LAUSR

Abstract SO42−, NO3− and NH4+ are the important components of PM2.5 in China. This study developed a modeling-monitoring coupled approach to determining the sources of SO42− and NO3− as well as the secondary oxidation ratio of SO2 to SO42−, through a monitoring of PM2.5 in Beijing and Tangshan, and a WRF-CAMx simulation over North China. Measurement showed that the concentrations of NO3−, SO42− and NH4+ in winter (summer) were 24.2 ± 25.2 μg/m3 (5.1 ± 2.9 μg/m3), 17.5 ± 18.5 μg/m3 (5.8 ± 3.7 μg/m3) and 14.5 ± 14.1 μg/m3 (4.7 ± 2.9 μg/m3) in Beijing, while 12.2 ± 11.0 μg/m3 (13.3 ± 4.5 μg/m3), 15.8 ± 11.9 μg/m3 (14.3 ± 4.9 μg/m3) and 14.6 ± 13.0 μg/m3 (9.6 ± 3.7 μg/m3) in Tangshan. CAMx-PSAT simulation showed that the concentrations of SO42− in Beijing was affected more by regional transport. In Tangshan, local SO2 was the main source of SO42−. As for NO3−, both cities were affected more by regional transport in both winter and summer. Sulfate/nitrate aged ratio (SAR and NAR) and sulfate oxidation ratio (SOR) calculated by the coupling approach stated above could reflect the regional and local atmospheric oxidation capacity respectively. The NAR in Beijing were 0.13 ± 0.08 in winter and 0.09 ± 0.06 in summer, 0.08 ± 0.05 and 0.15 ± 0.05 in Tangshan. The SAR were 0.32 ± 0.17 and 0.51 ± 0.17 in Beijing, 0.13 ± 0.11 and 0.19 ± 0.11 in Tangshan. Then, we further designed several scenarios in CAMx to differentiated the SO42− converted from outside SO2 in the studied city from that in outside areas, and to further evaluated the SOR. It was 0.22 ± 0.15 and 0.23 ± 0.11 in winter and summer in Beijing, and 0.11 ± 0.10 and 0.14 ± 0.08 in Tangshan. It implies a stronger atmospheric oxidation ability in Beijing compared with Tangshan, and the impact of regional transport of sulfur pollutants on Beijing is much more obvious. Moreover, the correlation coefficient between RH and SOR was 0.94 and 0.93 in winter, 0.05 and 0.17 in summer in two cities. It indicates that SO42− is transformed mainly through liquid-phase and heterogeneous reaction in winter and gaseous reaction in summer.