SCIENCE CHINA Chemistry

© Science China Press and Springer-Verlag Berlin Heidelberg 2012 chem.scichina.com www.springerlink.com

*Corresponding author (email: linweili@pku.org.cn)

• ARTICLES • July 2012 Vol.55 No.7: 1451–1458

doi: 10.1007/s11426-012-4524-y

Significant downward trend of SO2 observed from 2005 to 2010 at a background station in the Yangtze Delta region, China

QI HuangXiong1, LIN WeiLi2,3*, XU XiaoBin2, YU XiangMing4 & MA QianLi4

1Institute of Urban Planning and Environmental Management, Zhejiang University of Finance and Economics, Hangzhou 310012, China 2Key Laboratory for Atmospheric Chemistry, Chinese Academy of Meteorological Sciences, Beijing 100081, China

3Center for Atmosphere Watch and Services, Meteorological Observation Centre, China Meteorological Administration, Beijing 100081, China 4Linan Regional Atmosphere Background Station, Linan 311307, China

Received July 16, 2011; accepted January 11, 2012; published online March 26, 2012

SO2 is an important gas in atmosphere with great environmental and climate implications. SO2 emission in China has been re-ceiving great attention as the economy grows and the amount of coal consumption has increased in the past few decades. SO2 has been observed from July 2005 to June 2010 at Linan WMO GAW regional station (30.3 °N, 119.73 °E, 138 m a.s.l.) lo-cated in the Yangtze Delta region in eastern China. These observation data are analyzed to understand the trend of regional SO2 background concentration. Strict quality controls are conducted to ensure the temporal comparability of the data. Signifi-cant downward trend with 2.4 ppb/yr (P < 0.0001) of surface SO2 is observed from 2005 to 2010, especially after 2008. The average concentration of SO2 from July 2005 to June 2008 is 14.2±3.1 ppb, which is slightly higher than the mean values of 13.5±5.1 ppb during 19992000 and is two folds of the average value (7.1±3.1 ppb) from July 2008 to June 2010. More than 50% of the SO2 has been cut down after 2008 in the Yangtze Delta region due to the implementation of stricter emission con-trol measures. The peak SO2 concentration appears around 10 o’clock in the morning after 2009 while appears at night before 2009. These diurnal variations of SO2 might indicate that after 2009, more SO2 is from the vertical exchange process than from the local accumulation.

SO2, background level, trend, Yangtz Delta Region

1 Introduction

Atmospheric SO2 is significant to health, acid deposition, and secondary fine aerosol formation [1–3]. Anthropogenic SO2 emissions in China are mainly from coal burning be-cause coal consumption accounts for about 70% of the total national energy consumption [4, 5]. Thermal power plants used 50% of the coal production and are the largest contrib-utor of anthropogenic SO2 in China [6]. For a long time, SO2 is one of the major gaseous pollutants in ambient air in China’s major cities and economically developed regions [7–16]. In addition, SO2 emission is the most important

cause of acid deposition in China [17]. As the SO2 pollution control measures continue to tighten, China’s SO2 emission began to decrease from 2006 [18] even though the total amount of coal consumption increased in the past few dec-ades. The major SO2 emissions control measures include the application of flue-gas desulfurization devices in power plants, the phase- out of small but high-emitting power gen-eration units, the technical innovation of coal-burning de-vices, and the alternative use of clean energy. The Chinese government set up a goal in the 11th Five Year Plan (20062010) to reduce SO2 emissions in 2010 by 10% from the level in 2005. Wang et al. [19] reported their assessment of the air quality benefits from Chinese air pollution control policies and showed that SO2 concentrations in highly pol-luted areas of east China in 2010 could be estimated to be

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decreased by30%–60% compared to the levels in the 2010 Business-As-Usual case. Li et al. [20] reported the dramatic decline in the SO2 column amount detected by OMI from 2007 to 2008 in northern China and assumed that the de-cline was attributable to the usage of flue-gas desulfuriza-tion (FGD) devices and their effectiveness in reducing SO2 emissions. In addition, environmental protection, especially for Beijing 2008 Olympics and for World Expo 2010 Shanghai, had been greatly enhanced and effectively im-plemented in Beijing, Shanghai, and their surrounding areas. Many publications [21–27] have focused on the effects of pollution control during these two events. However, it is scarce to use long-term monitoring data from individual stations in China to show and assess the recent trend or ef-fect of pollution control. Lin et al. [16] used the observa-tional data at urban, rural, and background sites in north China to study the recent trends of surface SO2 and the ef-fectiveness of control measures, and found a reduction of up to 40% for SO2 in polluted areas and a reduction of 20% for regional backgournd SO2 before and after the Beijing 2008 Olympic Games.

Surface SO2 concentration measurements are often con-fined in polluted ambient air in cities and there are fewer results in background regions in China. The concentrations of SO2 and other pollutants at atmospheric background sites can better reflect the impacts of human activities and natural processes on the compositions of the atmosphere because their observation results can better represent a “well- mixing” status of the air [28]. In this study, SO2 observed from July 2005 to June 2010 at Linan WMO GAW regional station, located in the economically blooming Yangtze Del-ta region in China, is analyzed to understand the level and the trend of regional SO2 background concentration and to discover the long-term impact of reductions in sulfur emis-sions in recent years. At this site, the enhanced variability of long-term trend of surface ozone has been reported recently and the most likely cause is believed to be the increase of NOx concentration [29].

2 Study site and data collection

Linan WMO GAW regional station (30.3°N, 119.73°E, 138 m a.s.l.) is on the southern edge of the Yangtze Delta region in China. It is 11 km north of the Linan Township where population is about 150 thousands. The station location is 50 km west of Hangzhou and 210 km southwest of Shang-hai. The populations of these two cities are 4 million and 20 million, respectively. The site is surrounded by hills well covered by vegetation and there are no big villages within 3 km surrounding area. The average annual temperature is 15.3 °C and annual precipitation is 1480 mm for this site. The prevailing wind directions are the northeast and south-west winds (NNE: 21.0%; NE: 12.2%; SW: 12.0%; SSW: 10.8%). It has a typical subtropical monsoon climate with

distinct four seasons. The SO2 mixing ratios have been continuously measured

since July 2005 using a 43CTL SO2 analyzer (Thermo En-vironmental, USA) by the pulsed fluorescence method. The lowest limit detection is 0.06 ppb in 5-min average. Daily zero and span checks are routinely carried out and mul-ti-point calibrations are done monthly. The SO2/N2 standard mixtures used at the sites for in-situ calibrations were pro-duced in different time by different factories of China. The differences among the standards can be significant [30] and were determined by comparing with the same NIST-trace- able SO2 standard produced by Scottgas, USA.

3 Backward trajectories analysis

Trajectories are often calculated and clustered to study the influence of airmass originated from different regions on the surface measurements. 3-day backward trajectories for 0, 6, 12, and 18 UTC every day during 2 years (2005–2006) were calculated by HYSLPIT model [31] using NCEP meteoro-logical data. The trajectory endpoint height was set to 100 meters above ground level at Linan station and the vertical motion method in the calculations was the default model selection, which uses the meteorological model’s vertical velocity fields and is terrain following. The clustering jobs were done by SAS/STAT software (SAS Institute Inc., Cary, NC, USA) using the method of gravity center (spatial coor-dinates: latitude, longitude, pressure height/10). When cal-culating Euler distances, the pressure altitude was divided by 10 to get a considerable magnitude level with that of latitude and longitude.

Figure 1 shows the results of the 9 mean trajectories and their ratios in the left part. The geographic cover area of the two major mean trajectories (No. 1 and No. 2) with ±1σ is also shown in the right part of Figure 1. The total trajectory number of the 9 clusters accounts to 95.2% of the all calcu-lated trajectories and the total number of trajectories in Cluster 1 and 2 accounts to 75.4% of the all trajectories. According to the geographic cover of the trajectories, more than 75% airmasses originated from or passing over the Yangtze River Delta region can influence the surface meas-urements at Linan station. Therefore, the observation results at Linan station can well represent the background changes of atmospheric components in the Yangtze River Delta re-gion.

4 The variations and trends of SO2 concentra-tions

4.1 The variations and trend of SO2 concentrations from 2005 to 2010

Figure 2 shows the time series variations of hourly average SO2 concentrations from July 2005 to August 2010. As

Qi HX, et al. Sci China Chem July (2012) Vol.55 No.7 1453

Figure 1 Mean trajectories with their ratios and geographic cover of the two major mean trajectories with ±1σ.

shown in Figure 2, a very significant downward trend with –2.4 ppb/yr of surface SO2 at Linan station was observed. The maximum hourly mean (128.6 ppb) and daily mean (56.0 ppb) of SO2 concentrations were observed in Decem-ber 2007. The mean value is 11.1±10.6 ppb and the median value is 8.0 ppb from July 2005 to August 2010. There are 4.4% of the data before July 2008 and only 0.1% of the data after June 2008 with the SO2 concentrations higher than 40 ppb.

Because the operational measurements were started in July 2005, the period from July to the next June is consid-ered as one year. Figure 3 shows the seasonal variations of SO2 concentrations in different periods. It shows much higher SO2 concentrations in winter (heating period) and lower SO2 concentrations in summer. In cold season, SO2 accumulate under more emissions from heating, weak pho-tochemical conversion and atmospheric mixing, and in warm and rainy seasons, SO2 are easily removed by rapid photochemical conversion, more precipitation, and better

Figure 2 The time series variations of SO2 concentrations from July 2005 to August 2010.

atmospheric diffusion. On average, the ratio of the winter (DJF) concentrations to summer (JJA) concentrations is 2.6, which is less than that (4.8–6.9) at sites in north China plain [14, 16]. The annual mean values before July 2008 and after June 2008 are 14.0 ppb and 7.1 ppb, respectively. Those values are higher than that (2.8 ppb) observed at Jinsha sta-tion (29°38’N, 114°12’E, 750 m a.s.l.) in Hubei province and that (2.5 ppb) at Longfengshan, a background station in northeast China [32] and also higher than that of 7 g/m3 (about 2.7 ppb) in years of 2003–2006 at Melpitz (51°32′N, 12°56′E; 86 m a.s.l.), Germany, which is well representative of the regional SO2 background in Europe [33]. The value after June 2008 is similar with the level of SO2 (7.5 ppb) at Shangdianzi (117.12°E, 40.65°N, 293.9 m a.s.l.) [16], a background station in North China.

Figure 4 shows the average diurnal variations of SO2

Figure 3 The seasonal variations of SO2 concentrations in different years.

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Figure 4 The average diurnal variations of SO2 concentrations in differ-ent years.

concentrations in different years at Linan Station. The characteristics of SO2 after June 2008 are quite different from that before July 2008. First, the average value of 7.1 ppb after June 2008 is much lower than that (14.2 ppb) be-fore July 2008. Second, the average diurnal amplitude of 3.0 ppb after June 2008 is much lower than that (6.6 ppb) be-fore July 2008. In the latest year, the SO2 peak appeared at 10:00 instead of night peaks as the previous years showed. On average, the SO2 peak at 10:00 is less than 1 ppb higher than the SO2 level at night, but the difference is significant-ly at the 0.05 level according the results of Two Sample Paired t-Test. Similar as the mean value, the median value at 10:00 is also higher than that during other time. In the previous years, the SO2 maximum usually occurred at night (2:00) with a slow decline until 10:00, and it began to de-cline rapidly after 10:00 and the lowest value were at about 15–16 o’clock. The change of night SO2 peak to daytime peak indicates that the SO2 concentration in the mixed layer has been lower than that above the mixed layer height, re-flecting the weaker intensity of SO2 emission from low-er-level sources than that from elevated sources. In general, daytime SO2 peak can be attributable to regular advection transport and/or downward mixing of SO2-richer air. Figure 5 shows the average surface wind vectors in different years at Linan station. As showed in Figure 5, from 4:00 to 11:00 the prevail winds are constantly from southwest. Although winds from southwset brought much higher concentrations of SO2 as showed in Figure 9 in the following context, it cannot completely unpuzzle the SO2 peak at 10:00. During the period of about 5–6 hours before 10 o’clock, the average wind speeds were similar but with weaker convection mixing. Sources in the nearby areas, such as sources in the Linan Township about 10 km away from the station, cannot explain the peak at 10 o’clock because the air could be transported more than 25 km within 5 hours under the average wind velocity of 1.5 m/s. The SO2 peak at 10:00 at Linan station may be more reasonably explained by the downward mixing of elevated plume layers. The

Figure 5 Average surface wind vectors in different year periods at Linan station.

phenomenon could also happen in the north China [16].

4.2 The historical trend of SO2 concentrations and its significance

In this section, SO2 data (Table 1) before July 2005, which have been obtained in previous researches in different peri-ods, are used together to study the historical trend of SO2 concentrations. The monthly mean SO2 concentrations with ±1σ are plotted in Figure 6. As shown in figure 5, SO2 con-centration increased at first, then kept at high levels for years and decreased after 2006. The monthly values of SO2 concentrations were 11.9–24.8 ppb with an overall mean value of 16.1±7.4 ppb from August 1994 to July 1995, 7.2–24.6 ppb with an overall mean value of 13.5±5.1 ppb from July 1999 to June 2000, 8.7–26.4 ppb with an overall mean value of 15.5±4.9 ppb from July 2005 to June 2006, 5.2–24.7 ppb with an overall mean value of 13.3±6.2 ppb from July 2006 to June 2007, 3.7–21.2 ppb with an overall mean value of 13.3±5.8 ppb from July 2007 to June 2008, 4.0–14.4 ppb with an overall mean value of 7.8±2.9 ppb from July 2008 to June 2009, and 1.9–14.9 ppb with an overall mean value of 6.5±3.9 ppb from July 2009 to June 2010, respectively. It suggests that both the average level and the variability of SO2 changed little at Linan station from 1994 to 2007. By the middle 1990s, concentrations of acidic gases in the Yangtze Delta region had already increased to rather high levels because of rapid economic development and dense population in the region. Although the coal consumption in the Yangtze Delta region has significantly increased, there seems to be no significant increase in SO2 emission as suggested by the SO2 data from Linan station. This may be attributable to better desulfurization in this region [34]. After 2008, surface concentrations of SO2 showed a signifiacant decrease which might be due to much more effective and stricter pollution control measures to meet the air quality. For example, as shown in Figure 7(a), in Shanghai, total SO2 emission began

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Table 1 The surface SO2 data collection periods in this paper

Period Project SO2 analyzer Data source / Reference

Aug. 1991 to Nov. 1991 PEM-WEST A TEI Model 43A Zhou et al. [35]

Feb. 1993 to Mar. 1993 PEM-WEST B TEI Model 43S Luo et al. [36]

Aug. 1994 to July 1995 Chinese Ozone Research Program TEI Model 43S Yan et al. [37]

June 1999 to June 2000 China-MAP TEI Model 43S Wang et al. [38, 39]

July 2005 to July 2010 Operational long-term observation TEI Model 43C this work

Figure 6 The change of monthly mean SO2 concentrations at Linan from 1991 to 2010.

Figure 7 (a) Total SO2 emissions, SO2 reductions and annual mean SO2 concentrations in different years in Shanghai city; (b) total SO2 emissions in different years in Shanghai, Jiangsu, Zhejiang, Anhui, Fujian, and Jiang-xi Province. Data from Shanghai Environmental Protection Bureau (http://www.sepb.gov.cn) and from the Yearbook 2010 of National Bureau of Statistics of China (http://www.stats.gov.cn/).

to decrease from 2006 and the reduction of SO2 dramatically enhanced after 2008, at the same time, the annuly mean SO2 concentrations in urban area also decreased down to 0.029 mg/m3 (about 11 ppb) in 2010,

which is 36% lower than that (0.055 mg/m3) in 2007. In Figure 7(b), as compared with the emission levels in 2005– 2006, the total SO2 emissions in 2009 in Shanghai, Jiangsu, Zhejiang, Anhui, Fujian, and Jiangxi province were reducted with –25.8%, –19.7%, –18.4%, –6.8%, –9.7%, and –9.5%, respectively. The two different datasets show quite consistent trend.

4.3 SO2 reductions in recent years

In this section, we try to quantify the effectiveness of SO2 control measures which have been implemented in the recent years. All SO2 data are divided into two data sets from 2005 to 2007 and from 2008 to 2010 according to the results of Figures 3 and 4, which show very clear drop of the SO2 level from 2007 to 2008. The difference of the two data sets are used to estimate the reduction effectiveness of SO2. Figure 8 shows the frequency distributions and their Lorentz curve fittings of SO2 before and after 2008 at Linan. The distribution curve has shifted towards lower values and there have less extremely high values in 2008–2010 when comparing the results in 2005–2007. A peak value of 7.3 ppb during 2005–2007 and a peak value of 2.1 ppb during

Figure 8 Frequency distributions and Lorentz curve fittings of SO2 be-fore and after 2008 at Linan.

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2008–2010 are obtained. The peak values of Lorentz curve are often used to represent regional background levels over a defined area and in a defined time period [28]. A 5 ppb (71%) decrease of background level is calculated.

Surface SO2 concentration is not only determined by local SO2 emissions, but also can be affected by the transport of external SO2. Besides, the variation of the SO2 concentration depends on weather conditions, atmospheric diffusion, transmission, and photochemical reactions. These factors can be different from year to year, and they affect the comparability of SO2 concentrations in different time periods. The variation of wind speed and direction is usually associated with atmospheric dispersion and transport. Temperature is one of the most important factors which could affect the chemical reactions in the atmosphere. Relative humidity indicates the relative water content in the atmosphere, and it’s closely associated with the weather pattern. For example, the humidity in rainy or cloudy days is normally higher than that in sunny days. Therefore, the meteorological conditions influence the lifetime and transport of SO2. To minimize the meteorological effects, the two data sets are reclassified according to temperature, relative humidity (RH), wind speed, and wind direction. The average mixing ratios of SO2 for different intervals are calculated and the mean reduction of SO2 for different meteorology classes before and after Year 2008 are determined.

Figure 9 shows the mixing ratios of SO2 before and after 2008 as a function of relative humidity (RH), wind speed (WS), wind direction (WD) and temperature (T) at Linan

station. As shown in Figure 9, in all the different ranges of RH, temperature, wind speed, and wind direction, the con-centrations of SO2 before 2008 are significantly higher than that after 2008. The overall differences are 50%±4%, 53%±7%, 51%±5%, and 51%±6%, respectively, between the two reclassified data according to RH, temperature, wind speed, and wind direction. There are about 51% reduction of SO2 after 2008 based on the the surface measurement data presented in Figure 9.

5 Conclusion

Linan station is a good representative site for studying the background changes of atmospheric components in the Yangtze River Delta region. It shows much higher SO2 concentrations in winter (heating period) and lower SO2 concentrations in summer. On average, the ratio of the win-ter (DJF) concentrations to summer (JJA) concentrations is 2.6. A significant downward trend (P < 0.0001) was ob-served from 2005 to 2010, especially after 2008, which in-dicated the significant effectiveness of reductions in sulfur emissions in recent years in the Yangtze River Delta region, the most economically vibrant regions in China. The aver-age concentration of SO2 from July 2005 to June 2008 was 14.2±3.1 ppb, which is slightly higher than the mean values of 13.5±5.1 ppb during 1999–2000 and is two folds of the average value (7.1±3.1 ppb) from July 2008 to June 2010. More than 50% of the SO2 has been cut down after 2008 in the Yangtze Delta region due to the implementation of

Figure 9 Mixing ratios of SO2 before and after year 2008 as a function of RH, wind speed, wind direction and temperature at Linan.

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stricter emission control measures. The diurnal variation of SO2 after 2009 is quite different from that in the previous years as the peak SO2 concentration appears around 10 o’clock in the morning instead of the night, which might indicate that more SO2 is from the vertical exchange process than from the local accumulation.

We thank the operators of the Linan station for carrying out the measure-ments. This work was supported by the Planning Project Fund of Humani-ties and Social Science, Ministry of Education, China (11YJAZH071), the Basic Research Fund of CAMS (2011Z003) and CMA (GYHY200706036), and Desert Meteorological Foundation of CMA (Sqj2010012).

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