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Abstract: A new hydroxyl radical formation pathway via photo-excited nitrogen dioxide chemistry is incorporated into a chemistry-only box model as well as a 3D air quality model to examine its potential role on ozone formation and emission control strategy over the Pearl River Delta region in China. While the box model results suggest that the photo-excited nitrogen dioxide chemistry can substantially enhance ozone at high NOx and VOC concentrations, results of 3D air quality model show only a moderate increase in ozone. Though the photo-excited nitrogen dioxide chemistry enhances ozone by a maximum of 10ppbV over the urban area in the vicinity of abundant NOx and VOC concentrations, its typical enhancements range between 2 and 3ppbV. It enhances ozone within the entire planetary boundary layer under conditions featured in weak synoptic wind, abundant water vapor and strong land–sea breeze circulation. No significant improvement in model performance statistics for ozone is found with the photo-excited nitrogen dioxide chemistry. The photo-excited nitrogen dioxide chemistry marginally changes ozone responses to NOx and VOC emission controls. [Copyright &y& Elsevier]

Sulfate plays an important role in atmospheric haze in China, which has received considerable attention in recent years. Various types of parameterization methods and heterogeneous oxidation rates of SO 2 have been used in previous studies. However, properly representing heterogeneous sulfate formation in air quality models remains a big challenge. In this study, we quantified the heterogeneous oxidation reaction using experimental results that approximate the haze conditions in China. Firstly, a series of experiments were conducted to investigate the heterogeneous uptake of SO 2 with different relative humidity (RH) levels and the presence of NH 3 and NO 2 on natural dust surfaces. Then the uptake coefficients for heterogeneous oxidation of SO 2 to sulfate at different RH under NH 3 and NO 2 coexistence were parameterized based on the experimental results and implemented in the Community Multiscale Air Quality modeling system (CMAQ). Simulation results suggested that this new parameterization improved model performance by 6.6% in the simulation of wintertime sulfate concentrations for Beijing. The simulated maximum growth rate of SO 4 2− during a heavy pollution period increased from 0.97 μg m−3 h−1 to 10.11 μg m−3 h−1. The heterogeneous oxidation of SO 2 in the presence of NH 3 contributed up to 23% of the sulfate concentration during heavy pollution periods. Image 1 • Uptake coefficients of SO 2 on dust with coexistence of NH 3 and NO 2 were obtained. • New parameterization was proposed based on experimental results under different RHs. • The new parameterization can improve sulfate simulation in wintertime in CMAQ. • Impact of NH 3 on heterogeneous formation of sulfate in wintertime is non-negligible. [ABSTRACT FROM AUTHOR]