Mixing state of oxalic acid containing particles in the rural area of Pearl River Delta, China: implication for seasonal formation mechanism of Secondary Organic Aerosol (SOA)

The formation of oxalic acid and its mixing state in atmospheric particulate matter (PM) were studied using a single particle aerosol mass spectrometer (SPAMS) in the summer and winter of 2014 in Heshan, a supersite in the rural area of the Pearl River Delta (PRD) region in China. Oxalic acid-containing particles accounted for 2.5 % and 2.7 % in total detected ambient particles in summer and winter, respectively. Oxalic acid was measured in particles classified as elemental carbon (EC), organic carbon (OC), elemental and organic carbon (ECOC), biomass burning (BB), heavy metal (HM), secondary (Sec), sodium-potassium (NaK) and dust. Oxalic acid was found predominantly mixing with sulfate and nitrate during the whole sampling period, likely due to aqueous phase reactions. In summer, oxalic acid-containing particle number and ozone concentration followed a very similar trend, which may reflect the significant contribution of photochemical reactions to oxalic acid formation. Furthermore, favorable in-situ pH (2–4) conditions were observed, which promote Fenton like reactions for efficient production of •OH in HM type particles. A mechanism in which products of photochemical oxidation of VOCs partitioned into the aqueous phase of HM particles, followed by multistep oxidation of •OH through Fenton like reactions to form oxalic acid is proposed. In wintertime, carbonaceous type particles contained a substantial amount of oxalic acid as well as abundant carbon clusters and biomass burning markers. The general existence of nitric acid in oxalic acid-containing particles indicates an acidic environment during the formation process of oxalic acid. Organosulfate-containing particles well correlated with oxalic acid-containing particles during the episode, which suggests the formation of oxalic acid is closely associated with acid-catalyzed reactions of organic precursors.