Assessment of biogenic secondary organic aerosol in the Himalayas

Environmental context Secondary organic aerosols derived from biogenic gases are ubiquitous in the atmosphere. We found that biogenic secondary organic aerosol in Nepal accounted for 6 to 23 % of organic carbon. Primary and secondary biogenic sources combined accounted for approximately half of the observed organic aerosol, suggesting that additional aerosol sources or precursors are significant in this region of the Himalayas. Abstract Biogenic contributions to secondary organic aerosol (SOA) in the South-East Asian regional haze were assessed by measurement of isoprene, monoterpene and sesquiterpene photooxidation products in fine particles (PM2.5) at a mid-latitude site in the Himalayas. Organic species were measured in solvent extracts of filter samples using gas chromatography–mass spectrometry (GCMS) and chemical derivatisation; this analysis was used to quantify molecular markers for primary aerosol sources – including motor vehicles, biomass burning and detritus – and SOA tracers. Authentic standards of most SOA products were not commercially available at the time of this study, so surrogate standards were used for semiquantitation. Using an empirical approach to uncertainty estimation based on homologous series of atmospherically relevant model compounds, analytical uncertainties ranged from 32 % for SOA tracers with structurally similar surrogates to more than 100 % for tracers with a poorly matched surrogate. Biogenic SOA contributions to PM2.5 organic carbon content in the 2005 monsoon and post-monsoon season ranged from 2–19 % for isoprene, 1–5 % for monoterpenes and 1–4 % for sesquiterpenes. High concentrations of isoprene derivatives in aerosol, particularly in the late summer months, point to biogenic SOA as a significant source of organic carbon in the Himalayan region.