Carbonaceous Aerosols over Lachung in the Eastern Himalayas: Primary Sources and Secondary Formation of Organic Aerosols in a Remote High-Altitude Environment

Abstract

The Himalayan and Tibetan Plateau, containing the largest ice mass outside the polar region, is very sensitive to the influence of carbonaceous aerosols. In this regard, year-round measurements of carbonaceous aerosols, along with major ionic species, were made over a remote high-altitude (2700 m a.s.l.) site Lachung in the eastern Himalayas to elucidate seasonal source signatures, transport, and secondary organic aerosol (SOA) formation pathways. The observation showed the dominance of organic carbon (OC) in winter (7.6 ± 2.6 μg m–3), having its highest fractional share (32%) to PM10 during both winter and summer. Elemental carbon (EC) concentrations as high as 1 μg m–3 and EC/PM10 > 5% indicated significant anthropogenic influence over this remote site. High OC/EC (5.5 ± 2.5) and the WSOC/OC (0.74 ± 0.15) ratios indicated the dominance of water-soluble secondary organic aerosol (SOA) throughout the year. During spring, the aqueous phase formation (APF) of aerosols was prominent, which is indicated by the strong correlation of aerosol liquid water content with WSOC and SO42–. Further, the positive matrix factorization model demonstrated the dominant contribution by biomass burning sources (>25%), followed by primary emission including mineral dust (22%) and vehicular and industrial emissions (20.5%). The role of north-westerly advection (∼88%) was highest in spring, increasing the OC and EC concentrations (∼70%). These observations univocally support the dominant contributions by anthropogenic aerosols to the eastern Himalayas.