Abstract
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 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, 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.
Highlights
Reactive nitrogen compounds play important roles in atmospheric chemistry and affect the formation of secondary pollutants including ozone (O3) and secondary aerosols
We showed that including these additional sources of HONO very well simulated the observed HONO at a suburban site in southern China
NO2, the main precursor of HONO and N2O5, was, overall, reasonably reproduced by our model compared with the observations as shown in Table S1 and Fig. S1 in the Supplement
Summary
Reactive nitrogen compounds play important roles in atmospheric chemistry and affect the formation of secondary pollutants including ozone (O3) and secondary aerosols. Nitrous acid (HONO), dinitrogen pentoxide (N2O5), and nitryl chloride (ClNO2) have received considerable attention, together with the traditionally known oxides of nitrogen (NO + NO2), due to their potentially significant impact on the oxidation capacity in the polluted portion of the troposphere Within a few hours after sunrise, the ClNO2 is photolyzed into NO2 and reactive chlorine atoms (Reaction R4), the latter of which further react with hydrocarbons to produce additional peroxy radicals (RO2 and HO2) and subsequently accelerate the cycling of oxidants in the atmosphere at both ground level and upper levels (e.g., Tham et al, 2016; Wang et al, 2016). The photolysis of HONO and ClNO2 during the daytime could have a significant effect on the formation of secondary pollutants in polluted regions (Osthoff et al, 2008; Tham et al, 2016; Tang et al, 2015; Thornton et al, 2010; Wang et al, 2016)
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