Anthropogenic sulfate-driven secondary organic aerosol formation over the eastern Himalayas through aqueous phase photochemical reactions

Abstract

The ubiquitous presence of organic aerosols (OA) and their dominant role in climate forcing via direct and indirect effects are increasingly recognized. Among the large spectrum of OAs, water-soluble secondary organic aerosols (SOA) have gained considerable scientific interest due to the in-cloud formation process and consequent climate effects. However, a lack of consensus on the molecular fingerprints and formation pathways of SOA limits our ability to assess their climate impact globally.In this study, a detail investigation is made on water-soluble SOA components such as dicarboxylic acids, oxocarboxylic acids and dicarbonyls in springtime (March to May 2019) PM10 samples collected from a remote high-altitude site Lachung (27.4˚N and 88.4˚E, 2700 m a.s.l.) in the eastern Himalayas. The molecular fingerprints of water-soluble SOA components showed the predominance of oxalic acid (C2) followed by phthalic (Ph) and succinic acid. The diagnostic mass ratio of Ph to azelaic (C9) acid (2.1-7.7) indicated that anthropogenic emissions are the major sources of water-soluble OA at this remote high-altitude site. Moreover, SO42- dominates total water-soluble inorganic constituents and correlates significantly (R2 = 0.66) with the ISORROPIA-II model-derived aerosol liquid water content (LWC), indicating a crucial role of anthropogenic SO42- in controlling LWC of aerosols over the eastern Himalayas. Notably, we found significant positive relationships of C2 acid with both SO42- (R2 = 0.94) and aerosol LWC (R2 = 0.76), suggesting oxalic acid formation in LWC-enriched aerosols largely controlled by anthropogenic SO42- via aqueous phase photochemical processes. The increased formation of oxalic acid and related polar compounds with increasing SO42- is indicative of the fact that the reduction in anthropogenic emissions at the regional scale or source regions could control the water-soluble SOA loading in the atmosphere of the eastern Himalayas.