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Abstract. In the past decade, average PM2.5 concentrations decreased rapidly under the strong pollution control measures in major cities in China; however, ozone (O3) pollution emerged as a significant problem. Here we examine a unique (for China) 12-year data set of ground-level O3 and precursor concentrations collected at an urban site in Beijing (PKUERS, campus of Peking University), where the maximum daily 8 h average (MDA8) O3 concentration and daytime Ox (O3+NO2) concentration in August increased by 2.3±1.2 ppbv (+3.3±1.8 %) yr−1 and 1.4±0.6 (+1.9±0.8 %) yr−1, respectively, from 2005 to 2016. In contrast, daytime concentrations of nitrogen oxides (NOx) and the OH reactivity of volatile organic compounds (VOCs) both decreased significantly. Over this same time, the decrease of particulate matter (and thus the aerosol optical depth) led to enhanced solar radiation and photolysis frequencies, with near-surface J(NO2) increasing at a rate of 3.6±0.8 % yr−1. We use an observation-based box model to analyze the combined effect of solar radiation and ozone precursor changes on ozone production rate, P(O3). The results indicate that the ratio of the rates of decrease of VOCs and NOx (about 1.1) is inefficient in reducing ozone production in Beijing. P(O3) increased during the decade due to more rapid atmospheric oxidation caused to a large extent by the decrease of particulate matter. This elevated ozone production was driven primarily by increased actinic flux due to PM2.5 decrease and to a lesser extent by reduced heterogeneous uptake of HO2. Therefore, the influence of PM2.5 on actinic flux and thus on the rate of oxidation of VOCs and NOx to ozone and to secondary aerosol (i.e., the major contributor to PM2.5) is important for determining the atmospheric effects of controlling the emissions of the common precursors of PM2.5 and ozone when attempting to control these two important air pollutants.

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