HONO chemistry at a suburban site during the EXPLORE-YRD campaign in 2018: HONO formation mechanisms and impacts on O3 production
Authors: Ye Can , Lu Keding , Ma Xuefei , Qiu Wanyi , Li Shule , Yang Xinping , Xue Chaoyang , Zhai Tianyu , Liu Yuhan , Li Xuan , Li Yang , Wang Haichao , Tan Zhaofeng , Chen Xiaorui , Dong Huabin , Zeng Limin , Hu Min , Zhang Yuanhang 
Published: July 2023

Abstract. HONO is an important precursor for OH radicals that impact secondary pollutants production. However, there are still large uncertainties about different HONO sources, which hinder accurate predictions of HONO concentration and hence atmospheric oxidation capacity. Here HONO was measured during the EXPLORE-YRD campaign, along with other important parameters, enabling us to comprehensively investigate HONO variation characteristics and evaluate the relative importance of different HONO sources by using a box model. HONO showed significant variations, ranging from several tens of ppt to 4.4 ppb. The average diurnal pattern of HONO/NOx showed a maximum of 0.17 around noon and resembled that of j(O1D), indicating the existence of photo-induced sources. Modeling simulations with only the default HONO source (OH+NO) largely underestimated HONO concentrations, with the modeled averaged noontime HONO concentration an order of magnitude lower than the observed concentration. The calculated unknown source strength (Punknown) of HONO showed a nearly symmetrical diurnal profile with a maximum of 2.5 ppb h-1 around noon. The correlation analysis and sensitivity tests showed that photo-induced NO2 conversion on the ground was able to explain Punknown. Additional HONO sources incorporated into the box model improved the model’s performance in simulating HONO concentrations. The revised box model well reproduced nighttime HONO concentration but still underestimated daytime HONO concentration. Further sensitivity tests indicated the underestimation of daytime HONO was not due to uncertainties of photo-induced NO2 uptake coefficients on the ground or aerosol surfaces or enhancement factor of nitrate photolysis but was more likely to other sources that were not considered in the model. Among the incorporated heterogeneous HONO sources and the gas-phase source, photo-induced NO2 conversion on the ground dominated the modeled HONO production during the daytime, accounting for 73 % of the total, followed by NO+OH (10 %), NO2 hydrolysis on the ground surface (9 %), photo-induced NO2 conversion on the aerosol surface (3 %), nitrate photolysis (3 %), and NO2 hydrolysis on the aerosol surface (2 %). NO2 hydrolysis on the ground surface was the major source of nighttime HONO, contributing to 65 % of total HONO production. HONO photolysis contributed to 43 % of ROx production during the daytime, followed by O3 photolysis (17 %), HCHO photolysis (14 %), ozonolysis of alkenes (12 %), and carbonyl photolysis (10 %). The net ozone production rate (12.6 ppb h-1) with observed HONO as a model constraint decreased by 45 % compared to that (6.7 ppb h-1) without HONO as a model constraint, indicating HONO evidently enhanced HONO production and hence aggravated O3 pollution in summer seasons. Our study emphasized the importance of NO2 heterogeneous conversion on the ground surface in HONO production and accurate parameterization of HONO sources in predicting secondary pollutants production.