Combined impacts of nitrous acid and nitryl chloride on lower-tropospheric ozone: new module development in WRF-Chem and application to China

Abstract. 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 the production of ozone (O₃), but their combined impact on the chemistry of the lower part of the troposphere has not been addressed yet. In this study, we updated the WRF-Chem model with the currently known sources and chemistry of HONO and chlorine in a new chemical mechanism (CBMZ_ReNOM), and applied it to a study of the combined effects of HONO and ClNO₂ on summertime O₃ in the boundary layer over China. We simulated the spatial distributions of HONO, ClNO₂, and related compounds at the surface and within the lower troposphere. The results showed that the modeled HONO levels reached up to 800–1800 ppt at the surface (0–30 m) over the North China Plain (NCP), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD) regions and that HONO was concentrated within a 0–200 m layer. In comparison, the simulated surface ClNO₂ mixing ratio was around 800–1500 ppt over the NCP, YRD, and central China regions and was predominantly present in a 0–600 m layer. HONO enhanced daytime RO_x (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, whereas ClNO₂ increased surface O₃ in the NCP and YRD regions by 2.4–3.3 ppb (or 5–6 %) and it also had a significant impact (3–6 %) on above-surface O₃ within 200–500 m. The combined effects increased surface O₃ by 11.5, 13.5, and 13.3 % in the NCP, YRD, and PRD regions, respectively. Over the boundary layer (0–1000 m), the HONO and ClNO₂ enhanced O₃ by up to 5.1 and 3.2 %, respectively, and their combined effect increased O₃ by 7.1–8.9 % in the three regions. The new module noticeably improved O₃ predictions at ∼ 900 monitoring stations throughout China by reducing the mean bias from −4.3 to 0.1 ppb. Our study suggests the importance of considering these reactive nitrogen species simultaneously into chemical transport models to better simulate the formation of summertime O₃ in polluted regions.