Bulk sulfur (S) deposition in China
Introduction
Rapid economic growth and increased energy command have resulted in severe air pollution problems throughout China (Larssen and Carmichael, 2000, Pan et al., 2013), indicated by the high levels of mineral acids (predominately sulfuric) observed in precipitation. In the early 1980s, the area affected by acid rain covered about 1.7 million km2, mainly in southeastern China; By the mid-1990s, the area had expanded by more than 1 million km2 to the areas of south of Yangtze River, east of Qinghai-Tibet Plateau, and the Sichuan Basin (Pu et al., 2000). Altogether, 40% of China experienced annual average precipitation with a pH less than 5.6, the threshold for pollution induced acidity.
To halt the trend of increasing SO2 emissions and worsening acid rain by 2000, China has embarked on an ambitious plan to mitigate acid rain within its borders by restricting emissions of SO2. The SO2 emissions have changed dramatically since 2000. During the Chinese 10th Five-Year Plan (FYP) period (2001–2005), the State Environmental Protection Administration (SEPA) – changed to the Ministry of Environmental Protection (MEP) in March 2008 – set a goal to reduce the national SO2 emission level in 2000 (20 Tg/year) by 10% by the year 2005 (i.e., to 18.0 Tg/year). However, this goal was not achieved. The national SO2 emissions in 2005 actually rose to 25.5 Tg/year, 42% higher than the goal, reported by the China MEP (Lu et al., 2011). Moreover, this value was believed to be underestimated due to the neglect of emission from rural industries and biofuels (Zhang et al., 2009).
The Chinese 11th FYP set a target of 10% reduction in SO2 emissions from 2005 to 2010. Although the 11th FYP finished in advance and the whole national SO2 emissions decreased, the remote sensed OMI SO2 vertical column densities (VCDs) monitored showed that the OMI SO2 had not reduced in the year 2010 but increased instead (Yan et al., 2014). The OMI SO2 columns increased from 614.78 molec./cm2 in 2006 to 904.71 × 1013 molec./cm2 in 2010, and also the OMI SO2 increased in four areas, (1) Hebei, Shandong and Shanxi provinces, (2) Anhui, Jiangsu, Shanghai and Zhejiang provinces, (3) Beijing provinces and (4) Xinjiang, Tibet and Qinghai provinces; decreased in two areas, (1) Sichuan and Chongqing provinces, (2) Pearl River Delta. Thus, although SO2 emissions in China decreased in year 2010, the SO2 observed by satellite has not decreased.
Previous studies on acid deposition have focused on the precipitation pH (Zhao et al., 2009). High levels of strong acids in precipitation were not widely detected in China due to the fact that in the northern China acid deposition is heavily influenced and modified by natural dust from dessert and semi-arid areas (Wang et al., 2012, Zhang et al., 2012). Due to the neutralizing capacity of the dust, precipitation pH in this part of China remained high despite the high S emissions and a large quantity of sulfuric acid in precipitation. In the southern China, the influence of the dessert areas in the north and northeast is less obvious and low precipitation pH has been reported in some local regions (Wang and Han, 2011, Zhao et al., 2013).
Most studies on acid deposition have focused on the distribution of pH, while a few studies concerned sulfur deposition (Sdep). Sdep originating largely from man-made emissions of sulfur dioxide (SO2), is a global environmental issue because of its transboundary effects on the biogeochemical cycles (Kuribayashi et al., 2012, Pan et al., 2013). Excessive Sdep can cause long-term harmful effects with several deleterious consequences, such as the eutrophication of coastal waters and acidification of lakes (Doney et al., 2007), streams and soils (Guo et al., 2010, Scherer, 2001), which also reduce species diversity (Aherne and Posch, 2013). With sulfur deposition posing a serious environmental hazard, it is critically important for decision makers to determine the emission control strategies for effective mitigation and the extent of emissions reduction that would promote ecosystem recovery from Sdep. Sulfur deposition has been recognized as a major environmental problem in Europe and North America for several decades, while in China it was recognized somewhat later and considered among the most pressing environmental issues (Zhao et al., 2007). To address these concerns, it is essential to gain a quantitative knowledge of Sdep throughout China.
Sulfur deposition occurs via wet and dry process. For quantification of atmospheric deposition on a national scale, long-term monitoring networks such as a nationwide campaign for acid rain measurement from 1981 to 1990 by the Chinese National Environment Bureau, and the four cities in the Acid Deposition Monitoring Network of East Asia (EANET) have been established; such networks are essential for quantifying both wet and dry deposition and revealing long-term trends and spatial patterns under major environmental and climate change (Xu et al., 2015). Also some publications regarding field experiments focused on some local regions (Pan et al., 2013, Qiao et al., 2015). However, it is still difficult to get knowledge of the status of Sdep throughout China, mainly because no shared, convincing or enough data could be obtained. Thus, recent efforts have been made to model Sdep on a regional scale based on emission data (Kuribayashi et al., 2012, Rodhe et al., 2002). Large uncertainties in their study resulting from large uncertainties in the chemistry transport model and very limited long-term observation data obtained (Kuribayashi et al., 2012, Larssen et al., 2011).
In the present study, we complemented the previous study by additional estimation of the atmospheric Sdep via precipitation (bulk deposition), systemically collected from the publication sources during 2000–2013, based on the fact that most measurements in China were bulk deposition including wet and dry deposition through particles (Liu et al., 2013, Liu et al., 2015b). The wet deposition refers strictly to wet-only deposition which is collected only during rainfall and snowfall events (the samplers are closed outside the periods of precipitation). The bulk deposition refers to rainfall and snowfall samples collected using traditional rain gauges which are open permanently. The objective of this study was to explore the spatial variations and the source apportionment of bulk Sdep throughout China. The knowledge gained from this study will provide a scientific basis for the crafting of control strategies to reduce emissions and ecological impact in China.
Section snippets
Data collection
To evaluate the status of bulk Sdep across China, it is essential to establish a systematic dataset of an observation network on a national scale. In this study, the data sets on chemical composition of atmospheric precipitation during 2000–2013 were collected. These studies were located by making a search through ISI Web of Knowledge using Keywords “sulfur deposition”, “chemical composition” or “precipitation” and “China”, and through CNKI website using the same Chinese keywords. Finally, 105
Chemical composition of atmospheric precipitation
A statistical summary of the mean concentrations of major ions in precipitation is presented in Table 1. The geometric mean rather than the arithmetic mean was used to do the analysis when calculating bulk concentration in different regions or the whole China because the methods that consider the corresponding area around observation sites should produce more accurate results than averaging methods (He et al., 2015). The concentrations of the volume weighted mean (VWM) of ionic species in the
Conclusion
Sulfur (S) deposition (Sdep) originating largely from man-made emissions of sulfur dioxide (SO2), is a global environmental issue because its transboundary effects on the biogeochemical cycles. The atmospheric Sdep via precipitation (bulk deposition), systemically collected from publication sources during 2000–2013, based on the fact that most measurements in China were bulk deposition. The major findings were following as:
- 1
High bulk Sdep occurred across the south of Middle Yellow River (MYR),
Acknowledgment
This study is supported by the National Natural Science Foundation of China (No. 41471343, 41101315 and 41501212).
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