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The atmospheric oxidizing capacity (AOC) plays a key role in atmospheric chemistry, determining the removal of most reduced gases in the atmosphere. In this study, we assess the summertime AOC from 7 to 22 June 2014 associated with a field campaign conducted at Wangdu in the Beijing‐Tianjin‐Hebei region (BTH), China using the WRF‐Chem model. The model reasonably well reproduces the incident solar radiation, ambient O3 and fine particulate matter (PM2.5) concentrations, the photolysis rates of O3 and NO2, and radical concentrations against measurements in the BTH. The model results show high daytime AOC levels in urban areas and relatively low AOC in remote areas of the BTH. The diurnal cycle of the AOC exhibits a unimodal pattern with peaks at noon. OH radical contributes about 68% of the daily average AOC in Beijing, followed by O3 (27%) and NO3 (5%). At Wangdu, the production rate of OH radical reaches around 13 ppb h−1 at noon, which is mainly resulted from the reaction of HO2 with NO. The OH radical loss is primarily contributed by reactions with volatile organic compounds, followed by CO and NO2. Statistically, the population hourly or afternoon average O3 and Ox (=O3 + NO2) concentrations are significantly correlated with the AOC in Beijing, constituting potential simplified tracers to indicate the AOC level. Key Points: The summertime atmospheric oxidizing capacity (AOC) over the Beijing‐Tianjin‐Hebei area is assessed combining a field campaign and WRF‐Chem simulationOH radical contributes about 68% of the daily average AOC in Beijing, followed by O3 (27%) and NO3 (5%)Population hourly or afternoon average O3 and Ox (=O3 + NO2) concentrations constitute potential simplified tracers for the AOC level [ABSTRACT FROM AUTHOR]

• Beijing haze is strongly related to enhanced atmospheric oxidizing capacity. • Two regimes separated by [O 3 ] of 80 μg m−3: AOC-sufficient and -deficient regimes. • Increasing oxidation capacity considerably weakens the emission mitigation effort. Although strict mitigation measures have been implemented since 2013 in Beijing-Tianjin-Hebei (BTH), China, air pollution still frequently occurs. Observations reveal that during pollution episodes in autumn, fine particulate matter (PM 2.5) concentrations have not decreased, and particularly, ozone (O 3) concentrations have increased remarkably from 2013 to 2015 in Beijing. Additionally, a concurrence of O 3 and particulate pollution with high secondary aerosol contributions has been observed frequently, indicating high atmospheric oxidizing capacity (AOC) during particulate pollution. The WRF-Chem model simulations show elevated O 3 concentrations and high fractions of oxygenated secondary aerosols (OSA) in PM 2.5 (0.53–0.73) during the severe pollution period. During daytime there exhibits an AOC-sufficient regime with the persistently high OSA fraction and an AOC-deficient regime with varied OSA fractions, separated by the O 3 level of 80 μg m−3. Our results suggest that increasing AOC can considerably weaken the emission mitigation effort by enhancing the secondary aerosol formation [ABSTRACT FROM AUTHOR]

Implementation of strict emission mitigation measures since 2013 has significantly changed air pollutants in the Beijing-Tianjin-Hebei region (BTH), China. Observations show that ozone (O 3) concentrations have increased by 62.40% (27.84%) and SO 2 concentrations have decreased by 56.42% (35.07%) during particulate pollution episodes in Beijing (BTH) in the autumn from 2013 to 2015. The measured nitrate concentration in Beijing has increased markedly, which to a large degree offsets the sulfate decrease caused by SO 2 emission mitigation. Using the WRF-Chem model, we demonstrate that the enhanced nitrate formation is primarily attributed to increasing atmospheric oxidizing capacity (AOC) and decreasing sulfate competition for base ions. A 9.41–46.24% (7.58–40.97%) decrease in OH radical (O 3) concentrations in October 2015 reduces nitrate and fine particulate matters (PM 2.5) concentrations by 2.51–18.18% and 3.15–18.90% in Beijing, respectively. Based on the scenario in October 2015, if the SO 2 emission increases by 20.00–100.00%, the PM 2.5 concentration increases by 3.02–11.21%, but the nitrate level decreases by 2.48–21.87% simultaneously. Our results suggest that the nitrate aerosol has become a dominant contributor to particulate pollution in Beijing and that decreasing AOC is critical to mitigate nitrate and PM 2.5 concentrations. ● Increasing nitrate concentration largely offsets the sulfate decrease since 2013. ● Increasing nitrate is mainly due to enhanced AOC and weakened sulfate competition. ● Decreasing AOC is beneficial to mitigate nitrate and PM 2.5 concentrations. [ABSTRACT FROM AUTHOR]