Elevated Levels of OH Observed in Haze Events During Wintertime in Central Beijing
Total Page：16
File Type：pdf, Size：1020Kb
Atmos. Chem. Phys., 20, 14847–14871, 2020 https://doi.org/10.5194/acp-20-14847-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. 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, Freya 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 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 of 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 Correspondence: Dwayne E. Heard ([email protected]) and Lisa K. Whalley ([email protected]) Received: 15 April 2020 – Discussion started: 6 May 2020 Revised: 18 August 2020 – Accepted: 24 September 2020 – Published: 2 December 2020 Abstract. Wintertime in situ measurements of OH, HO2 and OH, HO2 and RO2, especially when NO mixing ratios were RO2 radicals and OH reactivity were made in central Beijing high (above 6 ppbv). The observation-to-model ratio of OH, during November and December 2016. Exceptionally ele- HO2 and RO2 increased from ∼ 1 (for all radicals) at 3 ppbv vated NO was observed on occasions, up to ∼ 250 ppbv. The of NO to a factor of ∼ 3, ∼ 20 and ∼ 91 for OH, HO2 and daily maximum mixing ratios for radical species varied sig- RO2, respectively, at ∼ 200 ppbv of NO. The significant un- nificantly day-to-day over the ranges 1–8 × 106 cm−3 (OH), derprediction of radical concentrations by the MCM sug- 8 −3 8 −3 0.2–1:5 × 10 cm (HO2) and 0.3–2:5 × 10 cm (RO2). gests a deficiency in the representation of gas-phase chem- Averaged over the full observation period, the mean day- istry at high NOx. The OH concentrations were surprisingly time peak in radicals was 2:7 × 106, 0:39 × 108 and 0:88 × similar (within 20 % during the day) in and outside of haze 8 −3 1 10 cm for OH, HO2 and total RO2, respectively. The main events, despite j(O D) decreasing by 50 % during haze pe- daytime source of new radicals via initiation processes (pri- riods. These observations provide strong evidence that gas- mary production) was the photolysis of HONO (∼ 83 %), phase oxidation by OH can continue to generate secondary and the dominant termination pathways were the reactions pollutants even under high-pollution episodes, despite the re- of OH with NO and NO2, particularly under polluted haze duction in photolysis rates within haze. conditions. The Master Chemical Mechanism (MCM) v3.3.1 operating within a box model was used to simulate the con- centrations of OH, HO2 and RO2. The model underpredicted Published by Copernicus Publications on behalf of the European Geosciences Union. 14848 E. J. Slater et al.: Elevated levels of OH observed in haze events during wintertime in central Beijing 1 Introduction in situ measurements of ambient OH concentration are re- quired. In China, especially in its capital city, Beijing, air pollution Measurements of OH and HO2 in northern China during and air quality are serious concerns (Tang et al., 2017). Bei- the wintertime have only recently been made. The first mea- jing can experience severe haze episodes (Hu et al., 2014; surements were made during the BEST-ONE campaign (Tan Lang et al., 2017) with high particulate matter loadings dur- et al., 2017) that took place in January 2016 in Huairou, ing winter months and high ozone episodes during the sum- which is a suburban site 60 km northeast of Beijing. The av- mer (Cheng et al., 2016; Wang et al., 2015). China has one erage daytime maximum concentrations observed during the 6 of the world’s fastest expanding economies and has rapidly BEST-ONE campaign for OH, HO2 and RO2 were 2:5×10 , increased its urban population to form numerous megacities. 0:8 × 108 (3.2 pptv) and 0:6 × 108 cm−3 (2.4 pptv), respec- From 1980 to 2005, the fraction of the population living in tively. The concentration of OH during the BEST-ONE cam- urban areas of China increased from 20 % to 40 %. China’s paign was an order of magnitude higher than predicted by economic growth has led to an increase in energy consump- global models over the North China Plain region (Lelieveld tion, with 50 % of the global demand for coal accounted for et al., 2016) and is consistent with the increase in secondary- by China in 2016 (Qi et al., 2016). The Chinese government aerosol contribution to PM2:5 observed during haze events have been implementing air quality controls in China (Zhang (Huang et al., 2014). The radical measurements during the et al., 2016a), and emission and concentrations of primary BEST-ONE campaign were separated into clean and polluted −1 pollutants have been decreasing nationwide; however, sec- periods (OH reactivity (kOH) > 15 s ) with an average daily ondary pollutants still remain a major concern (Huang et al., maximum OH concentration for these periods of 4×106 and 2014). 2:3 × 106 cm−3, respectively. The RACM2–LIM1 (Regional The OH radical mediates virtually all oxidative chem- Atmospheric Chemistry Mechanism coupled with Leuven istry during the daytime and converts primary pollutants into Isoprene Mechanism 1) box model was used to simulate secondary pollutants, as shown in Fig. 1. The reaction of the radical concentrations measured during BEST-ONE (Tan OH with primary-pollutant emissions (particularly NOx – et al., 2018), but these could not reproduce the OH concentra- NO C NO2 – SO2 and VOCs – volatile organic compounds) tion observed when NO was above 1 ppbv or below 0.6 ppbv, can form secondary pollutants such as HNO3,H2SO4 and consistent with previous campaigns when OH was measured secondary oxygenated organic compounds (OVOCs). These and modelled under NO concentrations > 1 ppbv (Emmer- secondary pollutants can lead to the formation of secondary son et al., 2005; Kanaya et al., 2007; Lu et al., 2013; Tan aerosols and contribute to the mass of PM2:5. During the et al., 2017; Zhou et al., 2003). More recently, OH and HO2 photochemical cycle initiated by OH, NO can be oxidised were measured in central Beijing during wintertime at the to form NO2 via reaction with HO2 and organic peroxy rad- Peking University (PKU) campus in November and Decem- icals, RO2, and the subsequent photolysis of NO2 can lead ber 2017 (Ma et al., 2019). The radical measurements were to the net formation of ozone. It has been shown in previ- simulated using the RACM2–LIM1 box model which high- ous field campaigns that measured mixing ratios of radicals lighted an underprediction of the OH concentration when NO have a strong dependence on j(O1D) (Ehhalt and Rohrer, exceeded 1 ppbv (Ma et al., 2019). Two further campaigns 2000; Ma et al., 2019; Stone et al., 2012; Tan et al., 2018). have taken place in northern China during the summertime. Hence, the radical concentrations measured during winter- The first took place in 2006 at a suburban site in Yufa (Lu time are typically expected to be lower than in the summer- et al., 2013), which is 40 km south of Beijing. The second time due to lower photolysis rates of primary radical sources took place in 2014 at the rural site in Wangdu (Tan et al., such as O3, HONO and HCHO. Here we define primary pro- 2017). In both the Wangdu and Yufa field campaigns, the duction as any process which initiates the formation of rad- box model calculations underestimated the OH concentration icals and hence the photochemical chain reaction. Also, the when NO was below 0.5 ppbv. When NO exceeded 2 ppbv, a lower temperatures experienced in the winter lead to lower missing peroxy radical source was found, leading to a large water vapour concentrations, and this is expected to further underestimation of local ozone production by the model. 1 limit primary OH formation via (O D) C H2O (Heard and To try to understand the link between radical chemistry Pilling, 2003). and the extremely high air pollution that is seen during Bei- In contrast to the expectation of limited photochemistry jing in the wintertime, a field campaign, Air Pollution and in winter, particularly during haze episodes when light lev- Human Health in Chinese Megacities (APHH), took place in els are reduced, aerosol composition analysis has highlighted central Beijing from November to December in 2016. Simul- that the contribution of secondary aerosols to the total par- taneous measurements of OH, HO2 and RO2 concentrations ticulate mass increases during pollution events in the North were performed during the APHH campaign.