Diagnostic Analysis of the Sulfate Aerosol Pollution in Spring over Pearl River Delta, China

The study of sulfate aerosols has important research significance to atmospheric chemistry and climate change. A diagnostic study on generation mechanism of sulfate aerosol over the PRD region was conducted using sulfate tracking method. Three pollution episodes occurred in March 2012, in which the first two episodes were mostly caused by fine particulate matters and their secondary aerosols were mainly inorganic sulfates, resulting in low visibility. In contrast, the third pollution episode was dominated by coarse particles with relatively greater visibility. The results of sulfate tracking showed that in addition to emission sources and contributions of initial and boundary conditions, sulfate-generating reaction processes were closely correlated with relative humidity. The first two episodes occurred under the influence of southerly warm and wet air flow system with relatively high humidity, in which the conditions were favorable for aqueous reaction processes. The primary type of liquid-phase oxidation was with hydrogen peroxide (H2O2), followed by catalytic oxidation with Fe and Mn and oxidation with ozone (O3), while peroxyacetic acid (PAA) and methylhydroperoxide (MHP) had very small contributions. Meanwhile, the third pollution episode happened during a warming period following passage of a cold front and was influenced by northerly wind system. Consequently, humidity was relatively low, and therefore, contribution of liquid-phase oxidation reactions was weaker while contributions by gas-phase oxidation between S (IV) and hydroxyl radical (OH) became more prominent. In addition, the simulated vertical distribution results showed that the mass concentration of sulfate aerosols decreased with increasing height. For near-surface layers, sulfates were mainly contributed by emission sources, boundary and initial conditions, liquid-phase oxidations with H_2O_2 and catalytic conversions with Fe and Mn; while for upper layers, sulfates were contributed by, in the order of significance, liquid-phase oxidations with H_2O_2, catalytic conversions with Fe and Mn, emission sources and gas-phase oxidations.