# Evaluating the sensitivity of radical chemistry and ozone formation to ambient VOCs and NO x in Beijing

Measurements of inline-formulaOH, inline-formulaHO2, complex inline-formulaRO2 (alkene- and aromatic-related inline-formulaRO2) and total inline-formulaRO2 radicals taken during the integrated Study of AIR Pollution PROcesses in Beijing (AIRPRO) campaign in central Beijing in the summer of 2017, alongside observations of inline-formulaOH reactivity, are presented. The concentrations of radicals were elevated, with inline-formulaOH reaching up to inline-formula $M10inlinescrollmathmlnormal 2.8×{normal 10}^{normal 7}\phantom{\rule{0ex}{0ex}}unit\mathrm{normal molecule}\phantom{\rule{0ex}{0ex}}{\mathrm{normal cm}}^{-normal 3}$ 117pt14ptsvg-formulamathimg4e909abbf8551399336f90e21388a47a acp-21-2125-2021-ie00001.svg117pt14ptacp-21-2125-2021-ie00001.png , inline-formulaHO2 peaking at inline-formula $M12inlinescrollmathmlnormal 1×{normal 10}^{normal 9}\phantom{\rule{0ex}{0ex}}unit\mathrm{normal molecule}\phantom{\rule{0ex}{0ex}}{\mathrm{normal cm}}^{-normal 3}$ 108pt14ptsvg-formulamathimg0b74abaa7182a6ed08496f31d68c0a08 acp-21-2125-2021-ie00002.svg108pt14ptacp-21-2125-2021-ie00002.png and the total inline-formulaRO2 concentration reaching inline-formula $M14inlinescrollmathmlnormal 5.5×{normal 10}^{normal 9}\phantom{\rule{0ex}{0ex}}unit\mathrm{normal molecule}\phantom{\rule{0ex}{0ex}}{\mathrm{normal cm}}^{-normal 3}$ 117pt14ptsvg-formulamathimgde9cfbd8569f0f81a6e6a2dec65912fd acp-21-2125-2021-ie00003.svg117pt14ptacp-21-2125-2021-ie00003.png . inline-formulaOH reactivity (inline-formulak(OH)) peaked at 89 inline-formulas−1 during the night, with a minimum during the afternoon of inline-formula $M18inlinescrollmathml\approx normal 22\phantom{\rule{0ex}{0ex}}unit{\mathrm{normal s}}^{-normal 1}$ 41pt13ptsvg-formulamathimgb72d46ec792fda104bb9323e7d3f3d34 acp-21-2125-2021-ie00004.svg41pt13ptacp-21-2125-2021-ie00004.png on average. An experimental budget analysis, in which the rates of production and destruction of the radicals are compared, highlighted that although the sources and sinks of inline-formulaOH were balanced under high inline-formulaNO concentrations, the inline-formulaOH sinks exceeded the known sources (by 15 inline-formulappbv h−1) under the very low inline-formulaNO conditions (inline-formula<0.5 ppbv) experienced in the afternoons, demonstrating a missing inline-formulaOH source consistent with previous studies under high volatile organic compound (VOC) emissions and low inline-formulaNO loadings. Under the highest inline-formulaNO mixing ratios (104 inline-formulappbv), the inline-formulaHO2 production rate exceeded the rate of destruction by inline-formula $M30inlinescrollmathml\approx normal 50\phantom{\rule{0ex}{0ex}}unit\mathrm{normal ppbv}\phantom{\rule{0ex}{0ex}}{\mathrm{normal h}}^{-normal 1}$ 67pt15ptsvg-formulamathimgf3058558c05f47e48b5ad0ecff7508df acp-21-2125-2021-ie00005.svg67pt15ptacp-21-2125-2021-ie00005.png , whilst the rate of destruction of total inline-formulaRO2 exceeded the production by the same rate, indicating that the net propagation rate of inline-formulaRO2 to inline-formulaHO2 may be substantially slower than assumed. If just 10 % of the inline-formulaRO2 radicals propagate to inline-formulaHO2 upon reaction with inline-formulaNO, the inline-formulaHO2 and inline-formulaRO2 budgets could be closed at high inline-formulaNO, but at low inline-formulaNO this lower inline-formulaRO2 to inline-formulaHO2 propagation rate revealed a missing inline-formulaRO2 sink that was similar in magnitude to the missing inline-formulaOH source. A detailed box model that incorporated the latest Master Chemical Mechanism (MCM3.3.1) reproduced the observed inline-formulaOH concentrations well but over-predicted the observed inline-formulaHO2 under low concentrations of inline-formulaNO (inline-formula<1 ppbv) and under-predicted inline-formulaRO2 (both the complex inline-formulaRO2 fraction and other inline-formulaRO2 types which we classify as simple inline-formulaRO2) most significantly at the highest inline-formulaNO concentrations. The model also under-predicted the observed inline-formulak(OH) consistently by inline-formula $M55inlinescrollmathml\approx normal 10\phantom{\rule{0ex}{0ex}}unit{\mathrm{normal s}}^{-normal 1}$ 41pt13ptsvg-formulamathimg23ddb0086302b8f58a9edac03fc13f5b acp-21-2125-2021-ie00006.svg41pt13ptacp-21-2125-2021-ie00006.png across all inline-formulaNOx levels, highlighting that the good agreement for inline-formulaOH was fortuitous due to a cancellation of missing inline-formulaOH source and sink terms in its budget. Including heterogeneous loss of inline-formulaHO2 to aerosol surfaces did reduce the modelled inline-formulaHO2 concentrations in line with the observations but only at inline-formulaNO mixing ratios inline-formula<0.3 ppbv. The inclusion of inline-formulaCl atoms, formed from the photolysis of nitryl chloride, enhanced the modelled inline-formulaRO2 concentration on several mornings when the inline-formulaCl atom concentration was calculated to exceed inline-formula $M66inlinescrollmathmlnormal 1×{normal 10}^{normal 4}\phantom{\rule{0ex}{0ex}}unit\mathrm{normal atoms}\phantom{\rule{0ex}{0ex}}{\mathrm{normal cm}}^{-normal 3}$ 95pt14ptsvg-formulamathimg98b77bc515d2967c9ddb21d1393c5808 acp-21-2125-2021-ie00007.svg95pt14ptacp-21-2125-2021-ie00007.png and could reconcile the modelled and measured inline-formulaRO2 concentrations at these times. However, on other mornings, when the inline-formulaCl atom concentration was lower, large under-predictions in total inline-formulaRO2 remained. Furthermore, the inclusion of inline-formulaCl atom chemistry did not enhance the modelled inline-formulaRO2 beyond the first few hours after sunrise and so was unable to resolve the modelled under-prediction in inline-formulaRO2 observed at other times of the day. Model scenarios, in which missing VOC reactivity was included as an additional reaction that converted inline-formulaOH to inline-formulaRO2, highlighted that the modelled inline-formulaOH, inline-formulaHO2 and inline-formulaRO2 concentrations were sensitive to the choice of inline-formulaRO2 product. The level of modelled to measured agreement for inline-formulaHO2 and inline-formulaRO2 (both complex and simple) could be improved if the missing inline-formulaOH reactivity formed a larger inline-formulaRO2 species that was able to undergo reaction with inline-formulaNO, followed by isomerisation reactions reforming other inline-formulaRO2 species, before eventually generating inline-formulaHO2. In this work an inline-formulaα-pinene-derived inline-formulaRO2 species was used as an example. In this simulation, consistent with the experimental budget analysis, the model underestimated the observed inline-formulaOH, indicating a missing inline-formulaOH source. The model uncertainty, with regards to the types of inline-formulaRO2 species present and the radicals they form upon reaction with inline-formulaNO (inline-formulaHO2 directly or another inline-formulaRO2 species), leads to over an order of magnitude less inline-formulaO3 production calculated from the predicted peroxy radicals than calculated from the observed peroxy radicals at the highest inline-formulaNO concentrations. This demonstrates the rate at which the larger inline-formulaRO2 species propagate to inline-formulaHO2, to another inline-formulaRO2 or indeed to inline-formulaOH needs to be understood to accurately simulate the rate of ozone production in environments such as Beijing, where large multifunctional VOCs are likely present.

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Whalley, Lisa K. / Slater, Eloise J. / Woodward-Massey, Robert / et al: Evaluating the sensitivity of radical chemistry and ozone formation to ambient VOCs and NOx in Beijing. 2021. Copernicus Publications.

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Rechteinhaber: Lisa K. Whalley et al.

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