investigating the impact of regional transport on pm2.5 formation

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  • Atmos. Chem. Phys., 16, 1545115460, 2016www.atmos-chem-phys.net/16/15451/2016/doi:10.5194/acp-16-15451-2016 Author(s) 2016. CC Attribution 3.0 License.

    Investigating the impact of regional transport on PM2.5 formationusing vertical observation during APEC 2014 Summit in BeijingYang Hua1,2, Shuxiao Wang1,2, Jiandong Wang1,2, Jingkun Jiang1,2, Tianshu Zhang3, Yu Song4, Ling Kang4,Wei Zhou1,2, Runlong Cai1,2, Di Wu1,2, Siwei Fan1,2, Tong Wang1,2, Xiaoqing Tang5, Qiang Wei6, Feng Sun6, andZhimei Xiao71State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University,Beijing 100084, China2State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China3Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China4College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China5Hebei Environmental Monitoring Center, Hebei 050051, China6Beijing Environmental Monitoring Center, Beijing 100048, China7Tianjin Environmental Monitoring Center, Tianjin 300191, China

    Correspondence to: Shuxiao Wang (shxwang@tsinghua.edu.cn)

    Received: 17 August 2016 Published in Atmos. Chem. Phys. Discuss.: 5 September 2016Revised: 13 November 2016 Accepted: 17 November 2016 Published: 15 December 2016

    Abstract. During the APEC (Asia-Pacific Economic Coop-eration) Economic Leaders 2014 Summit in Beijing, strictregional air emission controls were implemented, providinga unique opportunity to investigate the transport and forma-tion mechanism of fine particulate matter (PM2.5). This studyexplores the use of vertical observation methods to investi-gate the influence of regional transport on PM2.5 pollution inBeijing before and during the APEC Summit. Vertical pro-files of extinction coefficient, wind, temperature and rela-tive humidity were monitored at a rural site on the border ofBeijing and Hebei Province. Three PM2.5 pollution episodeswere analyzed. In episode 1 (27 October to 1 November),regional transport accompanied by the accumulation of pol-lutants under unfavorable meteorological conditions led tothe pollution. In episode 2 (25 November), pollutants leftfrom episode 1 were retained in the boundary layer of the re-gion for 2 days and then settled down to the surface, leadingto an explosive increase of PM2.5. The regional transport ofaged aerosols played a crucial role in the heavy PM2.5 pol-lution. In episode 3 (611 November), emissions from largepoint sources had been controlled for several days while pri-mary emissions from diesel vehicles might have led to thepollution. It is found that ground-level observation of mete-orological conditions and air quality could not fully explain

    the pollution process, while vertical parameters (aerosol op-tical properties, winds, relative humidity and temperature)improved the understanding of regional transport influenceon heavy pollution processes. Future studies may considerincluding vertical observations to aid investigation of pollu-tant transport, especially during episodic events of rapidlyincreasing concentrations.

    1 Introduction

    With rapid economic development and increases in energyconsumption, a large quantity of emissions has caused se-rious particulate matter pollution in China. Monitoring datashows that the BeijingTianjinHebei (BTH) region is oneof the most polluted regions in China (B. Zhao et al., 2013;Wang et al., 2014). The region was home to 8 of the 10 mostpolluted Chinese cities in 2014 according to the reportspublished by Ministry of Environment Protection (MEP,2015). In 2014, the annual average PM2.5 (particulate mat-ter with aerodynamic diameter less than 2.5 m) concentra-tion reached 95 g m3 in the BTH region. With 21.5 mil-lion residents and 5.3 million vehicles in 2014, Beijing hasfrequently been burdened with severe pollution episodes in

    Published by Copernicus Publications on behalf of the European Geosciences Union.

  • 15452 Y. Hua et al.: Investigating the impact of regional transport on PM2.5 formation

    recent years (Beijing Municipal Bureau of Statistics, 2015).The capital is surrounded by mountains in three directions(north, west and east). The top three most polluted citiesin China (Baoding, Xingtai and Shijiazhuang) are locatedsouth of Beijing. Polluted air masses from the south con-tribute to PM2.5 pollution in Beijing (Wang et al., 2015).Source apportionment by the Beijing Environmental Pro-tection Bureau indicates that regional transport contributed2836 % to PM2.5 in Beijing in 20122013. During somesevere pollution periods, regional contribution was morethan 50 % (http://www.bjepb.gov.cn/bjepb/413526/331443/331937/333896/396191/index.html). Quite a few studieshave researched the causes of heavy pollution episodes inthe BTH region, and they show that regional transport playsan important role in pollution formation. The sharp PM2.5buildup events in Beijing were unique, while pollution accu-mulated more slowly in other cities in the region. This indi-cated that PM2.5 was probably transported to Beijing fromother cities (Zheng et al., 2015; Ji et al., 2014; Tao et al.,2014; X. J. Zhao et al., 2013). Meanwhile, most severe pollu-tion occurs under stable synoptic meteorological conditionsin Beijing (Sun et al., 2015; Zheng et al., 2015; X. J. Zhao etal., 2013). The low wind speed and stable synoptic meteoro-logical conditions at ground level cannot explain the reasonwhy regional transport contributes significantly to severe pol-lution. A previous study showed that the secondary aerosolin Beijing probably mainly forms over regional transport, ac-cording to a vertical observation from the ground to 260 mheight (Sun et al., 2015). Therefore, vertical profiles of me-teorology and air quality might help us to understand the im-pacts of regional transport on heavy pollution during stagnantconditions.

    As in other megacities with local sources and regionaltransport, air quality in Beijing is affected by many factors,including emissions inside the city, formation of secondarypollutants, atmospheric mixing and regional transport. It iswell known that the strength of each factor varies accord-ing to emissions and/or weather conditions. Therefore, it ischallenging to pinpoint the major contributors to clean orpolluted episodes in any given time period. This is espe-cially difficult in the BTH region, considering the compli-cated emission sources and transport processes.

    Emission control measures implemented during someevents provide a unique opportunity to investigate the impactof various factors that influence air quality. One of them wasthe APEC (Asia-Pacific Economic Cooperation) EconomicLeaders 2014 Summit held in Beijing from 5 to 11 Novem-ber 2014. A strict emission pollution control plan was car-ried out in the BTH region to improve air quality in Beijingfrom 3 to 11 November for APEC. According to a conserva-tive estimate by MEP, production of 9289 plants was pausedand 3900 plants were running at reduced capacity in sixprovinces (Beijing, Tianjin, Hebei, Shanxi, Shandong and In-ner Mongolia), and more than 40 thousand construction siteswere shut down temporarily (http://www.zhb.gov.cn/gkml/

    hbb/qt/201411/t20141115_291482.htm). Other measures in-cluded traffic control (50 % of private passenger vehicles and70 % of buses were off-road) and frequent road sweeping andcleaning in Beijing. More detailed emission control measuresare supplied in the Supplement. Studies found that regionalemission control effectively reduced air pollutant concentra-tions during the summit (Wen et al., 2015; Tang et al., 2015;Han et al., 2015; C. Chen et al., 2015; Sun et al., 2016a). Thesignificantly reduced local emissions led to reduced com-plexity of particulate matter pollution processes, thus pro-viding a unique opportunity to investigate the influence oftransport events on PM2.5 levels in Beijing.

    The objective of the study is to investigate the impact ofregional transport on PM2.5 in Beijing using both ground-level and vertical observations. Field observation was con-ducted at a rural site (Liulihe) southwest of Beijing beforeand during the control period of the APEC 2014 Summit.Vertical profiles of temperature, RH (relative humidity), windspeed and direction, and extinction coefficient were observedas well as pollutant concentration and meteorological param-eters on the ground. The characteristics of three PM2.5 pol-lution episodes were analyzed. Findings of this study willhelp explore vertical observation methods for in-depth anal-ysis of meteorological and transport influence. Furthermore,these findings can aid the development of future air qualitymanagement strategies in BTH and other regions around theglobe, including emission control and air surveillance.

    2 Field observation and analysis methods

    2.1 Field observation site and sampling methods

    Beijing is surrounded by mountains in the west, north andeast, which blocks pollutants from spreading. The open aircorridor in the south exposes the capital to air masses thatare passing Hebei Province (Fig. S1 in the Supplement),a heavily polluted area in China. To investigate the im-pact of regional transport on Beijing, a rural site (Liulihesite, 1162 E, 3936 N) southwest of Beijing was chosen.It was located on the border of Beijing and Hebei Province(Fig. S1).

    The field campaign was conducted from 27 October to12 November 2014, including both ground-level and verti-cal observations. All the times discussed in this article areLocal Time. Detailed information of instruments at the Li-ulihe site is provided in Table S1. Ground-level observa-tions included meteorological parameters, mass concentra-tion of PM2.5/PM10, SO2, NOx and O3 as well as physicaland chemical properties of PM. PM2.5PM10 mass concen-tration was determined by the tapered element oscillat

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