Impacts of an unknown daytime nitrous acid source on its daytime concentration and budget, as well as those of hydroxyl, hydroperoxyl, and organic peroxy radicals, in the coastal regions of China

Abstract

Abstract. Many field experiments have found high nitrous acid (HONO) mixing ratios in both urban and rural areas during daytime, but these high daytime HONO mixing ratios cannot be explained well by gas-phase production, suggesting that an unknown daytime HONO source (Punknown) could exist. The formula Punknown ≈ 19.60 × NO2 × J(NO2) was obtained using observed data from 13 field experiments across the globe. The additional HONO sources (i.e. the Punknown, HONO emissions, and nighttime hydrolysis conversion of nitrogen dioxide (NO2) on aerosols) were coupled into the WRF-Chem model (Weather Research and Forecasting model coupled with Chemistry) to assess the Punknown impacts on the concentrations and budgets of HONO and peroxy (hydroxyl, hydroperoxyl, and organic peroxy) radicals (ROx) (= OH + HO2 + RO2) in the coastal regions of China. Results indicated that the additional HONO sources produced a significant improvement in HONO and OH simulations, particularly in the daytime. Elevated daytime-mean Punknown values were found in the coastal regions of China, with a maximum of 2.5 ppb h−1 in the Beijing–Tianjin–Hebei region. The Punknown produced a 60–250% increase of OH, HO2 and RO2 near the ground in the major cities of the coastal regions of China, and a 5–48% increase of OH, HO2 and RO2 in the daytime meridional-mean mixing ratios within 1000 m above the ground. When the additional HONO sources were included, the photolysis of HONO was dominated in the OH production rate in Beijing, Shanghai and Guangzhou before 10:00 LST with a maximum of 10.01 [7.26 due to the Punknown] ppb h−1 in Beijing, whereas the reaction of HO2 + NO (nitric oxide) was dominated after 10:00 LST with a maximum of 9.38 [7.23] ppb h−1 in Beijing. The whole ROx cycle was accelerated by the additional HONO sources, especially the Punknown. The OH production rate was enhanced by 0.67 [0.64] to 4.32 [3.86] ppb h−1 via the reaction of HO2 + NO, and by 0.85 [0.69] to 4.11 [3.61] ppb h−1 via the photolysis of HONO, and the OH loss rate was enhanced by 0.58 [0.55] to 2.03 [1.92] ppb h−1 via the reaction of OH + NO2 and by 0.31 [0.28] to 1.78 [1.64] ppb h−1 via the reaction of OH + CO (carbon monoxide) in Beijing, Shanghai and Guangzhou. Similarly, the additional HONO sources produced an increase of 0.31 [0.28] to 1.78 [1.64] ppb h−1 via the reaction of OH + CO and 0.10 [0.09] to 0.63 [0.59] ppb h−1 via the reaction of CH3O2 (methylperoxy radical) + NO in the HO2 production rate, and 0.67 [0.61] to 4.32 [4.27] ppb h−1 via the reaction of HO2 + NO in the HO2 loss rate in Beijing, Shanghai and Guangzhou. The above results suggest that the Punknown considerably enhanced the ROx concentrations and accelerated ROx cycles in the coastal regions of China, and could produce significant increases in concentrations of inorganic aerosols and secondary organic aerosols and further aggravate haze events in these regions.