Missing OH source in a suburban environment near Beijing: observed and modelled OH and HO<sub>2</sub> concentrations in summer 2006

Abstract

Abstract. 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 concentration maxima were in the range of (4–17) × 106 cm−3 for OH and (2–24) × 108 cm−3 for HO2 (including an estimated interference of 25% from RO2). During daytime, OH reactivities were generally high (10–30 s−1) and mainly contributed by observed VOCs and their calculated 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 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 (about 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.

How the OH reactivity affects the ozone production efficiency: case studies in Beijing and Heshan, China

“…The boundary layer height was set as constant to 1000 m in the model due to the lack of measurements. This was similar to model setups in Lu et al (2013) and field measurement results in Guo et al (2016).…”

Investigation of potential interferences in the detection of atmospheric RO<sub><i>x</i></sub> radicals by laser-induced fluorescence under dark conditions

“…This instrument is similar to the instruments that were deployed in previous field campaigns on the ground and on board of a Zeppelin (e.g. Lu et al, 2012Lu et al, , 2013Li et al, 2014). It contains three separate fluorescence cells for the detection of OH, HO 2 , and RO 2 radicals.…”

“…In a previous study , we investigated potential OH interferences in our LIF instrument (Forschungszentrum Jülich) during the photo-oxidation of alkenes (including trans-2-butene and isoprene) and aromatics for similar chemical conditions as found in the field campaigns in the Pearl River Delta and Beijing in China, during which measured OH concentrations were larger than expected Lu et al, 2012Lu et al, , 2013. The experiments were performed with sunlight in the atmosphere simulation chamber SAPHIR in Jülich.…”

Development of an instrument for direct ozone production rate measurements: measurement reliability and current limitations

“…In contrast, models tend to overestimate HO 2 in forested areas and regions characterized by large concentrations of biogenic VOCs (Griffith et al, 2013;Mao et al, 2012;Pugh et al, 2010). Disagreements are also present in the modeling of OH, with the models underestimating the measurements at forested environments (Lelieveld et al, 2008;Tan et al, 2001;Whalley et al, 2011;Hofzumahaus et al, 2009;Lu et al, 2013;Pugh et al, 2010), while the agreement may be better when colder temperatures lead to lower concentrations of isoprene and other VOCs (Griffith et al, 2013). The discrepancies between models and measurements question our ability to successfully measure radical species or indicate that there are still unknowns in our understanding of the radical and ozone production chemistry, which in turn could lead to erroneous P (O 3 ) calculations by atmospheric models.…”