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

<p>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.</p> <p>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&#730;N and 88.4&#730;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, SO₄^2- 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 SO₄^2- in controlling LWC of aerosols over the eastern Himalayas. Notably, we found significant positive relationships of C₂ acid with both SO₄^2- (R² = 0.94) and aerosol LWC (R² = 0.76), suggesting oxalic acid formation in LWC-enriched aerosols largely controlled by anthropogenic SO₄^2- via aqueous phase photochemical processes. The increased formation of oxalic acid and related polar compounds with increasing SO₄^2- 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.</p>