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Atmospheric oxidizing capacity (AOC) is the fundamental driving factors of chemistry process (e.g., the formation of ozone (O 3) and secondary organic aerosols (SOA)) in the troposphere. However, accurate quantification of AOC still remains uncertainty. In this study, a comprehensive field campaign was conducted during autumn 2019 in downtown of Beijing, where O 3 and PM 2.5 episodes had been experienced successively. The observation-based model (OBM) is used to quantify the AOC at O 3 and PM 2.5 episodes. The strong intensity of AOC is found at O 3 and PM 2.5 episodes, and hydroxyl radical (OH) is the dominating daytime oxidant for both episodes. The photolysis of O 3 is main source of OH at O 3 episode; the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) plays important role in OH formation at PM 2.5 episode. The radicals loss routines vary according to precursor pollutants, resulting in different types of air pollution. O 3 budgets and sensitivity analysis indicates that O 3 production is transition regime (both VOC and NO x -limited) at O 3 episode. The heterogeneous reaction of hydroperoxy radicals (HO 2) on aerosol surfaces has significant influence on OH and O 3 production rates. The HO 2 uptake coefficient (γHO 2) is the determining factor and required accurate measurement in real atmospheric environment. Our findings could provide the important bases for coordinated control of PM 2.5 and O 3 pollution. [Display omitted] [ABSTRACT FROM AUTHOR]

The atmospheric chemistry of nitrous acid (HONO) has received extensive attention because of its significant contribution to hydroxyl (OH) radicals. Heterogeneous reaction of NO 2 is an important HONO source, and its reaction mechanism is affected by many factors, such as concentration of gaseous NO 2 , surface adsorbed water, relative humidity and temperature. Although laboratory studies have confirmed the effect of temperature on heterogeneous reaction of NO 2 , there are few field observations reporting about it. We have conducted a field observation in the early spring 2021 when the temperature ranges widely (−0.1–24.7 °C). Concentrations of HONO and related pollutants at the urban area of Beijing are obtained. The hourly averaged HONO concentration reaches 4.87 ppb with a mean value of 1.48 ± 1.09 ppb. Combined with box model and RACM2 mechanism, we found an optimal temperature (∼10 °C) existing for heterogeneous reaction of NO 2 during this measurement. When considering the promotion effect of optimal temperature, the contribution of heterogeneous reaction of NO 2 to HONO can increase by 10%. This result will provide essential information for developing an accurate model of HONO chemistry in the atmosphere especially for certain periods or regions with temperature changing largely. Moreover, heterogeneous reaction of NO 2 is the vital source of HONO, contributing 63–76% to simulated HONO during this measurement. Note that HONO photolysis is the most important formation pathway of OH radicals, and ambient HONO concentration is the obbligato constraint for evaluating atmospheric oxidation by model simulations. [Display omitted] • Hourly averaged HONO concentration reaches 4.87 ppb with a mean of 1.48 ± 1.09 ppb. • An optimal temperature of ∼10 °C exists for heterogeneous reaction of NO 2. • Heterogeneous reaction of NO 2 contributes 63–76% to simulated HONO. • HONO photolysis is the most important formation pathway of OH radicals. [ABSTRACT FROM AUTHOR]

Abstract: The atmospheric concentrations of nitrous acid (HONO) have been measured during two field campaigns in the winter and summer of 2007 at Beijing (China). The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. The daily average HONO concentrations were in the range of 0.03–2.91ppb and didn''t show temporal variation between the winter and summer periods. The temporal trends seemed to be largely affected by meteorological conditions. HONO concentrations showed very typical diurnal variations during intensive winter and summer periods. Nitrogen oxides were key precursors of HONO formation and the HONO/NO2 values were higher than those reported for direct emission (<1%), indicating the prevalence of secondary chemical HONO formation on direct emission during both periods. We used a pseudo steady state approach (PSS), which included homogeneous and heterogeneous reactions and direct emission, explaining on average about 83% and 48% of the observed HONO levels during the intensive winter and summer periods, respectively. The daytime unknown HONO production was on average 2.58ppbh−1 during the summer period. The HNO3 and fine particulate NO3 − photolysis contributed weakly as HONO source. Including these sources in the PSS calculation, we explained about 53% of the observed HONO levels. The results showed that heterogeneous JNO2 dependent processes on aerosol and ground surfaces, involving NO2 as HONO precursor, were HONO sources during the summer measurements. [Copyright &y& Elsevier]