Exploring the drivers of the increased ozone production in Beijing in summertime during 2005–2016

In the past decade, average PMinline-formula_2.5 concentrations decreased rapidly under the strong pollution control measures in major cities in China; however, ozone (inline-formulaO₃) pollution emerged as a significant problem. Here we examine a unique (for China) 12-year data set of ground-level inline-formulaO₃ and precursor concentrations collected at an urban site in Beijing (PKUERS, campus of Peking University), where the maximum daily 8 h average (MDA8) inline-formulaO₃ concentration and daytime inline-formulaO_x (inline-formulaO₃+NO₂) concentration in August increased by inline-formula2.3±1.2 ppbv (inline-formula $M8inlinescrollmathml+\mathrm{normal\; 3.3}\pm \mathrm{normal\; 1.8}$ 52pt10ptsvg-formulamathimgb582e1e37f5014bd16cb2a8a8cc0a132 acp-20-15617-2020-ie00001.svg52pt10ptacp-20-15617-2020-ie00001.png  %) yrinline-formula⁻¹ and inline-formula1.4±0.6 (inline-formula $M11inlinescrollmathml+\mathrm{normal\; 1.9}\pm \mathrm{normal\; 0.8}$ 52pt10ptsvg-formulamathimg87a37ea378aa54100288ae60112cc605 acp-20-15617-2020-ie00002.svg52pt10ptacp-20-15617-2020-ie00002.png  %) yrinline-formula⁻¹, respectively, from 2005 to 2016. In contrast, daytime concentrations of nitrogen oxides (inline-formulaNO_x) 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 inline-formulaJ(NO₂) increasing at a rate of inline-formula3.6±0.8 % yrinline-formula⁻¹. We use an observation-based box model to analyze the combined effect of solar radiation and ozone precursor changes on ozone production rate, inline-formulaP(O₃). The results indicate that the ratio of the rates of decrease of VOCs and inline-formulaNO_x (about 1.1) is inefficient in reducing ozone production in Beijing. inline-formulaP(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 PMinline-formula_2.5 decrease and to a lesser extent by reduced heterogeneous uptake of inline-formulaHO₂. Therefore, the influence of PMinline-formula_2.5 on actinic flux and thus on the rate of oxidation of VOCs and inline-formulaNO_x to ozone and to secondary aerosol (i.e., the major contributor to PMinline-formula_2.5) is important for determining the atmospheric effects of controlling the emissions of the common precursors of PMinline-formula_2.5 and ozone when attempting to control these two important air pollutants.