7A.3 Combined Impacts of Nitrous Acid and Nitryl Chloride on Lower Tropospheric Ozone: New Module Development in WRF-Chem and Application to China

Zhang, Li, Qinyi Li, Tao Wang, Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China, Ravan Ahmadov, Cooperative Institute for Research in Environmental Sciences, University of Colorado, and National Oceanic and Atmospheric Administration/Earth Systems Reserach Laboratory, Qiang Zhang, Meng Li, Center for Earth System Science, Tsinghua University, Beijing, China, and Mengyao Lv, National Meterological Center, China Meterological Administration, Beijing, China

Nitrous acid (HONO) and nitryl chloride (ClNO2)—through their photolysis—can have profound effects on the nitrogen cycle and oxidation capacity of the lower troposphere. Previous numerical studies have separately considered and investigated the sources/processes of these compounds and their roles in the fate of reactive nitrogen and ozone (O3) production, but their combined impact on the chemistry in the lower part of the troposphere have not been addressed in these modeling studies. In this study, we have updated the WRF-Chem model with currently known source and chemistry of HONO and chlorine in a new chemical mechanism (CBMZ_ReNOM), and applied it to study the combined effects of HONO and ClNO2 on summertime O3 in the boundary layer of China. We simulate the spatial distributions of HONO, ClNO2 and related compounds at surface and within the lower troposphere. The results show that the modeled HONO levels reach up to 800-1800 ppt at the surface (0-30 m) over the Northern China Plain (NCP), Yangtze River Delta (YRD), and Pearl River Delta (PRD) regions and that HONO is concentrated within the layer of 0-200 m. In comparison, simulated surface ClNO2 mixing ratio is around 800-1500 ppt over the NCP, YRD, central China which is predominantly present in the layer of 0-600 m. HONO enhances daytime ROX (OH+HO2+RO2) and O3 at the surface (0-30 m) by 2.8-4.6 ppt (28-37%) and 2.9-6.2 ppb (6-13%), respectively, over the three most developed regions, while ClNO2 increases surface O3 in the NCP and YRD by 2.4-3.3 ppb (or 5-6%) with smaller effect on the PRD and also lays significant impacts (3-6%) on above-surface O3 within 200-500 m. The combined effect increases surface O3 by 11.5%, 13.5%, and 13.3% in the NCP, YRD and PRD, respectively. Over the boundary layer (0-1000m), the HONO and ClNO2 enhance O3 by up to 5.1% and 3.2%, respectively, and their combined effect increases O3 by 7.1-8.9% in the three regions. The new module has noticeably improved O3 predictions at ~900 monitoring stations over China by reducing the mean bias from -4.3 ppb to 0.1 ppb. Our study suggests the importance for considering these reactive nitrogen species simultaneously into chemical transport models to better simulate the formation of summertime O3 in polluted regions.