Summertime atmospheric dicarboxylic acids and related SOA in the background region of Yangtze River Delta, China: Implications for heterogeneous reaction of oxalic acid with sea salts

The diacid chemistry of summertime PM_2.5 and the size-segregated aerosols (9-stages) in Chongming Island, a coastal site in the Yangtze River Delta (YRD), China, were investigated. Our results showed that oxalic acid (C₂) was the dominant dicarboxylic acid, followed by succinic acid (C₄), malonic acid (C₃), adipic acid (C₆) and phthalic acid (Ph). Two types of haze pollution events were identified during the sampling period, i.e., Event I, which was mainly caused by the local biomass burning emission, and Event II, which was caused by a long-distance transport of the YRD urban pollution. C₂ linearly correlated with SO₄²⁻ and NO₃⁻ in Event I but only with O₃ in Event II, indicating that oxalic acid formation was dominated by the aerosol aqueous phase oxidation in Event I and by the gaseous phase oxidation in Event II, respectively. 65.5% of Cl⁻ in sea salts at the site in the clean period was depleted and robustly correlated with oxalic acid (R² = 0.74). We proposed a mechanism to explain such a significant Cl⁻ depletion, in which anthropogenic VOC oxidize into oxalic acid and its precursors such as glyoxal and methyglyoxal by a photochemical oxidation, and then oxalic acid and the related compounds subsequently react with sea salts and release HCl into the troposphere. The significant Cl⁻ depletion of sea salts related with the organic acid (C₂) in coastal China was found for the first time and should be considered in future studies, because oxalic acid and related SOA in the country are abundant and the released HCl may effectively enhance the oxidation capacity of the atmosphere by photolytically producing Cl radicals.