Size-resolved aerosol water-soluble ions during the summer and winter seasons in Beijing: Formation mechanisms of secondary inorganic aerosols.

Size-segregated water-soluble ionic species (WSIs) were measured using an Anderson cascade impactor from Jul. to Aug. 2008 and from Dec. 2009 to Feb. 2010 in urban Beijing. The results showed that fine particles (PM2.1, Dp < 2.1 μm) accounted for ∼49% (summer) and ∼34% (winter) of the total particulate mass, and WSIs accounted for 23-82% of the mass concentration of PM2.1. Secondary inorganic aerosols (SIAs, the sum of SO42-, NO3- and NH4+) accounted for more than 30% of the fine particles, which were greatly elevated during particle pollution events (PM events), thereby leading to an alteration of the size distributions of SO42- and NO3- to nearly single fine-mode distributions peaking at 0.65-2.1 μm. This finding suggests that heterogeneous aqueous reactions were enhanced at high RH values. SIAs also increased during dust events, particularly for coarse mode SO42-, which indicated enhanced heterogeneous reactions on the dust surface. The positive matrix factorization (PMF) model was used to resolve the bulk mass size distributions into condensation, droplet, and coarse modes, representing the three major sources of the particles. The formation of SO42- was attributed primarily to in-cloud or aerosol droplet processes during summer (45%), and the heterogeneous reaction of SO2 on mineral dust surfaces was an important formation pathway during winter (45%). The formation pathways of NO3- in fine particles were similar to those of SO42-, where over 30% were formed by in-cloud processes. This work provides important field measurement-based evidence for understanding the formation pathway of secondary inorganic aerosols in the megacity of Beijing.