Bull Environ Contam Toxicol DOI 10.1007/s00128-014-1451-y
Assessment of Ozone Variations and Meteorological Influences at a Rural Site in Northern Xinjiang Huiqin Wang • Jianming Ma • Yanjie Shen Yanan Wang
•
Received: 16 April 2014 / Accepted: 23 December 2014 Ó Springer Science+Business Media New York 2015
Abstract Ozone concentrations and meteorological data were continuously monitored online from June to December 2013 at the Akedala regional atmosphere station in an arid region of Central Asia. We present daily, monthly, and seasonal variations of ozone concentrations in the atmosphere and elucidate possible emission sources. The ozone concentrations of this region varied from 14.7 to 58.6 ppb. A remarkable seasonal variation of ozone in aerosols was observed with highest level in summer, followed by autumn and winter. The daily peak value of ozone was observed at 9:00–11:00 a.m. while the lowest was at 17:00–19:00 p.m. The backward trajectories of air masses showed potential emission sources to be from the northwest and south during the measurement period. The backward trajectory also revealed that ozone concentrations during the measurement period were likely attributable to the emission from anthropogenic activities, and medium-range atmospheric transport from cities in central Asia and the northern slope of the Tian Shan Mountains. Keywords Akedala Ozone concentration Meteorological influences Seasonal characteristics Diurnal variations Transport pathway Many studies on ozone (O3) chemistry and associated meteorology have been carried out in China, particularly in the highly populated and prosperous eastern seaboard
H. Wang (&) J. Ma (&) Y. Shen Y. Wang Key Laboratory of Environmental Pollution Prediction and Control of Gansu, College of Earth and Environmental Science, Lanzhou University, Lanzhou 730000, China e-mail: [email protected] J. Ma e-mail: [email protected]
region (Cheung and Wang 2001; Ding and Wang 2006; Ouyang et al. 2012). These studies have improved our understanding of the factors affecting O3 formation, accumulation, and its adverse effects upon vegetation and human health. Over the past 20 years, high O3 concentrations have also been reported in eastern China’s major cities, such as Shanghai (Xu et al. 1999), Taipei (Liu et al. 1994), Hong Kong (Wang et al. 1998). In these cities, O3 has posed a serious health risk to environment and local populations during summers. Some studies also found that the mountainous region of northwestern China appeared to be a source area for O3, where O3 and OX (=O3 ? NO2) levels were remarkably higher than in the Northern China Plain (Zheng et al. 2005; Xin et al. 2010). The atmospheric environment monitoring network of the North China Plain was established to conduct routine O3 monitoring. The results from the network showed that high O3 concentrations, fine particles, and oxidation process contributed considerably to the mixture of atmospheric pollutants in this area in summer, with ubiquitous regional sources (Xin et al. 2010). This poses a great challenge to reduce atmospheric levels of O3 and particulate matter in this region (Xin et al. 2010; Tang et al. 2012). A number of regional atmosphere background stations were set up in western China in recent years. O3 measurements have been routinely measured at these regional background observatories and other monitoring sites in metropolitan area. These O3 monitoring data have improved our understandings of O3 levels in different areas, and the effects of O3 on regional climate (Lin et al. 2010). However, current O3 measurement data are still very scarce in Xinjiang, western China, which covers 17 % of the area of China. The present study presents a detailed assessment of O3 pollution in Akedala (north Xinjiang), a background atmospheric monitoring site, including diurnal
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and seasonal variations in O3 concentration and its relation with NOx and meteorological conditions. The emission characteristics were also examined to trace potential sources of O3-laden air masses.
Materials and Methods The sampling site is located within Fuhai County in Altay Region, China. Dominated by the mid-latitude westerlies, the local climate is typically continental (dry and arid) with several precipitation events occurring from May to September. Recorded meteorological data from a weather station in Fuhai (47°070 N, 87°280 E, 502 m a.s.l.) showed that the mean annual precipitation averaged over the time period between 1960 and 2009 was 121 mm, and the mean annual temperature was 4.0°C. July is the hottest month with mean temperature at 23.1°C and January is the coldest month with mean temperature at -24.3°C. The length of the frost-free growing season is 144 days or around 8 months with mean relative humidity at 55 %. Prevailing wind directions are NW in spring, summer and autumn; SE and N in winter. Precipitation and higher relative humidity occurred primarily in wet months (May–September). Precipitation is mainly associated with the mixing of air masses from NNW directions. Monthly averaged wind speed is lower (1.7 m/s) in winter (December, January, and February), and higher in spring (March, April and May) at 3.2 m/s. The sampling location for O3, NOX, and meteorological parameters was the Akedala Meteorological Station (47°060 N, 87°580 E, 563.3 m a.s.l.), located near the northern edges of the Gurbantonggut Desert (Fig. 1). This region is characterized by flat plains with low vegetation cover. Sampling was carried out at 3 m height above the ground level. Hourly ozone concentration was monitored from June 2013 to December 2013. During the same period, NOX and meteorological parameters were also measured to elucidate the relationship between O3 precursors and meteorological parameters, and O3 concentrations during the lower and higher photochemical reaction periods. O3 and NOX concentrations were simultaneously measured every 5 min using a model 9841T nitrogen oxide analyzer and a model 9810B ozone analyzer (Ecotech Pty Ltd., Blackburn, VIC, AU). The detection limits were 0.1 and 0.001 mg/m3 for O3 and NOX, respectively. Instruments were calibrated using the gas standards provided by the TSI Beijing branch and Beijing Magee Scientific Corporation, Beijing, China. Ozone and NOX samples were collected from 00:00 to 23:55 Eastern China Time. Climate data, including temperature, precipitation, relative humidity, wind speed and direction, and solar radiation data were obtained from Xinjiang Meteorological Bureau.
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Results and Discussion Figure 2 shows the variations of daily mean O3 concentration, ranging from 14.7 to 58.6 ppb. Several peak values were greater than 50 ppb, occurring in summer (June, July and August) 2013. These peak values occurred during conditions of relatively higher temperature and low relative humidity, or during time periods when the prevailing wind directions favored atmospheric transport of O3 from its emission sources. Five-day moving averages of O3 concentration were from 18.4 to 47.3 ppb. The daily mean O3 concentration was usually less than 45 ppb, suggesting weak local emissions. Ozone background concentration, defined as the O3 concentration in a pristine air mass when the contribution from anthropogenic sources was absent, varied in the range of 14.7–42.0 ppb in Akedala, which was close to the background concentration in Shangdianzi (13.8–52.1 ppb), Linan (17.5–44.8 ppb), and Longfengshan (25.2–47.3 ppb) in highly populated and industrialized eastern China under the east Asian monsoon regime (Xu et al. 2008; Meng et al. 2009; Lin et al. 2010). It has been found that the mean O3 concentrations at those sampling sites with low altitudes in China were lower than those in the northern Tibetan Plateau (Jin et al. 2008). This may be linked with high intensity solar radiation in northern Tibetan Plateau (Zha 1996). Mean daily O3 concentration measured in Akedala during the measurement period was 33.4 ppb. The low O3 concentrations in Akedala were much lower than those measured at Tazhong Observatory (38°580 N, 83°390 E, 1090 m a.s.l.) in the center of the Taklimakan Desert (49 ppb, Liu et al. 2014), suggesting that the area was still not heavily contaminated by O3 pollution. This may be linked with the different terrain and climate conditions. The Taklimakan Desert is enclosed by high mountains (Tian Shan and Kunlun Mountains). Such mountain-basin terrain may trap local air pollutants (i.e. NO, NO2, NOX) emitted from surrounding oasis areas via atmospheric transport. The changes in O3 concentrations at Tazhong were associated to some extent with the solar radiation intensity over the Taklimakan Desert, resulting in higher O3 levels during the sunny days. This higher level O3 concentration during the sunny days has been attributed to photochemical processes of O3 formation (Liu et al. 2014). Compared with Tazhong, the flat open terrain of Akedala is more conducive to the dispersion of pollutants. Further, warm moist air masses were more readily transported by the westerly, causing low solar radiation and high cloudiness in the study region (Chen et al. 2013). The monthly variation of the daily averaged concentrations of O3 during the field sampling period is presented in Fig. 3. This shows that the highest monthly mean O3 concentrations were observed in the summer months
Bull Environ Contam Toxicol Fig. 1 Location map of atmosphere background stations and meteorological stations (cities)
Fig. 2 a Daily mean O3 concentration during measurement period at Akedala station. The thick line is the 5-day moving average of O3 concentration. b The first differences (day-to-day changes) of mean O3 concentration during the measurement period in Akedala station
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(June–August), whereas the lowest concentrations were observed in the winter months (November–December). Monthly averaged values of ground level O3 varied from 21.6 ppb (December) to 44.1 ppb (July) on a daily basis. In addition, the highest daily value for O3 concentration was 58.6 ppb in July, whereas the lowest was 14.7 ppb in November. The O3 concentrations in Akedala were significantly lower as compared with those reported in east China’s major cities, such as Shanghai (Xu et al. 1999), Taipei (Liu et al. 1994), and Hong Kong (Wang et al. 1998). Table 1 presents mean daytime, nighttime, and daily concentrations of O3 during the three seasons. The highest daytime and daily O3 concentrations were again recorded during the summertime, whereas the lowest levels were found during winter season. The difference in mean concentrations was statistically significant (p \ 0.001). The mean values of O3 were 44.8, 32.1, and 22.1 ppb in the daytime hours, and 41.9, 29.7, and 21.5 ppb as 24 h daily means during summer, autumn and winter, respectively. The corresponding summer/winter ratios of O3 concentrations were 2.02 during the daytime hours and 1.95 ppb for the 24 h daily means, respectively. The averaged daytime O3 concentration in Akedala was below the European Union air quality standard (Duen˜as et al. 2002) and the National Ambient Air Quality Standards (NAAQS) of the
United States Environmental Protection Agency (U.S. EPA, Tong and Mauzerall 2006). It is known that O3 concentrations can be determined as a result of source and sink mechanisms which rely on the prevailing levels of O3 precursors and meteorological factors, such as temperature, precipitation, relative humidity, solar radiation intensity, and wind speed (Vukovich and Sherwell 2003; Pereira et al. 2005; Pudasainee et al. 2006; Alvim-Ferraz et al. 2006; Khoder 2009). A recent investigation of seasonal variations in O3 concentrations at a desert site in south Xinjiang also observed that the highest levels of O3 were recorded in the summer season and the lowest levels were in the winter season (Liu et al. 2014). The higher O3 concentrations in Akedala during summer can be attributed to the higher temperature and strong solar radiation that promote photochemical generation of O3, as previously reported by others (Derwent and Davies 1994; Vukovich and Sherwell 2003; Khoder 2009). Hourly variations of O3 concentrations on a daily basis are presented for July, September and November, as representatives of summer, autumn and winter seasons, respectively (Fig. 4). This shows a peak of O3 concentrations appearing from 9:00 to 11:00, and a valley from 17:00 to 19:00, which is similar to that observed in the Taklimakan Desert, where the O3 concentration increased
Fig. 3 Variation of the monthly mean O3 concentration during the measurement period in Akedala station
Fig. 4 Variation of the hourly mean O3 concentration in three seasons
Table 1 Mean daytime, nighttime and daily O3 concentrations during summer to December 2013 Season
Daytime (7:00–19:00) Mean concentrations (ppb)
Nighttime (19:05–6:55) Mean concentrations (ppb)
Daily (0:00–24:00) Mean concentrations (ppb)
Summer
44.8
39.0
41.9
Autumn
32.1
27.3
29.7
Winter
22.1
21.0
21.5
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between 8:00 and 11:00 (Liu et al. 2014). The strongest solar radiation and most intense atmospheric convection take place from 10:00 to 13:00 during which aerosol particles cannot be concentrated within the atmospheric boundary layer. As a result, elevated O3 concentrations in the atmosphere would not be readily degraded, and hence are maintained at a relatively high level. Different from the result in south Xinjiang (Liu et al. 2014), our data show that diurnal variation of O3 concentration was characterized by a pronounced single peak pattern during the observation period, and concentrations decreased after 14:00. Interestingly, while previous studies have shown that the lowest ozone values were often observed during
18:00–20:00 due to reduction of human activity, in our case, the O3 concentrations in Akedala exhibited an upward trend after 20:00, which may be linked with local atmospheric convection at nighttime (Wang et al. 2006). Ozone concentrations produced from photochemical reactions were influenced by meteorological factors (Table 2; significant correlations (p \ 0.001) indicated in bold font). Ozone concentrations were positively correlated with temperature, suggesting that high temperature led to an increase in the formation of O3. The favorable meteorological conditions (mild winds, high temperature, and clear skies) have been reported to exert a great influence on O3 levels (Vecchi and Valli 1999). In the present study, a
Table 2 Correlation coefficients between O3 concentration, temperature, relative humidity, solar radiation, mean wind speed and cloudiness during the observation period
O3
N
Temperature
Relative humidity
Solar radiation
Mean wind speed
Cloudiness
184
0.87
-0.46
0.42
0.22
0.17
Fig. 5 Air masses backward trajectories during high episodes of O3 and NOX concentration for different seasons in Akedala
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significant negative correlation coefficient was found between O3 concentration and relative humidity (Table 2). These results indicate that high relative humidity with strong solar radiation were important factors causing higher O3 concentrations during the observation period. Comparison of O3 concentration with O3 precursors (NOx) suggested no statistically significant relationship between the two. However, in several cases the monitored data did show high O3 concentration following relatively high NOx concentration. We calculated backward trajectories of 4-day air masses during the sampling period for O3 levels at Akedala by the HYSPLIT model provided by Air Resources Laboratory of NOAA, combined with reanalysis grid meteorological data of 2.5° 9 2.5° provided by Climate Diagnostics Center NCEP/NCAR (Rolph 2013). These transport paths are illustrated in Fig. 5. In high O3 and NOx concentration episodes, air masses mainly come from the northwest and south of the measurement site. The wind flows at the mid-troposphere and surface exhibited almost the same pattern, consistent with the frequency of NNW and S winds mostly recorded in local weather station. This indicates that that part of O3 and other air pollutants (e.g., NOx) emission sources may be traced back to Central Asia and cities on the northern slope of the Tian Shan (Fig. 1), especially in Kazakhstan. The present study revealed that O3 concentrations at a background monitoring site in north Xinjiang ranged from 14.7 to 58.6 ppb from June to December 2013. Ozone concentrations in the atmosphere in Akedala were the highest in summer and the lowest in winter. The monthly mean O3 concentration was the highest at 21.6 ppb in December and the lowest at 44.1 ppb in July. The O3 concentration in the warm season was higher than that in the cold season. Results revealed that the meteorological factors, such as temperature, solar radiation, and relative humidity were the dominant causes of changes in ozone concentration. Ozone concentrations exhibited a peak at 9:00–11:00 and a valley at 17:00–19:00, respectively. Compared with the cases in south Xinjiang, the diurnal variation of ozone concentration was characterized by a single peak pattern during the observation period, and increasing after 20:00. Backward trajectories of 4-day air masses calculated by the HYSPLIT model showed that O3 levels were affected to a large extent by the emission sources from the northwest and south. This indicates that emissions from industrial and urban activities in Central Asia and cities along the northern slope of the Tian Shan Mountains influence O3 background levels at Akedala. Acknowledgments This study was supported by NSFC (No. 41371478), the Fundamental Research Funds for the Central Universities (223000-861694). We thank the reviewers’ comments which improved greatly this manuscript.
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Bull Environ Contam Toxicol implications of an evaluation of the CMAQ model. Atmos Environ 40:3041–3056 Vecchi R, Valli G (1999) Ozone assessment in the southern part of the Alps. Atmos Environ 33:97–109 Vukovich FM, Sherwell J (2003) An examination of the relationship between certain meteorological parameters and surface ozone variations in the Baltimore–Washington corridor. Atmos Environ 37:971–981 Wang T, Lam KS, Lee ASY, Pang SW, Tsui WS (1998) Meteorological and chemical characteristics of the photochemical ozone episodes observed at Cape D’Aguilar in Hong Kong. J Appl Meteorol 37:1167–1178 Wang T, Wong HLA, Tang J, Ding A, Wu WS, Zhang XC (2006) On the origin of surface ozone and reactive nitrogen observed at a remote mountain site in the northeastern Qinghai-Tibetan Plateau, western China. J Geophy Res 111:D08303. doi:10. 1029/2005JD006527
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Assessment of Ozone Variations and Meteorological Influences at a Rural Site in Northern Xinjiang Huiqin Wang • Jianming Ma • Yanjie Shen Yanan Wang
•
Received: 16 April 2014 / Accepted: 23 December 2014 Ó Springer Science+Business Media New York 2015
Abstract Ozone concentrations and meteorological data were continuously monitored online from June to December 2013 at the Akedala regional atmosphere station in an arid region of Central Asia. We present daily, monthly, and seasonal variations of ozone concentrations in the atmosphere and elucidate possible emission sources. The ozone concentrations of this region varied from 14.7 to 58.6 ppb. A remarkable seasonal variation of ozone in aerosols was observed with highest level in summer, followed by autumn and winter. The daily peak value of ozone was observed at 9:00–11:00 a.m. while the lowest was at 17:00–19:00 p.m. The backward trajectories of air masses showed potential emission sources to be from the northwest and south during the measurement period. The backward trajectory also revealed that ozone concentrations during the measurement period were likely attributable to the emission from anthropogenic activities, and medium-range atmospheric transport from cities in central Asia and the northern slope of the Tian Shan Mountains. Keywords Akedala Ozone concentration Meteorological influences Seasonal characteristics Diurnal variations Transport pathway Many studies on ozone (O3) chemistry and associated meteorology have been carried out in China, particularly in the highly populated and prosperous eastern seaboard
H. Wang (&) J. Ma (&) Y. Shen Y. Wang Key Laboratory of Environmental Pollution Prediction and Control of Gansu, College of Earth and Environmental Science, Lanzhou University, Lanzhou 730000, China e-mail: [email protected] J. Ma e-mail: [email protected]
region (Cheung and Wang 2001; Ding and Wang 2006; Ouyang et al. 2012). These studies have improved our understanding of the factors affecting O3 formation, accumulation, and its adverse effects upon vegetation and human health. Over the past 20 years, high O3 concentrations have also been reported in eastern China’s major cities, such as Shanghai (Xu et al. 1999), Taipei (Liu et al. 1994), Hong Kong (Wang et al. 1998). In these cities, O3 has posed a serious health risk to environment and local populations during summers. Some studies also found that the mountainous region of northwestern China appeared to be a source area for O3, where O3 and OX (=O3 ? NO2) levels were remarkably higher than in the Northern China Plain (Zheng et al. 2005; Xin et al. 2010). The atmospheric environment monitoring network of the North China Plain was established to conduct routine O3 monitoring. The results from the network showed that high O3 concentrations, fine particles, and oxidation process contributed considerably to the mixture of atmospheric pollutants in this area in summer, with ubiquitous regional sources (Xin et al. 2010). This poses a great challenge to reduce atmospheric levels of O3 and particulate matter in this region (Xin et al. 2010; Tang et al. 2012). A number of regional atmosphere background stations were set up in western China in recent years. O3 measurements have been routinely measured at these regional background observatories and other monitoring sites in metropolitan area. These O3 monitoring data have improved our understandings of O3 levels in different areas, and the effects of O3 on regional climate (Lin et al. 2010). However, current O3 measurement data are still very scarce in Xinjiang, western China, which covers 17 % of the area of China. The present study presents a detailed assessment of O3 pollution in Akedala (north Xinjiang), a background atmospheric monitoring site, including diurnal
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and seasonal variations in O3 concentration and its relation with NOx and meteorological conditions. The emission characteristics were also examined to trace potential sources of O3-laden air masses.
Materials and Methods The sampling site is located within Fuhai County in Altay Region, China. Dominated by the mid-latitude westerlies, the local climate is typically continental (dry and arid) with several precipitation events occurring from May to September. Recorded meteorological data from a weather station in Fuhai (47°070 N, 87°280 E, 502 m a.s.l.) showed that the mean annual precipitation averaged over the time period between 1960 and 2009 was 121 mm, and the mean annual temperature was 4.0°C. July is the hottest month with mean temperature at 23.1°C and January is the coldest month with mean temperature at -24.3°C. The length of the frost-free growing season is 144 days or around 8 months with mean relative humidity at 55 %. Prevailing wind directions are NW in spring, summer and autumn; SE and N in winter. Precipitation and higher relative humidity occurred primarily in wet months (May–September). Precipitation is mainly associated with the mixing of air masses from NNW directions. Monthly averaged wind speed is lower (1.7 m/s) in winter (December, January, and February), and higher in spring (March, April and May) at 3.2 m/s. The sampling location for O3, NOX, and meteorological parameters was the Akedala Meteorological Station (47°060 N, 87°580 E, 563.3 m a.s.l.), located near the northern edges of the Gurbantonggut Desert (Fig. 1). This region is characterized by flat plains with low vegetation cover. Sampling was carried out at 3 m height above the ground level. Hourly ozone concentration was monitored from June 2013 to December 2013. During the same period, NOX and meteorological parameters were also measured to elucidate the relationship between O3 precursors and meteorological parameters, and O3 concentrations during the lower and higher photochemical reaction periods. O3 and NOX concentrations were simultaneously measured every 5 min using a model 9841T nitrogen oxide analyzer and a model 9810B ozone analyzer (Ecotech Pty Ltd., Blackburn, VIC, AU). The detection limits were 0.1 and 0.001 mg/m3 for O3 and NOX, respectively. Instruments were calibrated using the gas standards provided by the TSI Beijing branch and Beijing Magee Scientific Corporation, Beijing, China. Ozone and NOX samples were collected from 00:00 to 23:55 Eastern China Time. Climate data, including temperature, precipitation, relative humidity, wind speed and direction, and solar radiation data were obtained from Xinjiang Meteorological Bureau.
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Results and Discussion Figure 2 shows the variations of daily mean O3 concentration, ranging from 14.7 to 58.6 ppb. Several peak values were greater than 50 ppb, occurring in summer (June, July and August) 2013. These peak values occurred during conditions of relatively higher temperature and low relative humidity, or during time periods when the prevailing wind directions favored atmospheric transport of O3 from its emission sources. Five-day moving averages of O3 concentration were from 18.4 to 47.3 ppb. The daily mean O3 concentration was usually less than 45 ppb, suggesting weak local emissions. Ozone background concentration, defined as the O3 concentration in a pristine air mass when the contribution from anthropogenic sources was absent, varied in the range of 14.7–42.0 ppb in Akedala, which was close to the background concentration in Shangdianzi (13.8–52.1 ppb), Linan (17.5–44.8 ppb), and Longfengshan (25.2–47.3 ppb) in highly populated and industrialized eastern China under the east Asian monsoon regime (Xu et al. 2008; Meng et al. 2009; Lin et al. 2010). It has been found that the mean O3 concentrations at those sampling sites with low altitudes in China were lower than those in the northern Tibetan Plateau (Jin et al. 2008). This may be linked with high intensity solar radiation in northern Tibetan Plateau (Zha 1996). Mean daily O3 concentration measured in Akedala during the measurement period was 33.4 ppb. The low O3 concentrations in Akedala were much lower than those measured at Tazhong Observatory (38°580 N, 83°390 E, 1090 m a.s.l.) in the center of the Taklimakan Desert (49 ppb, Liu et al. 2014), suggesting that the area was still not heavily contaminated by O3 pollution. This may be linked with the different terrain and climate conditions. The Taklimakan Desert is enclosed by high mountains (Tian Shan and Kunlun Mountains). Such mountain-basin terrain may trap local air pollutants (i.e. NO, NO2, NOX) emitted from surrounding oasis areas via atmospheric transport. The changes in O3 concentrations at Tazhong were associated to some extent with the solar radiation intensity over the Taklimakan Desert, resulting in higher O3 levels during the sunny days. This higher level O3 concentration during the sunny days has been attributed to photochemical processes of O3 formation (Liu et al. 2014). Compared with Tazhong, the flat open terrain of Akedala is more conducive to the dispersion of pollutants. Further, warm moist air masses were more readily transported by the westerly, causing low solar radiation and high cloudiness in the study region (Chen et al. 2013). The monthly variation of the daily averaged concentrations of O3 during the field sampling period is presented in Fig. 3. This shows that the highest monthly mean O3 concentrations were observed in the summer months
Bull Environ Contam Toxicol Fig. 1 Location map of atmosphere background stations and meteorological stations (cities)
Fig. 2 a Daily mean O3 concentration during measurement period at Akedala station. The thick line is the 5-day moving average of O3 concentration. b The first differences (day-to-day changes) of mean O3 concentration during the measurement period in Akedala station
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(June–August), whereas the lowest concentrations were observed in the winter months (November–December). Monthly averaged values of ground level O3 varied from 21.6 ppb (December) to 44.1 ppb (July) on a daily basis. In addition, the highest daily value for O3 concentration was 58.6 ppb in July, whereas the lowest was 14.7 ppb in November. The O3 concentrations in Akedala were significantly lower as compared with those reported in east China’s major cities, such as Shanghai (Xu et al. 1999), Taipei (Liu et al. 1994), and Hong Kong (Wang et al. 1998). Table 1 presents mean daytime, nighttime, and daily concentrations of O3 during the three seasons. The highest daytime and daily O3 concentrations were again recorded during the summertime, whereas the lowest levels were found during winter season. The difference in mean concentrations was statistically significant (p \ 0.001). The mean values of O3 were 44.8, 32.1, and 22.1 ppb in the daytime hours, and 41.9, 29.7, and 21.5 ppb as 24 h daily means during summer, autumn and winter, respectively. The corresponding summer/winter ratios of O3 concentrations were 2.02 during the daytime hours and 1.95 ppb for the 24 h daily means, respectively. The averaged daytime O3 concentration in Akedala was below the European Union air quality standard (Duen˜as et al. 2002) and the National Ambient Air Quality Standards (NAAQS) of the
United States Environmental Protection Agency (U.S. EPA, Tong and Mauzerall 2006). It is known that O3 concentrations can be determined as a result of source and sink mechanisms which rely on the prevailing levels of O3 precursors and meteorological factors, such as temperature, precipitation, relative humidity, solar radiation intensity, and wind speed (Vukovich and Sherwell 2003; Pereira et al. 2005; Pudasainee et al. 2006; Alvim-Ferraz et al. 2006; Khoder 2009). A recent investigation of seasonal variations in O3 concentrations at a desert site in south Xinjiang also observed that the highest levels of O3 were recorded in the summer season and the lowest levels were in the winter season (Liu et al. 2014). The higher O3 concentrations in Akedala during summer can be attributed to the higher temperature and strong solar radiation that promote photochemical generation of O3, as previously reported by others (Derwent and Davies 1994; Vukovich and Sherwell 2003; Khoder 2009). Hourly variations of O3 concentrations on a daily basis are presented for July, September and November, as representatives of summer, autumn and winter seasons, respectively (Fig. 4). This shows a peak of O3 concentrations appearing from 9:00 to 11:00, and a valley from 17:00 to 19:00, which is similar to that observed in the Taklimakan Desert, where the O3 concentration increased
Fig. 3 Variation of the monthly mean O3 concentration during the measurement period in Akedala station
Fig. 4 Variation of the hourly mean O3 concentration in three seasons
Table 1 Mean daytime, nighttime and daily O3 concentrations during summer to December 2013 Season
Daytime (7:00–19:00) Mean concentrations (ppb)
Nighttime (19:05–6:55) Mean concentrations (ppb)
Daily (0:00–24:00) Mean concentrations (ppb)
Summer
44.8
39.0
41.9
Autumn
32.1
27.3
29.7
Winter
22.1
21.0
21.5
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between 8:00 and 11:00 (Liu et al. 2014). The strongest solar radiation and most intense atmospheric convection take place from 10:00 to 13:00 during which aerosol particles cannot be concentrated within the atmospheric boundary layer. As a result, elevated O3 concentrations in the atmosphere would not be readily degraded, and hence are maintained at a relatively high level. Different from the result in south Xinjiang (Liu et al. 2014), our data show that diurnal variation of O3 concentration was characterized by a pronounced single peak pattern during the observation period, and concentrations decreased after 14:00. Interestingly, while previous studies have shown that the lowest ozone values were often observed during
18:00–20:00 due to reduction of human activity, in our case, the O3 concentrations in Akedala exhibited an upward trend after 20:00, which may be linked with local atmospheric convection at nighttime (Wang et al. 2006). Ozone concentrations produced from photochemical reactions were influenced by meteorological factors (Table 2; significant correlations (p \ 0.001) indicated in bold font). Ozone concentrations were positively correlated with temperature, suggesting that high temperature led to an increase in the formation of O3. The favorable meteorological conditions (mild winds, high temperature, and clear skies) have been reported to exert a great influence on O3 levels (Vecchi and Valli 1999). In the present study, a
Table 2 Correlation coefficients between O3 concentration, temperature, relative humidity, solar radiation, mean wind speed and cloudiness during the observation period
O3
N
Temperature
Relative humidity
Solar radiation
Mean wind speed
Cloudiness
184
0.87
-0.46
0.42
0.22
0.17
Fig. 5 Air masses backward trajectories during high episodes of O3 and NOX concentration for different seasons in Akedala
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significant negative correlation coefficient was found between O3 concentration and relative humidity (Table 2). These results indicate that high relative humidity with strong solar radiation were important factors causing higher O3 concentrations during the observation period. Comparison of O3 concentration with O3 precursors (NOx) suggested no statistically significant relationship between the two. However, in several cases the monitored data did show high O3 concentration following relatively high NOx concentration. We calculated backward trajectories of 4-day air masses during the sampling period for O3 levels at Akedala by the HYSPLIT model provided by Air Resources Laboratory of NOAA, combined with reanalysis grid meteorological data of 2.5° 9 2.5° provided by Climate Diagnostics Center NCEP/NCAR (Rolph 2013). These transport paths are illustrated in Fig. 5. In high O3 and NOx concentration episodes, air masses mainly come from the northwest and south of the measurement site. The wind flows at the mid-troposphere and surface exhibited almost the same pattern, consistent with the frequency of NNW and S winds mostly recorded in local weather station. This indicates that that part of O3 and other air pollutants (e.g., NOx) emission sources may be traced back to Central Asia and cities on the northern slope of the Tian Shan (Fig. 1), especially in Kazakhstan. The present study revealed that O3 concentrations at a background monitoring site in north Xinjiang ranged from 14.7 to 58.6 ppb from June to December 2013. Ozone concentrations in the atmosphere in Akedala were the highest in summer and the lowest in winter. The monthly mean O3 concentration was the highest at 21.6 ppb in December and the lowest at 44.1 ppb in July. The O3 concentration in the warm season was higher than that in the cold season. Results revealed that the meteorological factors, such as temperature, solar radiation, and relative humidity were the dominant causes of changes in ozone concentration. Ozone concentrations exhibited a peak at 9:00–11:00 and a valley at 17:00–19:00, respectively. Compared with the cases in south Xinjiang, the diurnal variation of ozone concentration was characterized by a single peak pattern during the observation period, and increasing after 20:00. Backward trajectories of 4-day air masses calculated by the HYSPLIT model showed that O3 levels were affected to a large extent by the emission sources from the northwest and south. This indicates that emissions from industrial and urban activities in Central Asia and cities along the northern slope of the Tian Shan Mountains influence O3 background levels at Akedala. Acknowledgments This study was supported by NSFC (No. 41371478), the Fundamental Research Funds for the Central Universities (223000-861694). We thank the reviewers’ comments which improved greatly this manuscript.
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