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ACPD

Atmospheric Chemistry and Physics Discussions

ACPD

1680-7375

Copernicus GmbH

Göttingen, Germany

10.5194/acpd-12-10879-2012

Missing OH source in a suburban environment near Beijing: observed and modelled OH and HO2 concentrations in summer 2006

K. D.

¹ ² Hofzumahaus

² Holland

² Bohn

² Brauers

² Fuchs

² Hu

¹ Häseler

² Kita

³ Kondo

⁴ Li

¹ ² Lou

S. R.

² ⁵ ⁶ Oebel

² ⁷ Shao

¹ Zeng

L. M.

¹ Wahner

² Zhu

¹ Zhang

Y. H.

¹ Rohrer

State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, China

Institut für Energie und Klimaforschung: Troposphäre, Forschungszentrum Jülich, Jülich, Germany

Faculty of Science, Ibaraki University, Ibaraki, Japan

University of Tokyo, Research Center for Advanced Science and Technology, Tokyo, Japan

School of Environmental Science and Technology, Shanghai Jiao Tong University, Shanghai, China

now at: Shanghai Academy Of Environmental Sciences, Shanghai, China

now at: Aixtron SE, Herzogenrath, Germany

27 04 2012

12 4 10879 10936

This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Measurements of ambient OH and HO2 radicals were performed by laser induced fluorescence (LIF) during CAREBeijing2006 (Campaigns of Air Quality Research in Beijing and Surrounding Region 2006) at the suburban site Yufa in the south of Beijing in summer 2006. On most days, local air chemistry was influenced by aged air pollution that was advected by a slow, almost stagnant wind from southern regions. Observed daily maxima of OH and HO2* were in the range of (4–17) × 106 cm−3 and (2–24) × 108 cm−3, respectively. During daytime, OH reactivities were generally high (10–30 s−1) and mainly contributed by VOCs and their oxidation products. The comparison of modelled and measured HOx concentrations reveals a systematic underprediction of OH as a function of NO. A large discrepancy of a factor 2.6 is found at the lowest NO concentration encountered (0.1 ppb), whereas the discrepancy becomes insignificant above 1 ppb NO. This study extends similar observations from the Pearl-River Delta (PRD) in South China to a more urban environment. The OH discrepancy at Yufa can be resolved, if NO-independent additional OH recycling is assumed in the model. The postulated Leuven Isoprene Mechanism (LIM) has the potential to explain the gap between modelled and measured OH at Beijing taking into account conservative error estimates, but still lacks experimental confirmation. This and the hereby unresolved discrepancy at PRD suggest that other VOCs besides isoprene might be involved in the required, additional OH recycling. Fast primary production of ROx radicals up to 7 ppb h−1 was determined at Beijing which was dominated by the photolysis of O3, HONO, HCHO, and dicarbonyls. For a special case, 20 August, when the plume of Beijing city was encountered, a missing primary HOx source (≈ 3 ppb h−1) was determined under high NOx conditions similar to other urban areas like Mexico city. CAREBeijing2006 emphasizes the important role of OVOCs as a radical source and sink, and the need for further investigation of the chemical degradation of VOCs in order to better understand radical chemistry in VOC-rich air.

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