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Gaseous and particulate pollutants in Lhasa, Tibet during 2013–2017: Spatial variability, temporal variations and implications
Environmental Pollution 253 (2019) 68e77
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Gaseous and particulate pollutants in Lhasa, Tibet during 2013e2017: Spatial variability, temporal variations and implications* Xiufeng Yin a, b, c, d, Benjamin de Foy d, Kunpeng Wu a, e, Chuan Feng d, Shichang Kang a, f, **, Qianggong Zhang b, f, * a
State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Science, Lanzhou, 730000, China Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China c University of Chinese Academy of Sciences, Beijing, 100039, China d Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, MO, 63108, USA e Institute of International Rivers and Eco-Security, Yunnan University, Kunming, 650091, China f CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing, 100101, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2019 Received in revised form 20 June 2019 Accepted 27 June 2019 Available online 5 July 2019
In recent decades, most big cities in China have experienced severe air pollution accompanied by rapid economic and social development. Analysis of measurements of air pollutants form a fundamental basis for understanding the characteristics of air pollution and are important references for policy-making. In this study, five-year measurements of air pollutants at 6 sites in Lhasa, a typical high altitude big city in southwestern China, were analyzed from January 2013 to December 2017. Air pollutants at all the 6 sites in Lhasa generally displayed similar patterns of both diurnal and monthly variations, indicating the mixed atmospheric environment and the overall effect of the meteorological conditions in the city. Spatially, the air pollutant concentrations at the 6 sites were generally characterized by high concentrations of SO2, NO2, CO, PM10 and PM2.5 at urban sites and high O3 concentrations at suburban sites. In comparison with other provincial capital cities in China, Lhasa has low concentrations of air pollutants, except for O3, and thus, better air quality. Although Lhasa has experienced rapid urbanization and economic development, air pollution conditions have remained rather stable and even decreased slightly in term of particular air pollutants. We suggested that the relatively isolated location, low air pollutant emissions associated with its industrial structure and renewable energy consumption, and effective air pollution control measures, collectively contributed to the synchronous improvement of the economy and air quality in Lhasa. Such “Lhasa pattern” may serve as a positive example for other regional hub cities in China and beyond that experience socioeconomic development and simultaneously seek to improve air quality. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Air pollution poses serious threats to human health and
* This Paper has been recommended for acceptance by Xiaoping Wang. * Corresponding author. Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China. ** Corresponding author. State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Science, Lanzhou, 730000, China. E-mail addresses: [email protected] (S. Kang), qianggong.zhang@itpcas. ac.cn (Q. Zhang).
https://doi.org/10.1016/j.envpol.2019.06.113 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
ecosystems and influences climate change (Brunekreef and Holgate, 2002; Zhao et al., 2018). With rapid economic expansion, industrialization and urbanization over the past several decades, China has increasingly faced extremely severe air pollution (Wang and Hao, 2012; Hao et al., 2007; He et al., 2002; Chan and Yao, 2008; Kan et al., 2012), which is one of the foremost environmental and social problems in China. Similar issues were observed in developed countries in Europe and the United States in the middle and late 20th century (Katsouyanni et al., 1997; Pope et al., 2009; Katsouyanni et al., 2001; Samet et al., 2000). Due to unbalanced urbanization over its large territory, the air quality and associated influential factors in different regions of China are highly
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inhomogeneous. Many reports and studies of air quality in the developed first-tier cities, mostly in eastern China (e.g., Beijing, Shanghai, and Guangzhou), have been published, and they serve as the scientific basis for evaluation of the atmospheric environment and provide references for policy making (Sun et al., 2004; Streets et al., 2007; Chan and Yao, 2008; Hao and Wang, 2005; Huang et al., 2009; Liu et al., 2007; He et al., 2001; Huang et al., 2014; Zhao et al., 2015). Nonetheless, there are limited studies of air quality in developing areas, primarily in western China and especially in the Tibet Autonomous Region, one of the least developed regions in southwestern China. Due to the small population and low industrial development in the area, the Tibet Autonomous Region has generally been regarded as a pristine region with a clean atmospheric environment (Chen et al., 2015; Cong et al., 2007; Cong et al., 2013; Wan et al., 2016). Lhasa is the largest city and administrative capital of the Tibet Autonomous Region, and it is the cultural and economic center of Tibet. The air quality in Lhasa has been reported for several pollutants. For example, based on ground observations in September 2007 and August 2008, Cong et al. (2011) stated that the annual average concentration of PM10 in Lhasa was lower than that in major Asian cities. A study of black carbon aerosols (Gao et al., 2007) and atmospheric polycyclic aromatic hydrocarbons (Ma et al., 2013) indicated that Lhasa was less impacted by anthropogenic sources than were major Chinese cities such as Beijing and Harbin. Li et al. (2016) found that the daily averages of organic carbon, elemental carbon and water-soluble organic carbon in PM2.5 and PM10 in Lhasa were lower than those in other megacities, indicating that the atmosphere of Lhasa was less polluted. Notably, in recent years, Lhasa has experienced rapid urbanization with accelerated economic development, especially in the tourism industry. The air quality in Lhasa has been increasingly threatened by aerosol pollution episodes (Cui et al., 2018; Duo et al., 2015), and the O3 concentrations are higher than those in most of the provincial capital cities in China (Zhao et al., 2016; Ran et al., 2014). Here, we analyzed the spatial and temporal variations in air quality based on up-to-date datasets of air pollutants at 6 sites in Lhasa covering 5 years from 2013 to 2017. This study presents the air pollutant measurements and the spatial variation of air pollutants in Lhasa to reflect the characteristics of the air pollutants in a typical high-altitude city. Our results highlight the relationship between air quality and socio-economic development in Lhasa and provide a city reference for air pollution control in regional hub cities in China and perhaps other developing countries. 2. Measurements and methods 2.1. Measurement sites Lhasa lies in the center of Tibet at an elevation of approximately 3600 m a.s.l. The city covers an area of approximately 60 km2. The entire city is located in a flat river valley, and the Lhasa River flows through the city from west to east, with surrounding mountain elevations reaching 5500 m a.s.l. Due to the high elevation, Lhasa has a humid continental climate, with very dry, frosty winters and wet, warm summers. Lhasa is called the “sunlit city” due to the strong solar radiation in the area, with ~3000 h of sunlight exposure annually. Air pollutant measurements were collected at 6 statecontrolled air sampling sites in Lhasa (Fig. 1), namely, Qufushezhan (LHASA-QF), Bakuojie (LHASA-BK), Shihuanbaoju (LHASASH), Qujiancezhan (LHASA-QJ), Lasahuochezhan (LHASA-LS), and Xizangdaxue (LHASA-XZ) (Table S1). LHASA-QF is in western Lhasa near several vehicle repair centers, car dealers, a west-east national road (G 109), and a goods distribution center in the center of
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Duilongdeqing District. LHASA-BK is in downtown Chengguan District in Lhasa and is adjacent to a west-east national road (G 318) and Jokhang Temple, the largest temple in Lhasa, with continuous and abundant incense burning for religious activities. LHASA-SH is in the residential-business area of the main city, with several large shopping malls and farmers' markets distributed throughout the area. Moreover, LHASA-SH is adjacent to one of the north-south main roads in the city and is 600 m to the east of the 6.25 km2 Lalu Wetlands National Nature Preserve, also known as the “Lung of Lhasa”. LHASA-QJ is located on the southern edge of the main city and on the northern bank of the Lhasa River. LHASALS is located in the southern suburban area of the city near Lhasa railway station, which is in a new district called Liuwu and is separated from the main city by the Lhasa River. LHASA-XZ is located at a university in the eastern suburban area with limited industry and traffic.
2.2. Air quality and meteorological data SO2 (sulfur dioxide), NO2 (nitrogen dioxide), CO (carbon monoxide), O3 (surface ozone or tropospheric ozone), PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 mm) and PM10 (particulate matter with an aerodynamic diameter less than 10 mm) were measured from January 2013 to December 2017, and hourly mean concentrations of these six air pollutants have been released by the China National Environmental Monitoring Center (http:// 106.37.208.233:20035/). Air pollutant data from 30 additional provincial capital cities in 2017 were also compared with those from Lhasa. The measurement of SO2, NO2, CO, O3, PM10 and PM2.5 was under the control of the state controlling air sampling sites, and the arrangement of sites followed the technical regulation for selection of ambient air quality monitoring stations (National Environmental Protection Standards of the People's Republic of China, HJ 6642013). Instruments information and methods were listed in Table S2. The specifications and test procedures of SO2, NO2, CO and O3 followed the Specifications and Test Procedures for Ambient Air Quality Continuous Automated Monitoring System for SO2, NO2, O3 and CO (National Environmental Protection Standards of the People's Republic of China, HJ 654-2013) and the specifications and test procedures of PM10 and PM2.5 followed the Specifications and Test Procedures for Ambient Air Quality Continuous Automated Monitoring System for PM10 and PM2.5 (National Environmental Protection Standards of the People's Republic of China, HJ 653-2013). The inlet of the instrument was 3e20 m above the ground surface, 1 m higher than the roof of the building or the wall. The stations were located at least 50 m from any obvious stationary pollution sources. The data quality assurance and controls followed technical guidelines on environmental monitoring quality management (National Environmental Protection Standards of the People's Republic of China, HJ 630-2011), and the data were checked for validity based on ambient air quality standards (National Standards of the People's Republic of China, GB 3095-2012). Meteorological data for Lhasa were obtained from the National Climatic Data Center provided by the National Oceanic and Atmospheric Administration (http://www.ncdc.noaa.gov/oa/climate/isd/ index.php), which is one of the largest active weather data archives in the world (Lott et al., 2001; Smith et al., 2011). The meteorology site is located in the center of Lhasa (Table S1). The planetary boundary layer height (PBLH) was obtained from ERA-Interim data (Dee et al., 2011) provided by the European Centre for MediumRange Weather Forecasts (http://apps.ecmwf.int/datasets/). Beijing Time (UTCþ8) was used in this study, and the local solar noon in Lhasa is 14:00 in Beijing Time.
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Fig. 1. The geographical location of the 6 state-controlled air sampling sites in Lhasa, China.
3. Results 3.1. Overview of gaseous and particulate pollutants in Lhasa The mean concentrations of air pollutants at 6 sites in Lhasa based on all measurements were as follows: SO2 ¼ 8.67 ± 10.40 mg m3, NO2 ¼ 20.51 ± 19.55 mg m3, CO ¼ 0.76 ± 0.69 mg m3, O3 ¼ 76.40 ± 39.48 mg m3, PM10 ¼ 56.83 ± 61.38 mg m3 and PM2.5 ¼ 22.74 ± 23.92 mg m3. The concentrations of the six air pollutants displayed wide ranges at 6 sites (Fig. 2). Generally, the urban sites (LHASA-QF, LHASA-BK and LHASA-SH) were mainly characterized by high concentrations of SO2, NO2, CO, PM10 and PM2.5 and the suburban sites (LHASA-QJ, LHASA-LS and LHASA-XZ) were characterized by high O3 concentrations. Among the 6 sites (Table S3), LHASA-XZ exhibited the lowest mean concentrations of SO2 (6.60 ± 5.94 mg m3), NO2 (13.04 ± 12.87 mg m3), CO (0.54 ± 0.48 mg m3) and PM10 (42.54 ± 47.28 mg m3). LHASA-LS displayed the lowest mean concentration of PM2.5 (17.08 ± 15.65 mg m3). LHASA-SH had the lowest mean concentration of O3 (63.33 ± 42.13 mg m3). The highest mean concentration of O3 was observed at LHASA-LS (82.29 ± 38.22 mg m3). LHASA-QF exhibited the highest mean concentrations of SO2 (10.47 ± 18.81 mg m3), PM10 3 (83.60 ± 75.60 mg m ) and PM2.5 (31.70 ± 28.41 mg m3), and LHASA-SH displayed the highest mean concentrations of NO2 (27.38 ± 25.79 mg m3) and CO (0.94 ± 0.98 mg m3). In the five years, concentrations of SO2 (0.51 mg m3 year1), CO (0.1 mg m3 year1), O3 (0.45 mg m3 year1) and PM2.5 (0.09 mg m3 year1) were relatively stable, while NO2 (1.48 mg m3 year1) and PM10 (4.08 mg m3 year1) displayed slight increases (Fig. S1). For the measurements in the past 5 years, when the values at all 6 sites were averaged, PM2.5, PM10, O3, CO and SO2 had low concentrations in Lhasa in 2017, while NO2 had a high concentration in 2017 that was only lower than that in 2016 (Fig. 3). Particulate pollutants (PM10 and PM2.5) in Lhasa increased from 2013 to 2016 but decreased in 2017. Both PM10 and PM2.5 peaked in 2016 and then decreased in 2017 (Table S4, PM10: from 78.34 ± 78.44 mg m3 in 2016 to 54.19 ± 54.59 mg m3 in 2017, and PM2.5: from 27.19 ± 27.32 mg m3 in 2016 to 19.09 ± 21.15 mg m3 in 2017). Similarly, NO2 reached a maximum in 2016 (24.01 ± 21.97 mg m3) and decreased in 2017 (22.45 ± 20.07 mg m3). O3 was stable from 2013 to 2016 (ranging from 76.19 ± 41.35 mg m3 to 77.78 ± 46.82 mg m3) but decreased
Fig. 2. Comparison of mean concentrations of air pollutants from January 2013 to December 2017 at 6 sites in Lhasa (LHASA-QF, BK, SH, QJ, LS and XZ). Error bars refer to one standard deviation.
in 2016 and reached a minimum in 2017 (73.89 ± 34.28 mg m3). CO decreased from 2013 (1.10 ± 0.91 mg m3) to 2016 3 (0.54 ± 0.56 mg m ) but exhibited a slight increase in 2017 (0.66 ± 0.44 mg m3). SO2 reached a maximum concentration in
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season, and the other three sites, namely, LHASA-QJ, LHASA-XZ and LHASA-LS, exhibited maximum concentrations in the warm season. Based on the averages at the 6 sites, SO2 reached a maximum concentration in the winter (Fig. 4). For particulate pollutants, PM10 and PM2.5 reached maximum concentrations in December, with mean concentrations of 96.57 ± 104.64 mg m3 and 40.88 ± 42.88 mg m3, respectively, and their minimum concentrations occurred in July, with mean concentrations of 33.80 ± 35.10 mg m3 and 14.58 ± 13.48 mg m3, respectively. 3.3. Diurnal variations All 6 sites in Lhasa exhibited similar diurnal variations for the 6 measured air pollutants. The diurnal variation patterns in Lhasa were similar in each season (Fig. S2) and characterized by unimodal variations in O3, with a peak in the afternoon, and bimodal variations in SO2, NO2, CO, PM10 and PM2.5, with peaks in the morning and evening. Fig. 6 shows the mean diurnal variations in the air pollutants, temperature, wind speed and PBLH at the 6 sites at all measurement times in Lhasa. The mean concentrations of particulate pollutants (PM10 and PM2.5) and SO2 reached morning peak concentrations from 10:00e11:00, and NO2 and CO reached peak concentrations at 9:00 (Fig. 6); additionally, these 5 air pollutants displayed evening peak concentrations at 22:00 and minimum concentrations between 15:00 and 18:00. The diurnal variation in the O3 concentration was opposite that of the other air pollutants, with high concentrations from 15:00e18:00 and a minimum concentration at 9:00. The diurnal variations in the air pollutants in Lhasa were similar to those in other cities in China (Zhao et al., 2016). 4. Discussion 4.1. Comparison with other cities
Fig. 3. Comparison of yearly mean concentrations of air pollutants at 6 sites in Lhasa from 2013 to 2017. Error bars refer to one standard deviation.
2014 (10.09 ± 8.88 mg m3) and then continually decreased to 7.78 ± 4.46 mg m3 in 2017.
3.2. Monthly variations All 6 sites in Lhasa displayed similar monthly mean variations in all air pollutants except SO2. Considering the 6 sites as a whole (Fig. 4), the concentration of NO2 was high in November and December and reached a maximum concentration in December (mean concentration reaching 34.00 ± 28.06 mg m3). In the remaining months, NO2 concentrations were stable and ranged from 15.52 ± 12.88 mg m3 to 23.71 ± 21.07 mg m3. Similar to NO2, the CO concentration in Lhasa displayed an upward trend from October to December and reached a maximum in December, with the mean concentration at the 6 sites reaching 1.12 ± 0.98 mg m3. Unlike these pollutants with peaks in the winter, O3 peaked in May, with a mean concentration of 108.57 ± 40.43 mg m3, and reached a minimum in December, with a mean concentration of 51.84 ± 30.88 mg m3. For SO2, the LHASA-BK, LHASA-SH and LHASA-QF sites displayed maximum concentrations in the cold
The annual mean concentrations of air pollutants in Lhasa in 2017 were compared with those from 30 other provincial capital cities in China (Fig. 8). The annual mean concentration of SO2 varied from 6.07 ± 4.32 mg m-3 (Haikou) to 51.85 ± 70.65 mg m-3 (Taiyuan) in all the provincial capital cities (Table S5). The SO2 concentration in Lhasa was the third lowest of the extremely low concentrations (7.78 ± 4.46 mg m3) and was only higher than those in Haikou and Fuzhou (6.27 ± 3.78 mg m3). This level met the Class 1 air quality standard (AQS) in China for SO2 (annual mean limit: 20 mg m3) (Table S6). For NO2, Haikou displayed the lowest annual mean concentration (11.51 ± 9.35 mg m3), and Lhasa had the second lowest concentration (22.45 ± 20.07 mg m3), which was much lower than the Class 1 AQS in China (annual mean limit: 40 mg m3). Of all the provincial capital cities, Xi'an displayed the highest yearly mean concentration of NO2 (58.42 ± 31.77 mg m3), which was 36 mg m3 higher than that in Lhasa. Lhasa displayed the lowest concentration of PM2.5 (19.09 ± 21.16 mg m3), which was less than one-quarter of the highest PM2.5 concentration in Shijiazhuang (81.87 ± 77.51 mg m3). No provincial capital city met the Class 1 Chinese AQS for PM2.5 (annual mean limit: 15 mg m3) in 2017. The PM2.5 concentration in Lhasa was much lower than the Class 2 AQS (annual mean limit: 35 mg m3). Moreover, the PM10 concentration in Lhasa (54.19 ± 54.62 mg m3) ranged between Class 1 (annual mean limit: 40 mg m3) and Class 2 (annual mean limit: 70 mg m3). Haikou had the lowest concentration of PM10 (37.38 ± 21.01 mg m3) and was the only provincial capital city that met the Class 1 AQS for PM10. Shijiazhuang displayed the highest concentration of PM10 (155.04 ± 120.63 mg m3), which was twice as high as the Class 2 standard. The annual mean concentration of
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Fig. 4. Monthly mean variations of mean concentrations of air pollutants at 6 sites, and temperature, wind speed and average of daily maximum PBLH (planetary boundary layer heights) in Lhasa during the entire measurements. Error bars refer to standard deviation. Temperature and wind speed data were obtained from the National Climatic Data Center provided by the National Oceanic and Atmospheric Administration (NOAA, 2018), and PBLH was obtained from ERA-Interim (Dee et al., 2011) provided by the European Centre for Medium-Range Weather Forecasts (ECMWF, 2018).
CO in Lhasa (0.66 ± 0.44 mg m3) was only less than that in Haikou (0.62 ± 0.18 mg m3) among all the cities, and Xi'an had the highest CO concentration (1.5 ± 0.79 mg m3). The Class 1 limit for the hourly mean CO concentration is 10 mg m3, and all hourly mean CO concentrations in Lhasa were below this limit in 2017. In terms of the five air pollutants discussed above, Lhasa ranked as one of the top three cities with the best air quality, behind only two coastal cities (i.e., Haikou and Fuzhou). However, it should be noted that Lhasa had the second highest annual mean O3 concentration (73.89 ± 34.23 mg m3), which was close to the highest O3 concentration in Shanghai (78.6 ± 48.48 mg m3). The daily 8-h maximum O3 concentrations in Lhasa were higher than 100 mg m3 (the Class 1 limit for the O3 concentration in the Chinese AQS) for 142 days (39% of 365 days) and higher than 160 mg m3 (the Class 2 limit for the O3 concentration in the Chinese AQS) for 3 days (1% of 365 days). Compared with other cities in the Tibetan Plateau (Shigatse, Ngari, Nagchu, Qamdo and Nyingchi) (Chen et al., 2019), the concentrations of almost all air pollutants in Lhasa were high, indicating that air pollutants in these cities in the Tibetan Plateau generally associated with the population and gross domestic product (GDP) of the city. In addition, compared to Mexico, a highaltitude and considerably polluted city in North America (RiveraGonz alez et al., 2015), Lhasa had a comparable concentration of PM10, and lower concentrations of SO2, NO2, CO and PM2.5. It should be highlighted that the concentration of O3 in Lhasa was comparable and even higher than those in Mexico. Extremely serious pollution of O3 in Lhasa is believed to be caused by strong photochemical production (Ran et al., 2014) and stratospheretroposphere exchange (Yin et al., 2017).
4.2. Seasonal variations of air pollutants in Lhasa Lhasa displayed elevated concentrations of NO2 and CO in the winter, and this result is related to the combined effects of primary emissions from domestic heating (Ran et al., 2014), weak photochemical reactions and adverse diffusion conditions (Chai et al., 2014; Zhao et al., 2015). SO2 has relatively low concentrations throughout the year at the 6 sites, likely caused by the limited emission sources in Lhasa such as residential coal-fired heating, incense burning and vehicular exhaust (Ran et al., 2014). For all monthly mean PM2.5/PM10 ratios in Lhasa (Fig. 5), they were lower than the average ratio of 31 Chinese provincial capital cities during 2014e2015 (0.58) (He et al., 2017), illustrating the fact that Lhasa is easily affected by dust events. The monthly mean PM2.5/PM10 ratios showed lower values in March, April and October, and the fugitive dust was the main cause of low PM2.5/PM10 ratios due to the low precipitation (Wang et al., 2015a,b) in Lhasa. O3 reached a maximum concentration in the spring in Lhasa, and this finding is similar to the results observed in Nam Co and Dangxiong (Yin et al., 2017; Lin et al., 2015). The maximum of O3 in Lhasa is earlier than those in most of the other capital cities in China (Zhao et al., 2016). This trend is likely due to the large-scale background of O3 in the spring (Monks, 2000). Stratosphere-troposphere exchange is an important factor contributing to the high concentration of O3 in the Tibetan Plateau and stratosphere-troposphere exchange in the spring is stronger than during other seasons in the central Tibetan Plateau (Yin et al., 2017), which can induce more O3 to surface in the spring leading the maximum O3 in the spring. Furthermore, under the influence of the Indian Monsoon, most precipitation occurs in the summer and leads to reduced photochemistry, which may remove O3 and its precursors in Lhasa, thereby decreasing O3 in the
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found between them: CO and NO2 (r ¼ 0.95), CO and SO2 (r ¼ 0.86), NO2 and SO2 (r ¼ 0.76), suggesting relatively identical sources for these gaseous pollutants. Moreover, gaseous pollutants (CO, SO2 and NO2) displayed negative correlations with temperature, PBLH and wind speed. The diurnal peaks of SO2, NO2, CO, PM10, and PM2.5 in the morning and evening were mainly attributed to traffic emissions during the rush hour periods and depressed air in the boundary layer (Fig. 6). The decrease in the afternoon is closely related to the increasing height of the boundary layer and wind speed, which result in the dilution of pollutants and photochemical reactions involving O3. With the decrease of NO2 and CO in the afternoon, the concentration of O3 was increased and it was suggested that the maximum O3 concentration in the afternoon was mainly due to photochemical reaction under intense solar radiation conditions, leading the consumption of NO2 and CO. 4.4. Wind-dependent pollutants at the 6 sites in Lhasa city
Fig. 5. Monthly mean PM2.5/PM10 ratios in Lhasa. Error bars refer to 95% confidence intervals.
summer (Ma et al., 2014). 4.3. Diurnal variations of air pollutants in Lhasa Hourly mean concentrations of two particulate pollutants (PM10 and PM2.5) had a strong positive correlation (r ¼ 0.96). For the three gaseous pollutants (CO, SO2 and NO2), positive correlations were
Affected by the valley setting, the prevailing winds in Lhasa are west winds and east winds (Fig. S3). Fig. 7 shows that the pollutant roses at the 6 sites in Lhasa based on all measurements. In contrast to other pollutants, surface O3 is a secondary pollutant that is produced by photochemical reactions involving nitrogen oxides (NOx), and volatile organic compounds (VOCs). The pollutant rose of O3, therefore, reflects afternoon winds and will not be discussed in this study. For LHASA-QF in the western part of the city, the concentrations of almost all air pollutants were high when an east wind was observed at the site. For LHASA-XZ in the east, the concentrations of SO2, CO, and NO2 were higher for a west wind than those for an east wind. LHASA-LS, another suburban site in the southern part of the
Fig. 6. Daily variations of mean concentrations of air pollutants at 6 sites, temperature, wind speed and PBLH in Lhasa during the entire measurements (Beijing Time, UTCþ8). Error bars refer to standard deviation. Temperature and wind speed data were obtained from the National Climatic Data Center provided by the National Oceanic and Atmospheric Administration (NOAA, 2018), and PBLH was obtained from ERA-Interim (Dee et al., 2011) provided by the European Centre for Medium-Range Weather Forecasts (ECMWF, 2018).
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Fig. 7. Pollutant roses of SO2, NO2, CO, PM10 and PM2.5 at 6 sites in Lhasa.
city, exhibited high concentrations of NO2, CO and PM2.5 associated with northeast winds. LHASA-QJ, which is on the edge of the main city, displayed increases in pollutants that were similar to those at LHASA-LS. The pollutants at these sites displayed higher concentrations when the wind blew from the downtown area compared to that from the suburban area, and this finding reflects the impact of pollutant emissions from the main city compared to those from suburban areas. At LHASA-SH, the lowest concentrations of all air pollutants were clearly associated with a west wind from the Lalu Wetlands National Nature Preserve, which serves as air filter in the city. For particulate pollutants, high concentrations of PM10 were associated with high wind speeds, indicating the effect of dust when the wind speed is high. High concentrations of PM2.5 were associated with low wind speeds, indicating the limited diffusion of the anthropogenic emissions of PM2.5 under stagnant atmospheric conditions. Similar correlations between wind speed and particulate pollutants were also observed at other sites in Lhasa. The pollutants increased at LHASA-BK, with high concentrations of SO2 and PM2.5 associated with north and east winds. The other air
pollutants in LHASA-BK exhibited no obvious directionality. Overall, the wind direction clearly impacted the air pollutants at the sites in Lhasa. Wind from urban areas contributed to high concentrations of air pollutants at suburban sites due to the high emissions in urban areas in Lhasa. The Lalu Wetlands National Nature Preserve has a positive effect on the air quality in Lhasa. 4.5. Air pollution versus economic growth: Lhasa pattern and implications The air quality in a city is influenced by complex factors such as the climatic and environmental conditions, industrial structure, energy consumption, and air pollution control policies/measures, etc. Most of the major cities, especially those in the North China Plain, have suffered from serious air pollution in recent decades due to rapid economic growth and urbanization with industrydominated development and a lack of atmospheric environmental protection (Li et al., 2016). Lhasa achieved rapid economic and population growth while
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Fig. 8. Comparison of annual mean concentrations of air pollutants in provincial capital cities in China in 2017.
maintaining good air quality. This trend differs from those in most other Chinese provincial capitals. Lhasa's GDP in 2017 was 25 times higher than that of 2001, and the population had increased by more than 130 000 in 16 years (Fig. 9). Meanwhile, a comparison
between measurements of SO2, NO2 and PM10 from 2001 to 2006 (Ga, 2009) and the measurements from 2013 to 2017 suggests that these three air pollutants remained low and stable over a decade in Lhasa (Fig. 9). The Lhasa pattern is distinctly different from that of
Fig. 9. Variation of air pollutants, GDP (gross domestic product) and population in Lhasa from 2001 to 2017. Air pollutants data from 2001 to 2006 were obtained from Ga (2009). GDP and population Source: National Bureau of Statistics of China (2018).
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major cities in eastern China, where deteriorating air quality is often associated with economic growth (e.g., Liu et al., 2018; Zhao et al., 2018). We suggested the following reasons. Firstly, Lhasa is isolated and far away from the heavily-polluted regions and it is located in the upwind of eastern China where serious air pollution could hardly reach and impact it. Air pollution from South Asia may impact Lhasa but is mostly long-range transport episodic events. Secondly, the tertiary industry is the major sector in the Lhasa's industrial structure, and contributed to 56.8% of the GDP in 2017, which is higher than that of China as a whole (51.9%) (Fig. S4). As a result, Lhasa consumes less energy and emits fewer air pollutants. Furthermore, different from most other cities in China that mainly rely on coal as energy generation (Wang et al., 2015a,b), Tibet has a high portion of renewable energy in the total energy consumption. For example, Tibet has abundant water resources (water resources totaled 475 billion m3) and has developed many hydropower stations as its major source of power generation (Fig. S5). The proportion of hydro energy reached 96.4% of all power generation in 2015 in Tibet, generating limited emission of air pollutants. Finally, the critical governmental air pollution control policies and measures at both the local and regional levels (Wang and Hao, 2012) contributed to the improvement of air quality in Lhasa. The Action Plan for the Prevention and Control of Air Pollution was issued by the State Council of China in 2013 to guide national efforts in preventing and controlling air pollution. Specific implementation rules were immediately formulated by the government of the Tibet Autonomous Region and have been strictly implemented since 2014 (http://www.xizang.gov.cn), and specific rules include rectifying small coal-fired boilers, controlling flying dust and cooking fume pollution from catering services, eliminating yellow-label vehicles and old vehicles, promoting public transport and newenergy vehicles, and providing central heating in the winter, all of which were favorable factors that supported good air quality. Notably, one of the overall goal settings of the Rules for the Implementation of Action Plan on Prevention and Control of Air Pollution in Lhasa is that the average annual concentrations of PM10 and PM2.5 in Lhasa in 2017 should not exceed those in 2013. The average annual concentration of PM2.5 in 2017 in Lhasa was 19.09 mg m3, lower than that in 2013 (21.16 mg m3), indicating that the goal was narrowly attained. However, the average annual concentration of PM10 in 2017 (54.19 mg m3) was higher than that in 2013 (46.58 mg m3). Furthermore, the concentration of O3 in Lhasa was ranked as one of the highest in China. Such high level was probably associated with the natural conditions and processes (e.g., strong solar radiation and strong stratosphere-troposphere ozone exchange process) in Lhasa. Special attention should be paid to PM10 and O3 to improve Lhasa's air quality in the future. Setting an appropriate and effective timeline and roadmap is key to improve air quality for Lhasa as well as for other cities. ScienceBased policy measures, with full consideration of the characteristics and trends of air pollution, should also be enacted for Lhasa. Additionally, more technical capacity and resources should be invested to sustain Lhasa's air quality. Overall, owing to its unique geographic location, limited air pollutant emissions induced from a low energy-consuming industry that utilizes a high portion of clean energy, and effective air pollution control measures, Lhasa is one of the cities in China with the best air quality, and achieved the synchronous improvement of economy and air quality in the last decade, such “Lhasa pattern” may serve as a positive example for other regional hub cities that experience rapid socioeconomic development and simultaneously seek to improve air quality.
spatial variations in gaseous and particulate pollutants over 5 years at 6 sites in Lhasa. Generally, the urban sites had higher concentrations of SO2, NO2, CO, PM10, and PM2.5 compared to those of the suburban sites, and the suburban sites had higher O3 concentrations. Elevated concentrations of air pollutants at suburban sites were associated with winds from urban areas, and this trend reflects the impact of emissions in urban areas. Air pollutant concentrations, including those of SO2, NO2, CO, PM10, and PM2.5, were significantly higher in the winter than in other seasons due to the combined effects of primary emissions from domestic heating, weak photochemical reactions and adverse diffusion conditions. However, the O3 concentration displayed a maximum in the late spring and early summer in Lhasa due to photochemical production and stratosphere-troposphere exchange. For the diurnal variations, SO2, NO2, CO, PM10, and PM2.5 exhibited bimodal patterns with peaks during the morning and evening rush hours due to the depressed air in the boundary layer. Unlike the other air pollutants, the diurnal variations in O3 displayed a unimodal pattern with a peak in the afternoon due to photochemical reactions and intense solar radiation. A comparison of all Chinese provincial capital cities showed that Lhasa was one of the cities with the best air quality in China based on the SO2, NO2, CO, PM10 and PM2.5 concentrations in 2017. Over the past decade, Lhasa achieved rapid economic and population growth while maintaining good air quality. This trend differs from that in most other Chinese provincial capitals. The unique geographical location, low emission of air pollutants associated with its industrial structure and energy consumption, and effective air pollution control measures contributed to the good air quality in Lhasa. The “Lhasa pattern” of synchronous improvement of the economy and air quality may serve as a positive example for other regional hub cities that experience rapid socioeconomic development and simultaneously seek to improve air quality. Conflicts of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Gaseous and particulate pollutants in Lhasa, Tibet during 2013e2017: spatial variability, temporal variations and implications”. Acknowledgments This study was supported by the Strategic Priority Research Program (A) of the Chinese Academy of Sciences (XDA20040501), the National Natural Science Foundation of China (41630754, 41630748) and State Key Laboratory of Cryospheric Science (SKLCSZZ-2019 and SKLCS-OP-2019-07). X. F. Yin acknowledges the China Scholarship Council. Q. G. Zhang acknowledges financial support from the Youth Innovation Promotion Association of CAS (2016070). We acknowledge the China National Environmental Monitoring Center, the National Oceanic and Atmospheric Administration, the European Centre for Medium-Range Weather Forecasts and the National Bureau of Statistics of China for providing the data. Appendix A. Supplementary data
5. Conclusions We performed a comprehensive analysis of the temporal and
Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.06.113.
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Gaseous and particulate pollutants in Lhasa, Tibet during 2013e2017: Spatial variability, temporal variations and implications* Xiufeng Yin a, b, c, d, Benjamin de Foy d, Kunpeng Wu a, e, Chuan Feng d, Shichang Kang a, f, **, Qianggong Zhang b, f, * a
State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Science, Lanzhou, 730000, China Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China c University of Chinese Academy of Sciences, Beijing, 100039, China d Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, MO, 63108, USA e Institute of International Rivers and Eco-Security, Yunnan University, Kunming, 650091, China f CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing, 100101, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2019 Received in revised form 20 June 2019 Accepted 27 June 2019 Available online 5 July 2019
In recent decades, most big cities in China have experienced severe air pollution accompanied by rapid economic and social development. Analysis of measurements of air pollutants form a fundamental basis for understanding the characteristics of air pollution and are important references for policy-making. In this study, five-year measurements of air pollutants at 6 sites in Lhasa, a typical high altitude big city in southwestern China, were analyzed from January 2013 to December 2017. Air pollutants at all the 6 sites in Lhasa generally displayed similar patterns of both diurnal and monthly variations, indicating the mixed atmospheric environment and the overall effect of the meteorological conditions in the city. Spatially, the air pollutant concentrations at the 6 sites were generally characterized by high concentrations of SO2, NO2, CO, PM10 and PM2.5 at urban sites and high O3 concentrations at suburban sites. In comparison with other provincial capital cities in China, Lhasa has low concentrations of air pollutants, except for O3, and thus, better air quality. Although Lhasa has experienced rapid urbanization and economic development, air pollution conditions have remained rather stable and even decreased slightly in term of particular air pollutants. We suggested that the relatively isolated location, low air pollutant emissions associated with its industrial structure and renewable energy consumption, and effective air pollution control measures, collectively contributed to the synchronous improvement of the economy and air quality in Lhasa. Such “Lhasa pattern” may serve as a positive example for other regional hub cities in China and beyond that experience socioeconomic development and simultaneously seek to improve air quality. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Air pollution poses serious threats to human health and
* This Paper has been recommended for acceptance by Xiaoping Wang. * Corresponding author. Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China. ** Corresponding author. State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Science, Lanzhou, 730000, China. E-mail addresses: [email protected] (S. Kang), qianggong.zhang@itpcas. ac.cn (Q. Zhang).
https://doi.org/10.1016/j.envpol.2019.06.113 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
ecosystems and influences climate change (Brunekreef and Holgate, 2002; Zhao et al., 2018). With rapid economic expansion, industrialization and urbanization over the past several decades, China has increasingly faced extremely severe air pollution (Wang and Hao, 2012; Hao et al., 2007; He et al., 2002; Chan and Yao, 2008; Kan et al., 2012), which is one of the foremost environmental and social problems in China. Similar issues were observed in developed countries in Europe and the United States in the middle and late 20th century (Katsouyanni et al., 1997; Pope et al., 2009; Katsouyanni et al., 2001; Samet et al., 2000). Due to unbalanced urbanization over its large territory, the air quality and associated influential factors in different regions of China are highly
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inhomogeneous. Many reports and studies of air quality in the developed first-tier cities, mostly in eastern China (e.g., Beijing, Shanghai, and Guangzhou), have been published, and they serve as the scientific basis for evaluation of the atmospheric environment and provide references for policy making (Sun et al., 2004; Streets et al., 2007; Chan and Yao, 2008; Hao and Wang, 2005; Huang et al., 2009; Liu et al., 2007; He et al., 2001; Huang et al., 2014; Zhao et al., 2015). Nonetheless, there are limited studies of air quality in developing areas, primarily in western China and especially in the Tibet Autonomous Region, one of the least developed regions in southwestern China. Due to the small population and low industrial development in the area, the Tibet Autonomous Region has generally been regarded as a pristine region with a clean atmospheric environment (Chen et al., 2015; Cong et al., 2007; Cong et al., 2013; Wan et al., 2016). Lhasa is the largest city and administrative capital of the Tibet Autonomous Region, and it is the cultural and economic center of Tibet. The air quality in Lhasa has been reported for several pollutants. For example, based on ground observations in September 2007 and August 2008, Cong et al. (2011) stated that the annual average concentration of PM10 in Lhasa was lower than that in major Asian cities. A study of black carbon aerosols (Gao et al., 2007) and atmospheric polycyclic aromatic hydrocarbons (Ma et al., 2013) indicated that Lhasa was less impacted by anthropogenic sources than were major Chinese cities such as Beijing and Harbin. Li et al. (2016) found that the daily averages of organic carbon, elemental carbon and water-soluble organic carbon in PM2.5 and PM10 in Lhasa were lower than those in other megacities, indicating that the atmosphere of Lhasa was less polluted. Notably, in recent years, Lhasa has experienced rapid urbanization with accelerated economic development, especially in the tourism industry. The air quality in Lhasa has been increasingly threatened by aerosol pollution episodes (Cui et al., 2018; Duo et al., 2015), and the O3 concentrations are higher than those in most of the provincial capital cities in China (Zhao et al., 2016; Ran et al., 2014). Here, we analyzed the spatial and temporal variations in air quality based on up-to-date datasets of air pollutants at 6 sites in Lhasa covering 5 years from 2013 to 2017. This study presents the air pollutant measurements and the spatial variation of air pollutants in Lhasa to reflect the characteristics of the air pollutants in a typical high-altitude city. Our results highlight the relationship between air quality and socio-economic development in Lhasa and provide a city reference for air pollution control in regional hub cities in China and perhaps other developing countries. 2. Measurements and methods 2.1. Measurement sites Lhasa lies in the center of Tibet at an elevation of approximately 3600 m a.s.l. The city covers an area of approximately 60 km2. The entire city is located in a flat river valley, and the Lhasa River flows through the city from west to east, with surrounding mountain elevations reaching 5500 m a.s.l. Due to the high elevation, Lhasa has a humid continental climate, with very dry, frosty winters and wet, warm summers. Lhasa is called the “sunlit city” due to the strong solar radiation in the area, with ~3000 h of sunlight exposure annually. Air pollutant measurements were collected at 6 statecontrolled air sampling sites in Lhasa (Fig. 1), namely, Qufushezhan (LHASA-QF), Bakuojie (LHASA-BK), Shihuanbaoju (LHASASH), Qujiancezhan (LHASA-QJ), Lasahuochezhan (LHASA-LS), and Xizangdaxue (LHASA-XZ) (Table S1). LHASA-QF is in western Lhasa near several vehicle repair centers, car dealers, a west-east national road (G 109), and a goods distribution center in the center of
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Duilongdeqing District. LHASA-BK is in downtown Chengguan District in Lhasa and is adjacent to a west-east national road (G 318) and Jokhang Temple, the largest temple in Lhasa, with continuous and abundant incense burning for religious activities. LHASA-SH is in the residential-business area of the main city, with several large shopping malls and farmers' markets distributed throughout the area. Moreover, LHASA-SH is adjacent to one of the north-south main roads in the city and is 600 m to the east of the 6.25 km2 Lalu Wetlands National Nature Preserve, also known as the “Lung of Lhasa”. LHASA-QJ is located on the southern edge of the main city and on the northern bank of the Lhasa River. LHASALS is located in the southern suburban area of the city near Lhasa railway station, which is in a new district called Liuwu and is separated from the main city by the Lhasa River. LHASA-XZ is located at a university in the eastern suburban area with limited industry and traffic.
2.2. Air quality and meteorological data SO2 (sulfur dioxide), NO2 (nitrogen dioxide), CO (carbon monoxide), O3 (surface ozone or tropospheric ozone), PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 mm) and PM10 (particulate matter with an aerodynamic diameter less than 10 mm) were measured from January 2013 to December 2017, and hourly mean concentrations of these six air pollutants have been released by the China National Environmental Monitoring Center (http:// 106.37.208.233:20035/). Air pollutant data from 30 additional provincial capital cities in 2017 were also compared with those from Lhasa. The measurement of SO2, NO2, CO, O3, PM10 and PM2.5 was under the control of the state controlling air sampling sites, and the arrangement of sites followed the technical regulation for selection of ambient air quality monitoring stations (National Environmental Protection Standards of the People's Republic of China, HJ 6642013). Instruments information and methods were listed in Table S2. The specifications and test procedures of SO2, NO2, CO and O3 followed the Specifications and Test Procedures for Ambient Air Quality Continuous Automated Monitoring System for SO2, NO2, O3 and CO (National Environmental Protection Standards of the People's Republic of China, HJ 654-2013) and the specifications and test procedures of PM10 and PM2.5 followed the Specifications and Test Procedures for Ambient Air Quality Continuous Automated Monitoring System for PM10 and PM2.5 (National Environmental Protection Standards of the People's Republic of China, HJ 653-2013). The inlet of the instrument was 3e20 m above the ground surface, 1 m higher than the roof of the building or the wall. The stations were located at least 50 m from any obvious stationary pollution sources. The data quality assurance and controls followed technical guidelines on environmental monitoring quality management (National Environmental Protection Standards of the People's Republic of China, HJ 630-2011), and the data were checked for validity based on ambient air quality standards (National Standards of the People's Republic of China, GB 3095-2012). Meteorological data for Lhasa were obtained from the National Climatic Data Center provided by the National Oceanic and Atmospheric Administration (http://www.ncdc.noaa.gov/oa/climate/isd/ index.php), which is one of the largest active weather data archives in the world (Lott et al., 2001; Smith et al., 2011). The meteorology site is located in the center of Lhasa (Table S1). The planetary boundary layer height (PBLH) was obtained from ERA-Interim data (Dee et al., 2011) provided by the European Centre for MediumRange Weather Forecasts (http://apps.ecmwf.int/datasets/). Beijing Time (UTCþ8) was used in this study, and the local solar noon in Lhasa is 14:00 in Beijing Time.
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Fig. 1. The geographical location of the 6 state-controlled air sampling sites in Lhasa, China.
3. Results 3.1. Overview of gaseous and particulate pollutants in Lhasa The mean concentrations of air pollutants at 6 sites in Lhasa based on all measurements were as follows: SO2 ¼ 8.67 ± 10.40 mg m3, NO2 ¼ 20.51 ± 19.55 mg m3, CO ¼ 0.76 ± 0.69 mg m3, O3 ¼ 76.40 ± 39.48 mg m3, PM10 ¼ 56.83 ± 61.38 mg m3 and PM2.5 ¼ 22.74 ± 23.92 mg m3. The concentrations of the six air pollutants displayed wide ranges at 6 sites (Fig. 2). Generally, the urban sites (LHASA-QF, LHASA-BK and LHASA-SH) were mainly characterized by high concentrations of SO2, NO2, CO, PM10 and PM2.5 and the suburban sites (LHASA-QJ, LHASA-LS and LHASA-XZ) were characterized by high O3 concentrations. Among the 6 sites (Table S3), LHASA-XZ exhibited the lowest mean concentrations of SO2 (6.60 ± 5.94 mg m3), NO2 (13.04 ± 12.87 mg m3), CO (0.54 ± 0.48 mg m3) and PM10 (42.54 ± 47.28 mg m3). LHASA-LS displayed the lowest mean concentration of PM2.5 (17.08 ± 15.65 mg m3). LHASA-SH had the lowest mean concentration of O3 (63.33 ± 42.13 mg m3). The highest mean concentration of O3 was observed at LHASA-LS (82.29 ± 38.22 mg m3). LHASA-QF exhibited the highest mean concentrations of SO2 (10.47 ± 18.81 mg m3), PM10 3 (83.60 ± 75.60 mg m ) and PM2.5 (31.70 ± 28.41 mg m3), and LHASA-SH displayed the highest mean concentrations of NO2 (27.38 ± 25.79 mg m3) and CO (0.94 ± 0.98 mg m3). In the five years, concentrations of SO2 (0.51 mg m3 year1), CO (0.1 mg m3 year1), O3 (0.45 mg m3 year1) and PM2.5 (0.09 mg m3 year1) were relatively stable, while NO2 (1.48 mg m3 year1) and PM10 (4.08 mg m3 year1) displayed slight increases (Fig. S1). For the measurements in the past 5 years, when the values at all 6 sites were averaged, PM2.5, PM10, O3, CO and SO2 had low concentrations in Lhasa in 2017, while NO2 had a high concentration in 2017 that was only lower than that in 2016 (Fig. 3). Particulate pollutants (PM10 and PM2.5) in Lhasa increased from 2013 to 2016 but decreased in 2017. Both PM10 and PM2.5 peaked in 2016 and then decreased in 2017 (Table S4, PM10: from 78.34 ± 78.44 mg m3 in 2016 to 54.19 ± 54.59 mg m3 in 2017, and PM2.5: from 27.19 ± 27.32 mg m3 in 2016 to 19.09 ± 21.15 mg m3 in 2017). Similarly, NO2 reached a maximum in 2016 (24.01 ± 21.97 mg m3) and decreased in 2017 (22.45 ± 20.07 mg m3). O3 was stable from 2013 to 2016 (ranging from 76.19 ± 41.35 mg m3 to 77.78 ± 46.82 mg m3) but decreased
Fig. 2. Comparison of mean concentrations of air pollutants from January 2013 to December 2017 at 6 sites in Lhasa (LHASA-QF, BK, SH, QJ, LS and XZ). Error bars refer to one standard deviation.
in 2016 and reached a minimum in 2017 (73.89 ± 34.28 mg m3). CO decreased from 2013 (1.10 ± 0.91 mg m3) to 2016 3 (0.54 ± 0.56 mg m ) but exhibited a slight increase in 2017 (0.66 ± 0.44 mg m3). SO2 reached a maximum concentration in
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season, and the other three sites, namely, LHASA-QJ, LHASA-XZ and LHASA-LS, exhibited maximum concentrations in the warm season. Based on the averages at the 6 sites, SO2 reached a maximum concentration in the winter (Fig. 4). For particulate pollutants, PM10 and PM2.5 reached maximum concentrations in December, with mean concentrations of 96.57 ± 104.64 mg m3 and 40.88 ± 42.88 mg m3, respectively, and their minimum concentrations occurred in July, with mean concentrations of 33.80 ± 35.10 mg m3 and 14.58 ± 13.48 mg m3, respectively. 3.3. Diurnal variations All 6 sites in Lhasa exhibited similar diurnal variations for the 6 measured air pollutants. The diurnal variation patterns in Lhasa were similar in each season (Fig. S2) and characterized by unimodal variations in O3, with a peak in the afternoon, and bimodal variations in SO2, NO2, CO, PM10 and PM2.5, with peaks in the morning and evening. Fig. 6 shows the mean diurnal variations in the air pollutants, temperature, wind speed and PBLH at the 6 sites at all measurement times in Lhasa. The mean concentrations of particulate pollutants (PM10 and PM2.5) and SO2 reached morning peak concentrations from 10:00e11:00, and NO2 and CO reached peak concentrations at 9:00 (Fig. 6); additionally, these 5 air pollutants displayed evening peak concentrations at 22:00 and minimum concentrations between 15:00 and 18:00. The diurnal variation in the O3 concentration was opposite that of the other air pollutants, with high concentrations from 15:00e18:00 and a minimum concentration at 9:00. The diurnal variations in the air pollutants in Lhasa were similar to those in other cities in China (Zhao et al., 2016). 4. Discussion 4.1. Comparison with other cities
Fig. 3. Comparison of yearly mean concentrations of air pollutants at 6 sites in Lhasa from 2013 to 2017. Error bars refer to one standard deviation.
2014 (10.09 ± 8.88 mg m3) and then continually decreased to 7.78 ± 4.46 mg m3 in 2017.
3.2. Monthly variations All 6 sites in Lhasa displayed similar monthly mean variations in all air pollutants except SO2. Considering the 6 sites as a whole (Fig. 4), the concentration of NO2 was high in November and December and reached a maximum concentration in December (mean concentration reaching 34.00 ± 28.06 mg m3). In the remaining months, NO2 concentrations were stable and ranged from 15.52 ± 12.88 mg m3 to 23.71 ± 21.07 mg m3. Similar to NO2, the CO concentration in Lhasa displayed an upward trend from October to December and reached a maximum in December, with the mean concentration at the 6 sites reaching 1.12 ± 0.98 mg m3. Unlike these pollutants with peaks in the winter, O3 peaked in May, with a mean concentration of 108.57 ± 40.43 mg m3, and reached a minimum in December, with a mean concentration of 51.84 ± 30.88 mg m3. For SO2, the LHASA-BK, LHASA-SH and LHASA-QF sites displayed maximum concentrations in the cold
The annual mean concentrations of air pollutants in Lhasa in 2017 were compared with those from 30 other provincial capital cities in China (Fig. 8). The annual mean concentration of SO2 varied from 6.07 ± 4.32 mg m-3 (Haikou) to 51.85 ± 70.65 mg m-3 (Taiyuan) in all the provincial capital cities (Table S5). The SO2 concentration in Lhasa was the third lowest of the extremely low concentrations (7.78 ± 4.46 mg m3) and was only higher than those in Haikou and Fuzhou (6.27 ± 3.78 mg m3). This level met the Class 1 air quality standard (AQS) in China for SO2 (annual mean limit: 20 mg m3) (Table S6). For NO2, Haikou displayed the lowest annual mean concentration (11.51 ± 9.35 mg m3), and Lhasa had the second lowest concentration (22.45 ± 20.07 mg m3), which was much lower than the Class 1 AQS in China (annual mean limit: 40 mg m3). Of all the provincial capital cities, Xi'an displayed the highest yearly mean concentration of NO2 (58.42 ± 31.77 mg m3), which was 36 mg m3 higher than that in Lhasa. Lhasa displayed the lowest concentration of PM2.5 (19.09 ± 21.16 mg m3), which was less than one-quarter of the highest PM2.5 concentration in Shijiazhuang (81.87 ± 77.51 mg m3). No provincial capital city met the Class 1 Chinese AQS for PM2.5 (annual mean limit: 15 mg m3) in 2017. The PM2.5 concentration in Lhasa was much lower than the Class 2 AQS (annual mean limit: 35 mg m3). Moreover, the PM10 concentration in Lhasa (54.19 ± 54.62 mg m3) ranged between Class 1 (annual mean limit: 40 mg m3) and Class 2 (annual mean limit: 70 mg m3). Haikou had the lowest concentration of PM10 (37.38 ± 21.01 mg m3) and was the only provincial capital city that met the Class 1 AQS for PM10. Shijiazhuang displayed the highest concentration of PM10 (155.04 ± 120.63 mg m3), which was twice as high as the Class 2 standard. The annual mean concentration of
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Fig. 4. Monthly mean variations of mean concentrations of air pollutants at 6 sites, and temperature, wind speed and average of daily maximum PBLH (planetary boundary layer heights) in Lhasa during the entire measurements. Error bars refer to standard deviation. Temperature and wind speed data were obtained from the National Climatic Data Center provided by the National Oceanic and Atmospheric Administration (NOAA, 2018), and PBLH was obtained from ERA-Interim (Dee et al., 2011) provided by the European Centre for Medium-Range Weather Forecasts (ECMWF, 2018).
CO in Lhasa (0.66 ± 0.44 mg m3) was only less than that in Haikou (0.62 ± 0.18 mg m3) among all the cities, and Xi'an had the highest CO concentration (1.5 ± 0.79 mg m3). The Class 1 limit for the hourly mean CO concentration is 10 mg m3, and all hourly mean CO concentrations in Lhasa were below this limit in 2017. In terms of the five air pollutants discussed above, Lhasa ranked as one of the top three cities with the best air quality, behind only two coastal cities (i.e., Haikou and Fuzhou). However, it should be noted that Lhasa had the second highest annual mean O3 concentration (73.89 ± 34.23 mg m3), which was close to the highest O3 concentration in Shanghai (78.6 ± 48.48 mg m3). The daily 8-h maximum O3 concentrations in Lhasa were higher than 100 mg m3 (the Class 1 limit for the O3 concentration in the Chinese AQS) for 142 days (39% of 365 days) and higher than 160 mg m3 (the Class 2 limit for the O3 concentration in the Chinese AQS) for 3 days (1% of 365 days). Compared with other cities in the Tibetan Plateau (Shigatse, Ngari, Nagchu, Qamdo and Nyingchi) (Chen et al., 2019), the concentrations of almost all air pollutants in Lhasa were high, indicating that air pollutants in these cities in the Tibetan Plateau generally associated with the population and gross domestic product (GDP) of the city. In addition, compared to Mexico, a highaltitude and considerably polluted city in North America (RiveraGonz alez et al., 2015), Lhasa had a comparable concentration of PM10, and lower concentrations of SO2, NO2, CO and PM2.5. It should be highlighted that the concentration of O3 in Lhasa was comparable and even higher than those in Mexico. Extremely serious pollution of O3 in Lhasa is believed to be caused by strong photochemical production (Ran et al., 2014) and stratospheretroposphere exchange (Yin et al., 2017).
4.2. Seasonal variations of air pollutants in Lhasa Lhasa displayed elevated concentrations of NO2 and CO in the winter, and this result is related to the combined effects of primary emissions from domestic heating (Ran et al., 2014), weak photochemical reactions and adverse diffusion conditions (Chai et al., 2014; Zhao et al., 2015). SO2 has relatively low concentrations throughout the year at the 6 sites, likely caused by the limited emission sources in Lhasa such as residential coal-fired heating, incense burning and vehicular exhaust (Ran et al., 2014). For all monthly mean PM2.5/PM10 ratios in Lhasa (Fig. 5), they were lower than the average ratio of 31 Chinese provincial capital cities during 2014e2015 (0.58) (He et al., 2017), illustrating the fact that Lhasa is easily affected by dust events. The monthly mean PM2.5/PM10 ratios showed lower values in March, April and October, and the fugitive dust was the main cause of low PM2.5/PM10 ratios due to the low precipitation (Wang et al., 2015a,b) in Lhasa. O3 reached a maximum concentration in the spring in Lhasa, and this finding is similar to the results observed in Nam Co and Dangxiong (Yin et al., 2017; Lin et al., 2015). The maximum of O3 in Lhasa is earlier than those in most of the other capital cities in China (Zhao et al., 2016). This trend is likely due to the large-scale background of O3 in the spring (Monks, 2000). Stratosphere-troposphere exchange is an important factor contributing to the high concentration of O3 in the Tibetan Plateau and stratosphere-troposphere exchange in the spring is stronger than during other seasons in the central Tibetan Plateau (Yin et al., 2017), which can induce more O3 to surface in the spring leading the maximum O3 in the spring. Furthermore, under the influence of the Indian Monsoon, most precipitation occurs in the summer and leads to reduced photochemistry, which may remove O3 and its precursors in Lhasa, thereby decreasing O3 in the
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found between them: CO and NO2 (r ¼ 0.95), CO and SO2 (r ¼ 0.86), NO2 and SO2 (r ¼ 0.76), suggesting relatively identical sources for these gaseous pollutants. Moreover, gaseous pollutants (CO, SO2 and NO2) displayed negative correlations with temperature, PBLH and wind speed. The diurnal peaks of SO2, NO2, CO, PM10, and PM2.5 in the morning and evening were mainly attributed to traffic emissions during the rush hour periods and depressed air in the boundary layer (Fig. 6). The decrease in the afternoon is closely related to the increasing height of the boundary layer and wind speed, which result in the dilution of pollutants and photochemical reactions involving O3. With the decrease of NO2 and CO in the afternoon, the concentration of O3 was increased and it was suggested that the maximum O3 concentration in the afternoon was mainly due to photochemical reaction under intense solar radiation conditions, leading the consumption of NO2 and CO. 4.4. Wind-dependent pollutants at the 6 sites in Lhasa city
Fig. 5. Monthly mean PM2.5/PM10 ratios in Lhasa. Error bars refer to 95% confidence intervals.
summer (Ma et al., 2014). 4.3. Diurnal variations of air pollutants in Lhasa Hourly mean concentrations of two particulate pollutants (PM10 and PM2.5) had a strong positive correlation (r ¼ 0.96). For the three gaseous pollutants (CO, SO2 and NO2), positive correlations were
Affected by the valley setting, the prevailing winds in Lhasa are west winds and east winds (Fig. S3). Fig. 7 shows that the pollutant roses at the 6 sites in Lhasa based on all measurements. In contrast to other pollutants, surface O3 is a secondary pollutant that is produced by photochemical reactions involving nitrogen oxides (NOx), and volatile organic compounds (VOCs). The pollutant rose of O3, therefore, reflects afternoon winds and will not be discussed in this study. For LHASA-QF in the western part of the city, the concentrations of almost all air pollutants were high when an east wind was observed at the site. For LHASA-XZ in the east, the concentrations of SO2, CO, and NO2 were higher for a west wind than those for an east wind. LHASA-LS, another suburban site in the southern part of the
Fig. 6. Daily variations of mean concentrations of air pollutants at 6 sites, temperature, wind speed and PBLH in Lhasa during the entire measurements (Beijing Time, UTCþ8). Error bars refer to standard deviation. Temperature and wind speed data were obtained from the National Climatic Data Center provided by the National Oceanic and Atmospheric Administration (NOAA, 2018), and PBLH was obtained from ERA-Interim (Dee et al., 2011) provided by the European Centre for Medium-Range Weather Forecasts (ECMWF, 2018).
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Fig. 7. Pollutant roses of SO2, NO2, CO, PM10 and PM2.5 at 6 sites in Lhasa.
city, exhibited high concentrations of NO2, CO and PM2.5 associated with northeast winds. LHASA-QJ, which is on the edge of the main city, displayed increases in pollutants that were similar to those at LHASA-LS. The pollutants at these sites displayed higher concentrations when the wind blew from the downtown area compared to that from the suburban area, and this finding reflects the impact of pollutant emissions from the main city compared to those from suburban areas. At LHASA-SH, the lowest concentrations of all air pollutants were clearly associated with a west wind from the Lalu Wetlands National Nature Preserve, which serves as air filter in the city. For particulate pollutants, high concentrations of PM10 were associated with high wind speeds, indicating the effect of dust when the wind speed is high. High concentrations of PM2.5 were associated with low wind speeds, indicating the limited diffusion of the anthropogenic emissions of PM2.5 under stagnant atmospheric conditions. Similar correlations between wind speed and particulate pollutants were also observed at other sites in Lhasa. The pollutants increased at LHASA-BK, with high concentrations of SO2 and PM2.5 associated with north and east winds. The other air
pollutants in LHASA-BK exhibited no obvious directionality. Overall, the wind direction clearly impacted the air pollutants at the sites in Lhasa. Wind from urban areas contributed to high concentrations of air pollutants at suburban sites due to the high emissions in urban areas in Lhasa. The Lalu Wetlands National Nature Preserve has a positive effect on the air quality in Lhasa. 4.5. Air pollution versus economic growth: Lhasa pattern and implications The air quality in a city is influenced by complex factors such as the climatic and environmental conditions, industrial structure, energy consumption, and air pollution control policies/measures, etc. Most of the major cities, especially those in the North China Plain, have suffered from serious air pollution in recent decades due to rapid economic growth and urbanization with industrydominated development and a lack of atmospheric environmental protection (Li et al., 2016). Lhasa achieved rapid economic and population growth while
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Fig. 8. Comparison of annual mean concentrations of air pollutants in provincial capital cities in China in 2017.
maintaining good air quality. This trend differs from those in most other Chinese provincial capitals. Lhasa's GDP in 2017 was 25 times higher than that of 2001, and the population had increased by more than 130 000 in 16 years (Fig. 9). Meanwhile, a comparison
between measurements of SO2, NO2 and PM10 from 2001 to 2006 (Ga, 2009) and the measurements from 2013 to 2017 suggests that these three air pollutants remained low and stable over a decade in Lhasa (Fig. 9). The Lhasa pattern is distinctly different from that of
Fig. 9. Variation of air pollutants, GDP (gross domestic product) and population in Lhasa from 2001 to 2017. Air pollutants data from 2001 to 2006 were obtained from Ga (2009). GDP and population Source: National Bureau of Statistics of China (2018).
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major cities in eastern China, where deteriorating air quality is often associated with economic growth (e.g., Liu et al., 2018; Zhao et al., 2018). We suggested the following reasons. Firstly, Lhasa is isolated and far away from the heavily-polluted regions and it is located in the upwind of eastern China where serious air pollution could hardly reach and impact it. Air pollution from South Asia may impact Lhasa but is mostly long-range transport episodic events. Secondly, the tertiary industry is the major sector in the Lhasa's industrial structure, and contributed to 56.8% of the GDP in 2017, which is higher than that of China as a whole (51.9%) (Fig. S4). As a result, Lhasa consumes less energy and emits fewer air pollutants. Furthermore, different from most other cities in China that mainly rely on coal as energy generation (Wang et al., 2015a,b), Tibet has a high portion of renewable energy in the total energy consumption. For example, Tibet has abundant water resources (water resources totaled 475 billion m3) and has developed many hydropower stations as its major source of power generation (Fig. S5). The proportion of hydro energy reached 96.4% of all power generation in 2015 in Tibet, generating limited emission of air pollutants. Finally, the critical governmental air pollution control policies and measures at both the local and regional levels (Wang and Hao, 2012) contributed to the improvement of air quality in Lhasa. The Action Plan for the Prevention and Control of Air Pollution was issued by the State Council of China in 2013 to guide national efforts in preventing and controlling air pollution. Specific implementation rules were immediately formulated by the government of the Tibet Autonomous Region and have been strictly implemented since 2014 (http://www.xizang.gov.cn), and specific rules include rectifying small coal-fired boilers, controlling flying dust and cooking fume pollution from catering services, eliminating yellow-label vehicles and old vehicles, promoting public transport and newenergy vehicles, and providing central heating in the winter, all of which were favorable factors that supported good air quality. Notably, one of the overall goal settings of the Rules for the Implementation of Action Plan on Prevention and Control of Air Pollution in Lhasa is that the average annual concentrations of PM10 and PM2.5 in Lhasa in 2017 should not exceed those in 2013. The average annual concentration of PM2.5 in 2017 in Lhasa was 19.09 mg m3, lower than that in 2013 (21.16 mg m3), indicating that the goal was narrowly attained. However, the average annual concentration of PM10 in 2017 (54.19 mg m3) was higher than that in 2013 (46.58 mg m3). Furthermore, the concentration of O3 in Lhasa was ranked as one of the highest in China. Such high level was probably associated with the natural conditions and processes (e.g., strong solar radiation and strong stratosphere-troposphere ozone exchange process) in Lhasa. Special attention should be paid to PM10 and O3 to improve Lhasa's air quality in the future. Setting an appropriate and effective timeline and roadmap is key to improve air quality for Lhasa as well as for other cities. ScienceBased policy measures, with full consideration of the characteristics and trends of air pollution, should also be enacted for Lhasa. Additionally, more technical capacity and resources should be invested to sustain Lhasa's air quality. Overall, owing to its unique geographic location, limited air pollutant emissions induced from a low energy-consuming industry that utilizes a high portion of clean energy, and effective air pollution control measures, Lhasa is one of the cities in China with the best air quality, and achieved the synchronous improvement of economy and air quality in the last decade, such “Lhasa pattern” may serve as a positive example for other regional hub cities that experience rapid socioeconomic development and simultaneously seek to improve air quality.
spatial variations in gaseous and particulate pollutants over 5 years at 6 sites in Lhasa. Generally, the urban sites had higher concentrations of SO2, NO2, CO, PM10, and PM2.5 compared to those of the suburban sites, and the suburban sites had higher O3 concentrations. Elevated concentrations of air pollutants at suburban sites were associated with winds from urban areas, and this trend reflects the impact of emissions in urban areas. Air pollutant concentrations, including those of SO2, NO2, CO, PM10, and PM2.5, were significantly higher in the winter than in other seasons due to the combined effects of primary emissions from domestic heating, weak photochemical reactions and adverse diffusion conditions. However, the O3 concentration displayed a maximum in the late spring and early summer in Lhasa due to photochemical production and stratosphere-troposphere exchange. For the diurnal variations, SO2, NO2, CO, PM10, and PM2.5 exhibited bimodal patterns with peaks during the morning and evening rush hours due to the depressed air in the boundary layer. Unlike the other air pollutants, the diurnal variations in O3 displayed a unimodal pattern with a peak in the afternoon due to photochemical reactions and intense solar radiation. A comparison of all Chinese provincial capital cities showed that Lhasa was one of the cities with the best air quality in China based on the SO2, NO2, CO, PM10 and PM2.5 concentrations in 2017. Over the past decade, Lhasa achieved rapid economic and population growth while maintaining good air quality. This trend differs from that in most other Chinese provincial capitals. The unique geographical location, low emission of air pollutants associated with its industrial structure and energy consumption, and effective air pollution control measures contributed to the good air quality in Lhasa. The “Lhasa pattern” of synchronous improvement of the economy and air quality may serve as a positive example for other regional hub cities that experience rapid socioeconomic development and simultaneously seek to improve air quality. Conflicts of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Gaseous and particulate pollutants in Lhasa, Tibet during 2013e2017: spatial variability, temporal variations and implications”. Acknowledgments This study was supported by the Strategic Priority Research Program (A) of the Chinese Academy of Sciences (XDA20040501), the National Natural Science Foundation of China (41630754, 41630748) and State Key Laboratory of Cryospheric Science (SKLCSZZ-2019 and SKLCS-OP-2019-07). X. F. Yin acknowledges the China Scholarship Council. Q. G. Zhang acknowledges financial support from the Youth Innovation Promotion Association of CAS (2016070). We acknowledge the China National Environmental Monitoring Center, the National Oceanic and Atmospheric Administration, the European Centre for Medium-Range Weather Forecasts and the National Bureau of Statistics of China for providing the data. Appendix A. Supplementary data
5. Conclusions We performed a comprehensive analysis of the temporal and
Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.06.113.
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