Characteristics and Formation Mechanisms of Sulfate and Nitrate in Size-segregated Atmospheric Particles from Urban Guangzhou, China

Abstract

Various water-soluble inorganic compounds, including Na+, NH4+, K+, Ca2+, Mg2+, Cl–, NO3–, PO43– and SO42–, were analyzed in 130 sets of size-segregated (< 0.49, 0.49–0.95, 0.95–1.5, 1.5–3.0, 3.0–7.2 and 7.2–10.0 µm) aerosol samples collected from March 2013 to April 2014 in Guangzhou, China. SO42– was unimodally distributed and peaked during a typical droplet mode (0.49–0.95 µm). However, the distribution of NO3– significantly varied across the four seasons. It was unimodally distributed in summer and autumn, peaking in the coarse mode (3.0–7.2 µm), and bimodally distributed in winter and spring, peaking in the size ranges of 0.49–0.95 µm and 3.0–7.2 µm, respectively. The coarse-mode NO3– was mainly related to the influence of soil/dust. The additional mode during winter and spring was attributable to the formation of ammonium nitrate. Compared to clean days, polluted days favored the formation of SO42– in summer and autumn and NO3– in winter and spring. The sulfur oxidation ratios (SORs) for < 0.49, 0.49–0.95 and 0.95–1.5 µm particles were negatively correlated with the relative humidity (RH) in spring, summer and autumn, respectively. However, the SORs for 0.49–3.0 µm particles were positively correlated with the RH in winter, implying an important contribution from the aqueous oxidation of SO2. Further analysis shows that the SO42– in < 0.49 µm particles was formed primarily through gas-phase photochemical oxidation of SO2 during all four seasons. The formation of NO3– was mainly attributable to heterogeneous reactions for 1.5–3.0 µm particles year-round and homogeneous gas-phase reactions for < 0.49 µm particles in winter. Correlation analysis also indicates a positive influence from biomass burning on the formation of nitrate and sulfate. The average pH of PM3 was calculated to be 2.6–5.6. Thus, the aqueous oxidation of SO2 by NO2 plays a limited role in the formation of sulfate in the atmosphere of Guangzhou.