Characterization, Sources, and Chemical Processes of Submicron Aerosols at a Mountain Site in Central China

A field campaign (29 May to 14 June 2018) was conducted at a mountain site in central China. The chemical composition of non-refractory submicron particulate matter (NR-PM1) and the particle number size distribution (PNSD) were measured, respectively. The mean NR-PM1 mass concentration was 20.94 +/- 10.14 mu g m-3, among which organics (47%) was the most abundant component, followed by sulfate (37%), ammonium (11%), nitrate (4%), and chloride (1%). Notably, sulfate accounted for more than 70% of the secondary inorganic aerosols. Positive matrix factorization (PMF) analysis of the composition data resulted in three organic aerosol (OA) factors: hydrocarbon-like OA (HOA), oxygenated OA I (OOA-I), and oxygenated OA II (OOA-II). The secondary organic aerosol (SOA) composed of the latter two factors (SOA: OOA-I + OOA-II) was dominant in OA (80.7%). The PMF analysis of the PNSD data yielded three factors: new particle formation related mode, growth mode, and accumulation mode, among which the last factor dominated both number and volume ratios. The sulfate formation was characterized by the sulfur oxidation ratio (SOR), heterogeneous sulfate production rate (Phet), and gaseous sulfuric acid concentration, representing secondary sulfate transformation, heterogeneous reactions, and gas phase reactions, respectively. The results showed that Phet was well correlated with both sulfate concentrations and SOR, especially during the polluted periods. Our study demonstrates that photochemically-driven heterogeneous reactions contribute dominantly to the sulfate formation and SOA is formed predominantly by photochemical oxidation of volatile organic compounds under high temperatures and ultraviolet (UV) intensities, and NOX concentrations on Mt. Wudang during the campaign period.