Aerosol and Air Quality Research, 18: 1720–1733, 2018 Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print / 2071-1409 online doi: 10.4209/aaqr.2017.10.0406 Understanding the Regional Transport Contributions of Primary and Secondary PM2.5 Components over Beijing during a Severe Pollution Episodes Wei Wen1,2, Xiaodong He1*, Xin Ma3**, Peng Wei4, Shuiyuan Cheng5, Xiaoqi Wang5, Lei Liu2 1Institute of Urban Meteorology, China Meteorological Administration, Beijing 100089, China 2 Key Laboratory of Atmospheric Chemistry, China Meteorological Administration, Beijing 100081, China 3 National Meteorological Center, Beijing 100081, China 4 Chinese Research Academy of Environment Science, Beijing 100012, China 5 Key Laboratory of Beijing on Regional Air Pollution Control, Beijing University of Technology, Beijing 100124, China ABSTRACT This study applied the CAMx model to study the regional transport of various PM2.5 components in Beijing during a severe pollution episodes. The results revealed that during the episodes, Beijing had the average PM2.5 pollution value of –3 –3 119 µg m . It was 1.58 times of the PM2.5 national air quality standard (75 µg m Level II). The wind speed was low (< 2 m s–1) and relative humidity reached 98%. The anticyclone in Eastern China showed weak local flow fields and southerly winds at the surface and strong temperature inversion under 1000 m, which promote pollution accumulation. The 2– – contribution of monthly regional transport to primary PM2.5 components and SO4 , NO3 , and secondary organic aerosol concentrations in Beijing were 29.6%, 41.5%, 58.7%, and 60.6%, respectively. The emissions from Baoding had the greatest effect on the primary components of PM2.5 (6.1%) in Beijing. The emissions from Tianjin had the greatest influence on the secondary components of PM2.5 concentrations. These values indicated that the secondary components of Beijing’s PM2.5 are more easily affected by transboundary transport than are the primary components. The present findings suggest that control strategies for PM2.5 pollution should include coordinated efforts aimed at reducing secondary aerosol precursors (SO2, NOx, and VOCs) from long-range transport and local generation in addition to primary particulate emissions. Keywords: PM2.5 pollution; Regional transport; Secondary component; Air quality modeling. INTRODUCTION NOx, and volatile organic compounds [VOCs], respectively) through not only photochemical reaction but also aqueous PM2.5 pollution is a severe environmental problem in and heterogeneous reaction etc. (Huang et al., 2014). PM2.5 China. It poses a considerable threat to human health because pollution can be controlled through coordinated regional it comprises various metallic elements and toxic organic control. Emphasis should be placed not only on controlling materials (Cao et al., 2012). PM2.5 pollution is a complex primary PM2.5 pollutants from local emissions but also on regional transport problem, as the particles can be transported coordinating the control of precursor emissions (SO2, over long distances (Hatakeyama et al., 2011; Squizzato et VOCs, and NOx) in the region. Beijing is the political and al., 2012; Tao et al., 2012; Lang et al., 2013). Previous cultural center of China. It’s located in the north of China, studies have reported that in North China, a large fraction the most polluted region. This region is characterized by high of PM2.5 is attributable to secondary aerosols (Huang et al., concentrations and frequencies of PM2.5 pollution (MEP, 2014; Zheng et al., 2015). Secondary components, such as 2015). The pollution in Beijing has attracted increasing 2– – SO4 , NO3 , and secondary organic aerosols (SOAs), are attention from the government and public of China. To transformed from precursor gases in the atmosphere (SO2, improve the overall air quality and control PM2.5 pollution in the region, a series of PM2.5 pollution control strategies were implemented by the government (China State Council 2013). For example, in 2014 and 2015, the government executed * Corresponding author. strict emission control strategies to ensure that the Asia- Tel.: +86 10 68400750 Pacific Economic Cooperation meeting and 70th anniversary E-mail address: [email protected] of the victory of the Anti-Japanese War celebration were ** Corresponding author. successfully. Emission control in Beijing and surrounding Tel.: +86 10 58993295 provinces (Hebei, Shanxin, Inner Mongolia, Shandong, and E-mail address: [email protected] Henan) has been correspondingly implemented (Wen et al., Wen et al., Aerosol and Air Quality Research, 18: 1720–1733, 2018 1721 2015; Wang et al., 2016). components to local PM2.5 concentrations. Several studies and projects have investigated the regional In this study, the offline model CAMx was applied to 2– transport of PM2.5. Chuang et al. (2008) reported that during study the transport contribution of primary PM2.5 and SO4 , 2– – a typical pollution episode in Taiwan, the SO4 concentration NO3 , and SOA. The particulate source apportionment (35%) was contributed by Shanghai. Wang et al. (2008) technology (PSAT) was implemented in CAMx. This model showed that 28.1% and 59.5% of PM pollution in urban was used to analyze the contributions of emission categories and suburban Beijing was contributed by PM transport from and different source regions. The model not only determines surrounding regions. Streets et al. (2007) found that 50%– the source region and contributions but also provides source 70% of PM pollutants in Beijing were transported from the apportionments for both primary and secondary particulate southern areas of Hebei Province. Wang et al. (2014) used matter. Furthermore, the relative importance of different the CAMx/PSAT (Comprehensive air quality model with source regions can be ranked and further provide a major extensions/ particulate source apportionment technology) contribution amount, which is important for air quality model to analyze a heavy pollution process (November management. Studies on the transport of primary and 2010) in Shanghai; the results revealed that nearly 50% of secondary PM2.5 components can provide scientific and PM2.5 pollution was from surrounding regions. Wang et al. technological support for the collaborative prevention of (2012) applied the CMAQ model in Hebei Province to air pollution. investigate haze pollution and found that regional transport accounted for approximately 35% and 41% of PM2.5 Model Setting and Application pollution in Shijiazhuang and Xingtai, respectively. Lin et Model System al. (2016) extended the CAMx model and reported that in In the present study, the transport components of primary August, local sources respectively accounted for 23.8% and secondary PM2.5 in the Beijing region were simulated and 16.6% of anthropogenic and biogenic SOAs in Beijing. using the CAMx model. The meteorological factors were Furthermore, Wang et al. (2015) applied the MM5–CMAQ simulated using The Weather Research and Forecasting model to estimate the regional source contribution to PM2.5 model (WRF; Version 3.3). The WRF initial conditions were concentrations in most polluted cities in Hebei Province. used as the final global tropospheric analysis data, which They reported that regional transport plays an important were provided by the National Centers for Environmental role in PM2.5 pollution and that the regional joint air Prediction. The PSAT module in CAMx is a framework for pollution controls are crucial. Concurrently, the long-distance apportioning six categories of PM components and gaseous transport of particulate matter and also the species has been precursors. PSAT has been widely used in the CAMx model widely investigated. For example, PM2.5 sulfate in Beirut is (Fann et al., 2012; Zhang et al., 2013). In the present study, mainly from the Eastern European region (Saliba et al., two nested grid architectures were designed to implement 2007). A study reported that pollution in Asia affects the air the modeling system (Fig. 1(a)). During October 2014, quality of the United States and Canada, showing that the several typical severe pollution episodes occurred in the long-distance emission from Asia can be transport and BTH region. The model was applied for a 35-day period in transmitted across the Pacific Ocean (Heald et al., 2006). Beijing, starting from September 25, 2014. The first 5 days However, most studies on the transport of particulate matter were considered as spin-up and were excluded from the are focused on total PM2.5. Few studies have used model analysis. Beijing is enclosed by Hebei Province except to the approaches to comprehensively analyze the transport of southeast, where Tianjin Province is located. The modeling primary and secondary PM2.5 components separately in the domains had the spatial resolution for the modeling Beijing-Tianjin-Hebei (BTH) region under one study domains are 27 km and 9 km respectively. The emission framework. The locations where secondary PM2.5 forms source regions included Beijing and other regions, which might differ from where its precursors are emitted, because were defined on the basis of geographical borders. The the precursors might not be directly converted into PM2.5 emission inventories were obtained from the Multi resolution after emitted into the atmosphere. The time for secondary Emission Inventory for China of the year 2012, which was particle formation to occur depends on the abundance of developed by Tsinghua University. We have previously precursor, temperature,