Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Atmos. Chem. Phys. Discuss., 15, 30563–30608, 2015 www.atmos-chem-phys-discuss.net/15/30563/2015/ doi:10.5194/acpd-15-30563-2015 ACPD © Author(s) 2015. CC Attribution 3.0 License. 15, 30563–30608, 2015 This discussion paper is/has been under review for the journal Atmospheric Chemistry Summertime ozone and Physics (ACP). Please refer to the corresponding final paper in ACP if available. formation in Xi’an and surrounding Summertime ozone formation in Xi’an and areas, China surrounding areas, China T. Feng et al. 1,2,3 1,2 2,4 2,3 5 3 2,3 T. Feng , N. Bei , R. Huang , J. Cao , Q. Zhang , W. Zhou , X. Tie , Title Page S. Liu2,3, T. Zhang2,3, X. Su2,3, W. Lei6, L. T. Molina6, and G. Li2,3 Abstract Introduction 1 School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an, China Conclusions References 2Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China Tables Figures 3State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China J I 4Laboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), Villigen, Switzerland 5Department of Environmental Sciences and Engineering, Tsinghua University, Beijing, China J I 6 Molina Center for Energy and the Environment, La Jolla, CA, USA Back Close Received: 3 June 2015 – Accepted: 20 October 2015 – Published: 4 November 2015 Full Screen / Esc Correspondence to: G. Li ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. Printer-friendly Version Interactive Discussion 30563 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract ACPD In the study, the ozone (O3) formation is investigated in Xi’an and surrounding areas, China using the WRF-CHEM model during the period from 22 to 24 August 2013 cor- 15, 30563–30608, 2015 responding to a heavy air pollution episode with high concentrations of O3 and PM2.5 5 (particulate matter with aerodynamic diameter less than 2.5 µm). The WRF-CHEM Summertime ozone model generally performs well in simulating the surface temperature and relative hu- formation in Xi’an midity compared to the observations and also reasonably reproduces the observed and surrounding temporal variations of the surface wind speed and direction. The convergence formed areas, China in Xi’an and surrounding areas is favorable for the accumulation of pollutants, causing T. Feng et al. 10 high concentrations of O3 and PM2.5. In general, the calculated spatial patterns and temporal variations of near-surface O3 and PM2.5 are consistent well with the mea- surement at the ambient monitoring stations. The simulated daily mass concentrations of aerosol constituents, including sulfate, nitrate, ammonium, elemental and organic Title Page carbon, are also in good agreement with the filter measurements. High aerosol concen- Abstract Introduction 15 trations in Xi’an and surrounding areas significantly decrease the photolysis frequen- −3 Conclusions References cies and can reduce near-surface O3 concentrations by more than 50 µgm (around 25 ppb) on average. Sensitivity studies show that the O3 production regime in Xi’an and Tables Figures surrounding areas is complicated, varying from NOx to VOC-sensitive chemistry. The industry emissions contribute the most to the O3 concentrations compared to the nat- J I 20 ural and other anthropogenic sources, but still do not play a determined role in the O3 J I formation. The complicated O3 production regime and high aerosol levels constitute a dilemma for O3 control strategies in Xi’an and surrounding areas. In the condition with Back Close high O3 and PM2.5 concentrations, decreasing various anthropogenic emissions can- Full Screen / Esc not efficiently mitigate the O3 pollution, and a 50 % reduction of all the anthropogenic 25 emissions only decreases near-surface O3 concentrations by less than 14 % during Printer-friendly Version daytime. Further studies need to be performed for O3 control strategies considering manifest changes of the emission inventory and uncertainties of meteorological field Interactive Discussion simulations. 30564 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 Introduction ACPD Ozone (O3) is a key species in the atmosphere due to its controlling role in the pho- 15, 30563–30608, 2015 tochemistry in the stratosphere and troposphere (Seinfeld and Pandis, 2006) since O3 and its photochemical derivative, OH, are the key oxidants for most reduced gases 5 (Brasseur et al., 1999). Atmospheric O3 also contributes to the global climate change Summertime ozone because of absorption of the infrared radiation, constituting one of the important short- formation in Xi’an lived climate pollutants. Additionally, high levels of surface O3 exert deleterious impacts and surrounding on ecosystems and human health (Cao et al., 2012; Zhou et al., 2011) and thence areas, China O3 becomes one of the criteria pollutants regulated by the environmental agencies 10 in many countries, such as US Environmental Protection Agency (US EPA) and the T. Feng et al. China’s Ministry of Environmental Protection (China MEP). Rapid industrialization and urbanization have caused severe air pollution recently in China (e.g., De Smedt et al., 2010; Lu et al., 2011) and numerous studies have been Title Page performed to investigate the severe O3 pollution, particularly in the Beijing–Tianjin- Abstract Introduction 15 Hebei region (e.g., Wang et al. 2006; Lin et al., 2008; Tang et al., 2009; Xu et al., 2011), Yangtze Delta region (e.g., Geng et al., 2009, 2011; Tie et al., 2009, 2012), Conclusions References and Pearl River Delta region (e.g., Zhang et al., 2008; T. Wang et al., 2009; Cheng et Tables Figures al., 2010; Wang et al., 2011; Li et al., 2013). For example, T. Wang et al. (2006) have observed strong O3 production in urban plumes from Beijing with a maximum O3 con- J I 20 centration of 286 ppb. Using a chemical transport model, Tie et al. (2009) have shown J I that unfavorable meteorological conditions cause high near-surface O3 level exceeding 100 ppb in the Shanghai region. T. Wang et al. (2009) have reported increasing surface Back Close O3 concentrations in the background atmosphere of Southern China during the period from 1994 to 2007. Full Screen / Esc 25 As the largest city in northwestern China with the population of more than 8 million, Xi’an is located in the Guanzhong basin. The basin is nestled between the Qinling Printer-friendly Version Mountains in the south and the Loess Plateau in the north, with a warm-humid climate. Interactive Discussion The unique topography is not favorable for the dispersion of air pollutants (Fig. 1a) 30565 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and, with the rapid increasing industries and city expansions, heavy air pollutions often attack the basin (Cao et al., 2005; Shen et al., 2008, 2009). Shen et al. (2009) have re- ACPD ported that the PM (particulate matter with aerodynamic diameter less than 2.5 µm) 2.5 15, 30563–30608, 2015 mass concentrations in Xi’an exceed 350 µgm−3 during haze episodes and the straw −3 5 combustion in suburban area of Xi’an increases PM2.5 level to 400 µgm . However, studies on O3 measurements and formation mechanism in Xi’an are still limited until Summertime ozone now. X. Wang et al. (2012) have performed one-year surface O3 measurement at an formation in Xi’an urban site in Xi’an in 2008. They have shown high O3 episodes with the O3 concen- and surrounding tration of more than 100 ppb in May and June and found that high O3 episodes are areas, China 10 associated with biogenic emissions from Qinling Mountains. Hence, considering the T. Feng et al. increasingly stringent air quality standards in China, studies are imperative to verify the O3 and other pollutants formation from both natural and anthropogenic emission sources to support the design and implementation of emission control strategies in the Title Page Guanzhong basin. 15 Since January 2013, the China MEP has commenced to release the real-time hourly Abstract Introduction observations of chemical species at the national ambient monitoring stations, includ- Conclusions References ing O3, NO2, CO, SO2, PM2.5, and PM10 (particulate matter with aerodynamic diam- eter less than 10 µm). Total thirteen national monitoring stations are distributed at the Tables Figures nine districts in Xi’an. In addition, continuous measurements of aerosol mass, chemi- 20 cal composition, and optical properties have been conducted at the Institute of Earth J I Environment, Chinese Academy of Sciences (IEECAS) in Xi’an, China since 2003. All J I those measurements provide an opportunity to investigate O3 formation in Xi’an and Back Close surrounding areas. The purpose of the present study is to evaluate the O3 formation from anthropogenic and natural sources and the difficulties in the O3 control strategy Full Screen / Esc 25 caused by the complicated nonlinear formation of O3 and the aerosol impact on the photochemistry. The WRF-CHEM model and the model configuration are described Printer-friendly Version in Sect. 2. Results of the modeling experiments and comparisons are presented in Sect. 3, and the Conclusions are given in Sect. 4. Interactive Discussion 30566 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 Model and method ACPD 2.1 WRF-CHEM model 15, 30563–30608, 2015 In this study, a specific version of the WRF-CHEM model (Grell et al., 2005) is applied to verify the O3 formation in Xi’an and surrounding areas, which is developed by Li et Summertime ozone 5 al. (2010, 2011a, b, 2012) at the Molina Center for Energy and the Environment, with a formation in Xi’an new flexible gas phase chemical module and the CMAQ (version 4.6) aerosol module and surrounding developed by US EPA (Binkowski and Roselle, 2003).