Secondary Organic Aerosol Formation in the Tropical Coastal Atmosphere: Impact of Land- and Sea-Breeze Circulations

The present study investigates secondary organic aerosol (SOA) formation in the tropical coastal atmosphere (Thumba: 8.5° N, 76.9° E, ∼3 m a.s.l.) in India during sea-breeze (SB) and land-breeze (LB) circulations by analyzing water-soluble organic compounds using gas chromatography (GC). The molecular distribution of organic compounds is characterized by a predominance of oxalic acid (C₂) during both circulations, with higher abundance during LB. While phthalic (Ph; an oxidation product of aromatics) and azelaic (C₉; an oxidation product of unsaturated fatty acids) acids are the next most abundant compounds during SB, terephthalic acid (tPh; a tracer of plastic-burning) is the second most dominant compound during LB. As markers of photochemical activity, the ratios of fumaric-to-maleic (F/M) and malonic-to-succinic (C₃/C₄) acids are doubled during SB. In contrast, C₂/Σ(C₂ – C₁₀) and water-soluble organic carbon (WSOC)/organic carbon (OC) ratios are higher during LB, suggesting secondary oxidation in an aqueous medium. The ISORROPIA II model’s results show that, consistent with relative humidity levels, aerosol-liquid water content (ALWC) in LB is 7 times larger than in SB. The 18-fold rise in the tPh/C₉ ratio during LB suggests a significant quantity of organics originated from plastic burning. Based on diagnostic mass ratios of organic and inorganic tracer compounds and their relationships, the present study emphasizes that SOA formation in the coastal atmosphere is likely caused by photochemical processes of marine-derived-organic precursors, such as unsaturated fatty acids and hydrolysis of phthalates, and related biogenic precursors during SB. However, the aqueous-phase formation of SOA relates to human-induced pollutants, including plastic burning dominant during LB. The impact of inorganics on daytime photooxidation of organic acids is also discussed.