Impacts of an unknown daytime HONO source on the mixing ratio and budget of HONO, and hydroxyl, hydroperoxyl, and organic peroxy radicals, in the coastal regions of China

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 (P_unknown) could exist. The formula P_unknown ≈ 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 P_unknown, 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 P_unknown impacts on the concentrations and budgets of HONO and peroxy (hydroxyl, hydroperoxyl, and organic peroxy) radicals (RO_x) (= 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 P_unknown 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 P_unknown 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 P_unknown 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 P_unknown contribution of 7.23 ppb h⁻¹ in Beijing. The whole RO_x cycle was accelerated by the three additional HONO sources, especially the P_unknown. The daytime average OH production rate was enhanced by 0.67 due to the three additional HONO sources; [0.64], due to the P_unknown, 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 P_unknown 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 P_unknown considerably enhanced the RO_x concentrations and accelerated RO_x 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.