Elevated levels of OH observed in haze events during wintertime in central Beijing

Slater, Eloise J.; Whalley, Lisa K.; Woodward-Massey, Robert; Ye, Chunxiang; Lee, James D.; Squires, Freya; Hopkins, James R.; Dunmore, Rachel E.; Shaw, Marvin; Hamilton, Jacqueline F.; Lewis, Alastair C.; Crilley, Leigh R.; Kramer, Louisa; Bloss, William; Vu, Tuan; Sun, Yele; Xu, Weiqi; Yue, Siyao; Ren, Lujie; Acton, W. Joe F.; Hewitt, C. Nicholas; Wang, Xinming; Fu, Pingqing; Heard, Dwayne E.

Wintertime in situ measurements of OH, inline-formulaHO2 and inline-formulaRO2 radicals and OH reactivity were made in central Beijing during November and December 2016. Exceptionally elevated NO was observed on occasions, up to inline-formula∼250inline-formulappbv. The daily maximum mixing ratios for radical species varied significantly day-to-day over the ranges 1–inline-formula8×106inline-formulacm−3 (OH), 0.2–inline-formula1.5×108inline-formulacm−3 (inline-formulaHO2) and 0.3–inline-formula2.5×108inline-formulacm−3 (inline-formulaRO2). Averaged over the full observation period, the mean daytime peak in radicals was inline-formula2.7×106, inline-formula0.39×108 and inline-formula0.88×108inline-formulacm−3 for OH, inline-formulaHO2 and total inline-formulaRO2, respectively. The main daytime source of new radicals via initiation processes (primary production) was the photolysis of HONO (inline-formula∼83 %), and the dominant termination pathways were the reactions of OH with NO and inline-formulaNO2, particularly under polluted haze conditions. The Master Chemical Mechanism (MCM) v3.3.1 operating within a box model was used to simulate the concentrations of OH, inline-formulaHO2 and inline-formulaRO2. The model underpredicted OH, inline-formulaHO2 and inline-formulaRO2, especially when NO mixing ratios were high (above 6 inline-formulappbv). The observation-to-model ratio of OH, inline-formulaHO2 and inline-formulaRO2 increased from inline-formula∼1 (for all radicals) at 3 inline-formulappbv of NO to a factor of inline-formula∼3, inline-formula∼20 and inline-formula∼91 for OH, inline-formulaHO2 and inline-formulaRO2, respectively, at inline-formula∼200inline-formulappbv of NO. The significant underprediction of radical concentrations by the MCM suggests a deficiency in the representation of gas-phase chemistry at high NOinline-formulax. The OH concentrations were surprisingly similar (within 20 % during the day) in and outside of haze events, despite inline-formulaj(inline-formulaO1D) decreasing by 50 % during haze periods. These observations provide strong evidence that gas-phase oxidation by OH can continue to generate secondary pollutants even under high-pollution episodes, despite the reduction in photolysis rates within haze.

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Slater, Eloise J. / Whalley, Lisa K. / Woodward-Massey, Robert / et al: Elevated levels of OH observed in haze events during wintertime in central Beijing. 2020. Copernicus Publications.

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