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Discussion papers | Copyright
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Research article 09 Apr 2018

Research article | 09 Apr 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Wintertime photochemistry in Beijing: Observations of ROx radical concentrations in the North China Plain during the BEST-ONE campaign

Zhaofeng Tan1,2, Franz Rohrer2, Keding Lu1, Xuefei Ma1, Birger Bohn2, Sebastian Broch2, Huabin Dong1, Hendrik Fuchs2, Georgios I. Gkatzelis2, Andreas Hofzumahaus2, Frank Holland2, Xin Li1, Ying Liu1, Yuhan Liu1, Anna Novelli2, Min Shao1, Haichao Wang1, Yusheng Wu1,a, Limin Zeng1, Min Hu1,3, Astrid Kiendler-Scharr2, Andreas Wahner2, and Yuanhang Zhang1,3 Zhaofeng Tan et al.
  • 1College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China
  • 2Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum Jülich GmbH, Jülich, Germany
  • 3Beijing Innovation Center for Engineering Sciences and Advanced Technology, Peking University, 100871, Beijing, China
  • anow at: Department of Physics, University of Helsinki, Helsinki, Finland

Abstract. The first wintertime in-situ measurements of hydroxyl (OH), hydroperoxy (HO2) and organic peroxy (RO2) radicals (ROx=OH+HO2+RO2) in combination with observations of total reactivity of OH radicals, kOH in Beijing are presented. The field campaign “Beijing winter finE particle STudy – Oxidation, Nucleation and light Extinctions” (BEST-ONE) was conducted at the suburban site Huairou near Beijing from January to March 2016. It aimed to understand oxidative capacity during wintertime and to elucidate the secondary pollutants formation mechanism in the North China Plain (NCP). OH radical concentrations at noontime ranged from 2.4×106cm−3 in severely polluted air (kOH~27s−1) to 3.6×106cm−3 in relatively clean air (kOH~5s−1). These values are nearly two-fold larger than OH concentrations observed in previous winter campaign in Birmingham, Tokyo, and New York City. During this campaign, the total primary production rate of ROx radicals was dominated by the photolysis of nitrous acid accounting for 46% of the identified primary production pathways for ROx radicals. Other important radical sources were alkene ozonolysis (28%) and photolysis of oxygenated organic compounds (24%). A box model was used to simulate the OH, HO2 and RO2 concentrations based on the observations of their long-lived precursors. The model was capable of reproducing the observed diurnal variation of the OH and peroxy radicals during clean days with a factor of 1.5. However, it largely underestimated HO2 and RO2 concentrations by factors up to 5 during pollution episodes. The HO2 and RO2 observed-to-modeled ratios increased with increasing NO concentrations, indicating a deficit in our understanding of the gas-phase chemistry in the high NOx regime. The OH concentrations observed in the presence of large OH reactivities indicate that atmospheric trace gas oxidation by photochemical processes can be highly effective even during wintertime, thereby facilitating the vigorous formation of secondary pollutants.

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Short summary
We present the first wintertime OH, HO2, and RO2 measurements in Beijing, China. OH concentrations are nearly two-fold larger than what observed in foreign cities during wintertime. The high OH and large OH reactivities indicate photochemical processes can be effective even during wintertime. A box model largely underestimated HO2 and RO2 concentrations during pollution episodes correlated with high NOx, indicating a deficit current chemistry in the high NOx regime.
We present the first wintertime OH, HO2, and RO2 measurements in Beijing, China. OH...