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Preprints
https://doi.org/10.5194/acp-2020-785
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-785
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  03 Sep 2020

03 Sep 2020

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This preprint is currently under review for the journal ACP.

Evaluating the sensitivity of radical chemistry and ozone formation to ambient VOCs and NOx in Beijing

Lisa K. Whalley1,2, Eloise J. Slater1, Robert Woodward-Massey1,a, Chunxiang Ye1,a, James D. Lee3,4, Freya Squires4, James R. Hopkins3,4, Rachel E. Dunmore4, Marvin Shaw3,4, Jacqueline F. Hamilton4, Alastair C. Lewis3,4, Archit Mehra5,b, Stephen D. Worrall5,c, Asan Bacak5,d, Thomas J. Bannan5, Hugh Coe5,6, Bin Ouyang7,e, Roderic L. Jones7, Leigh R. Crilley8,f, Louisa J. Kramer8, William J. Bloss8, Tuan Vu8, Simone Kotthaus9,10, Sue Grimmond9, Yele Sun11, Weiqi Xu11, Siyao Yue11, Lujie Ren11, W. Joe F. Acton12, C. Nicholas Hewitt12, Xinming Wang13, Pingqing Fu14, and Dwayne E. Heard1 Lisa K. Whalley 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, 10 YO10 5DD, UK
  • 5Centre for Atmospheric Science, School of Earth and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK
  • 6National Centre for Atmospheric Science, University of Manchester, Manchester, M13 9PL, UK
  • 7Department of Chemistry, University of Cambridge, UK
  • 8School of Geography, Earth and Environmental Sciences, University of Birmingham, B15 2TT, Birmingham, UK
  • 9Department of Meteorology, University of Reading, Reading, UK
  • 10Institut Pierre Simon Laplace, École Polytechnique, Palaiseau, France
  • 11State 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
  • 12Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YW, UK
  • 13State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Wushan, Tianhe District, Guangzhou, GD 510640 , China
  • 14Institute 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: Faculty of Science and Engineering, University of Chester, CH2 4NU, UK
  • cnow at: Aston Institute of Materials Research, School of Engineering and Applied Science, Aston University, Birmingham, B4 7ET, UK
  • dnow at: Turkish Accelerator & Radiation Laboratory, Ankara University Institute of Accelerator Technologies, Atmospheric and Environmemtal Chemistry Laboratory, Gölbaşı Campus, Ankara, Turkey
  • enow at: Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YW, UK
  • fnow at: Department of Chemistry, York University, Toronto ON, M3J 1P3, Canada

Abstract. Measurements of OH, HO2, RO2-complex (alkene and aromatic-related RO2) and total RO2 radicals taken during the AIRPRO campaign in central Beijing in the summer of 2017, alongside observations of OH reactivity are presented. The concentrations of radicals were elevated with OH reaching up to 2.8 × 107 molecule cm−3, HO2 peaked at 1 × 109 molecule cm−3 and the total RO2 concentration reached 5.5 × 109 molecule cm−3. OH reactivity (k(OH)) peaked at 89 s−1 during the night, with a minimum during the afternoons of ~ 22 s−1 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 OH were balanced under high NO concentrations, the OH sinks exceeded the known sources (by 15 ppbv hr−1) under the very low NO conditions (< 0.5 ppbv) experienced in the afternoons, demonstrating a missing OH source consistent with previous studies under high volatile organic compound (VOC), low NO loadings. Under the highest NO mixing ratios (104 ppbv), the HO2 production rate exceeded the rate of destruction by ~ 50 ppbv hr−1, whilst the rate of destruction of total-RO2 exceeded the production by the same rate indicating that the net propagation rate of RO2 to HO2 may be substantially slower than assumed. If just 10 % of the RO2 radicals propagate to HO2 upon reaction with NO, the HO2 and RO2 budgets could be closed at high NO, but at low NO this lower RO2 to HO2 propagation rate revealed a missing RO2 sink that was similar in magnitude to the missing OH source. A detailed box model that incorporated the latest MCM chemical mechanism (MCM3.3.1) reproduced the observed OH concentrations well, but over-predicted the observed HO2 under low concentrations of NO (< 1 ppbv) and under-predicted RO2 (both the complex-RO2 fraction and other RO2 types which we classify as simple-RO2) most significantly at the highest NO concentrations. The model also under-predicted the observed k(OH) consistently by ~ 10 s−1 across all NOx levels highlighting that the good agreement for OH was fortuitous due to a cancellation of missing OH source and sink terms in its budget. Including heterogeneous loss of HO2 to aerosol surfaces did reduce the modelled HO2 concentrations in-line with the observations, but only at NO mixing ratios < 0.3 ppbv. The inclusion of Cl atoms, formed from the photolysis of nitryl chloride, enhanced the modelled RO2 concentration on several mornings when the Cl atom concentration was calculated to exceed 1 × 104 atoms cm−3 and could reconcile the modelled and measured RO2 concentrations at these times. However, on other mornings, when the Cl atom concentration was lower, large under-predictions in total RO2 remained. Furthermore, the inclusion of Cl atom chemistry did not enhance the modelled RO2 beyond the first few hours after sunrise and so was unable to resolve the modelled under-prediction in RO2 observed at other times of the day. Model scenarios, in which missing VOC reactivity was included as an additional reaction that converted OH to RO2, highlighted that the modelled OH, HO2 and RO2 concentrations were sensitive to the choice of RO2 product. The level of modelled to measured agreement for HO2 and RO2 (both complex and simple) could be improved if the missing OH reactivity formed a larger RO2 species that was able to undergo reaction with NO, followed by isomerisation reactions reforming other RO2 species, before eventually generating HO2. In this work an α-pinene-derived RO2 species was used as an example. In this simulation, consistent with the experimental budget analysis, the model underestimated the observed OH indicating a missing OH source. The model uncertainty, with regards to the types of RO2 species present and the radicals they form upon reaction with NO (HO2 directly or another RO2 species), leads to over an order of magnitude less O3 production calculated from the predicted peroxy radicals than calculated from the observed peroxy radicals at the highest NO concentrations. This demonstrates the rate at which the larger RO2 species propagate to HO2 or to another RO2 or indeed to OH needs to be understood to accurately simulate the rate of ozone production in environments such as Beijing where large multifunctional VOCs are likely present.

Lisa K. Whalley et al.

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Lisa K. Whalley et al.

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Short summary
To understand how emission controls will impact ozone, an understanding of the sources and sinks of OH and the chemical cycling between peroxy radicals is needed. This paper presents measurements of OH, HO2, and total RO2 taken in central Beijing. The radical observations are compared to a detailed chemistry model which shows that under low NO conditions there is a missing OH source. Under high NOx conditions, the model under-predicts RO2 and impacts our ability to model ozone.
To understand how emission controls will impact ozone, an understanding of the sources and sinks...
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