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

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 mu m) 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 mu m). 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 mu m), and bimodally distributed in winter and spring, peaking in the size ranges of 0.49-0.95 mu m and 3.0-7.2 mu m, 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 mu m particles were negatively correlated with the relative humidity (RH) in spring, summer and autumn, respectively. However, the SORs for 0.49-3.0 mu m 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 mu m particles was formed primarily through gas-phase photochemical oxidation of SO2 during all four seasons. The formation of NO(3)(- )was mainly attributable to heterogeneous reactions for 1.5-3.0 mu m particles year-round and homogeneous gas-phase reactions for < 0.49 mu m 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.