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https://doi.org/10.5194/acp-2020-362
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-362
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 06 May 2020

Submitted as: research article | 06 May 2020

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This preprint is currently under review for the journal ACP.

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

Eloise J. Slater1, Lisa K. Whalley1,2, Robert Woodward-Massey1,a, Chunxiang Ye1,a, James D. Lee3,4, Freja Squires4, James R. Hopkins3,4, Rachel E. Dunmore4, Marvin Shaw3,4, Jacqueline F. Hamilton4, Alastair C. Lewis3,4, Leigh R. Crilley5,b, Louisa Kramer5, William Bloss5, Tuan Vu5, Yele Sun6, Weiqi Xu6, Siyao Yue6, Lujie Ren6, W. Joe F. Acton7, C. Nicholas Hewitt7, Xinming Wang8, Pingqing Fu9, and Dwayne E. Heard1 Eloise J. Slater et al.
  • 1School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
  • 2National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK
  • 3National Centre for Atmospheric Science, University of York, Heslington, York, YO10 5DD, UK
  • 4Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK
  • 5School of Geography, Earth and Environmental Sciences, University of Birmingham, B15 2TT, Birmingham, UK
  • 6State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute for Atmospheric Physics, Chinese Academy of Sciences, 40 Huayanli, Chaoyang District, Beijing 100029, China
  • 7Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YW, UK
  • 8State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Wushan, Tianhe District, Guangzhou, GD 510640, China
  • 9Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China
  • anow at: College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China
  • bnow at: Department of Chemistry, Faculty of Science, York University, 4700 Keele Street, Toronto ON, M3J 1P3, Canada

Abstract. Wintertime in situ measurements of OH, HO2 and RO2 radicals and OH reactivity were made in central Beijing during November and December 2016. Exceptionally elevated NO was observed on occasions, up to ~ 250 ppbv, believed to be the highest mole fraction for which there have then co-located radical observations. The daily maximum mixing ratios for radical species varied significantly day-to-day over the range 1–8 × 106 cm−3 (OH), 0.2–1.5 × 108 cm−3 (HO2) and 0.3–2.5 × 108 cm−3 (RO2). Averaged over the full observation period, the mean daytime peak in radicals was 2.7 × 106 cm−3, 0.39 × 108 cm−3 and 0.88 × 108 cm−3 for OH, HO2 and total RO2, respectively. The main daytime source of new radicals via initiation processes (primary production) was the photolysis of HONO (~ 83 %), and the dominant termination pathways were the reactions of OH with NO and NO2, 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, HO2 and RO2. The model underpredicted OH, HO2 and RO2, especially when NO mixing ratios were high (above 6 ppbv). The observation-to-model ratio of OH, HO2 and RO2 increased from ~ 1 (for all radicals) at 3 ppbv of NO to a factor of ~ 3, ~ 20 and ~ 91 for OH, HO2 and RO2, respectively, at ~ 200 ppbv of NO. The significant underprediction of radical concentrations by the MCM suggests a deficiency in the representation of gas-phase chemistry at high NOx. The OH concentrations were surprisingly similar (within 20 % during the day) inside and outside of haze events, despite j(O1D) 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.

Eloise J. Slater et al.

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Short summary
The paper details atmospheric chemistry in a mega-city (Beijing), focussing on radicals which mediate the formation of secondary pollutants such as ozone and particles. Highly polluted conditions were experienced, including the highest ever levels of nitric oxide (NO) with simultaneous radical measurements. Radical concentrations were large during “haze” events, demonstrating active photochemistry was occurring. Modelling showed that our understanding of the chemistry at high NOx is incomplete.
The paper details atmospheric chemistry in a mega-city (Beijing), focussing on radicals which...
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