation and growth rates were calculated for clear new particle formation (NPF) event days. The NPF events showed a pronounced seasonal cycle. The frequency of the events peaked in spring, when the ratio between event and non-event days was 53 %, whereas the events were truly sporadic on any other seasons. The annual mean particle formation and growth rates were 0.40 cm−3 s−1 and 2.43 nm h−1, respectively. The clear annual cycle was found to be mainly controlled by the seasonal evolution of the Planetary Boundary Layer (PBL) height together with local meteorological conditions. Spring NPF events were connected with increased PBL height, and therefore characterised as boundary layer events, while the rare events in other seasons represented lower free tropospheric particle formation.