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, HONO emissions, and nighttime hydrolysis conversion of nitrogen dioxide (NO2) on aerosols, 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 three additional HONO sources (i.e., the Punknown, nighttime hydrolysis conversion of NO2 on aerosols, and HONO emissions) 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. High daytime average 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 three additional HONO sources were included, the photolysis of HONO was the second most important source in the OH production rate in Beijing, Shanghai, and Guangzhou before 10:00 LST with a maximum of 3.72 ppb h−1 and a corresponding Punknown contribution of 3.06 ppb h−1 in Beijing, whereas the reaction of HO2 + NO (nitric oxide) was dominant after 10:00 LST with a maximum of 9.38 ppb h−1 and a corresponding Punknown contribution of 7.23 ppb h−1 in Beijing. The whole ROx cycle was accelerated by the three additional HONO sources, especially the Punknown. The daytime average OH production rate was enhanced by 0.67 due to the three additional HONO sources; [0.64], due to the Punknown, to 4.32 [3.86] ppb h−1, via the reaction of HO2 + NO, and by 0.49 [0.47] to 1.86 [1.86] ppb h−1, via the photolysis of HONO. The OH daytime average 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 three additional HONO sources produced an increase of 0.31 [0.28] (with a corresponding Punknown contribution) 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 daytime average HO2 production rate, and 0.67 [0.61] to 4.32 [4.27] ppb h−1, via the reaction of HO2 + NO in the daytime average 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.
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