Characteristics of the Acid Rain Variation in China During and Associated Causes

Transcription

1 NO.2 ZHAO Yanxia and HOU Qing 239 Characteristics of the Acid Rain Variation in China During and Associated Causes ZHAO Yanxia 1 ( ) and HOU Qing 1,2 ( ) 1 Chinese Academy of Meteorological Sciences, China Meteorological Administration, Beijing Key Laboratory for Atmospheric Chemistry, China Meteorological Administration, Beijing (Received July 1, 2009) ABSTRACT The acid rain observation network of China Meteorological Administration was established in 1989 with 22 stations. From 1993 to 2005, more than 80 stations were included and maintained in the network. In , the number of stations in the network went up to 294. In consideration of the data continuity, data used in this paper are the 14-yr observations of the 80 stations from 1993 to Based on the 14-yr observation of acid rain, analysis shows that the acid rain in China dominates in the vast regions south of the Yangtze River. Limited presence of acid rain is observed in the northern part of China. The 14-yr acid rain data reveal an expanding tendency for acid rain area, with the north of China being a growing zone, and the South China remaining virtually unchanged. The most severely polluted zone of acid rain gradually moves from Southwest China to Central China and the middle part of South China. With regard to the acid intensity of rain, the period of bears the highest acid intensity; the period of shows a bit weakening intensity; and in the period of , the acid intensity of rain increases again, basically up to the average acidity of the period of by the end of In addition, rain acidity in the north of China increases markedly. As to the causes of the acid rain situation in China, this paper examined the sulfur dioxide emissions as well as the rainwater chemicals monitoring data. Key words: China, acid rain, ph value, frequency of acid rain, sulfur dioxide emission, rainwater chemicals Citation: Zhao Yanxia and Hou Qing, 2009: Characteristics of the acid rain variation in China during and associated causes. Acta Meteor. Sinica, 24(2), Introduction Acid rain is atmospheric precipitation with a ph value lower than 5.6. Acidity exists in rain, freezing rain, snow, hail, dew, and so on. Acid rain is mainly caused by sulfuric acid and nitric acid in water droplets, converted from SO 2 and NO x in the atmosphere (Fan, 2002). The term acid rain was first used by Smith (Cowling, 1982). In 1972, the Swedish government made acid rain an international environmental issue, for which the attention of governments around the world was called (Ottar, 1976). China started to pay more attention to acid rain problems in the end of 1970s. Chinese environmental authorities established in the first place an acid rain monitoring network across the country (Liu et al., 1997). After that, Chinese meteorological authorities set up a long term acid rain monitoring network in the country as well (Ding et al., 2004). Monitoring results show that in the mid 1980s, averaged annual rainfall with a ph value less than 5.6 was mainly found in the southwest, south, and southeast coastal areas of China (Dai et al., 1997). Acid rain catchments have witnessed a noticeable change since the 1990s, compared with the 1980s, featured with more areas affected by acid rain. For example, acid rain areas surrounding Nanchang and Changsha in the middle part of the country registered a contamination level exceeding the one that occurred in the southwest, a traditional catchment of acid rain. The southern part of China saw acid rain mainly in the Pearl River Delta and the east part of Guangxi Province, with a basic distribution pattern unchanged. The acid rain areas in the eastern part of China, including the lower and middle reaches of the Yangtze River and the south coastal areas along Xiamen, had small fluctuations in contamination patterns (Tian et al., 2001). As a result, the areas with a ph value lower than 5.6 for Supported by the National Natural Science Foundation of China under Grant No Corresponding author: zyx@cams.cma.gov.cn. (Chinese version published in Vol. 66, No. 6, 2008)

2 240 ACTA METEOROLOGICA SINICA VOL.24 averaged annual rainfall accounted for 40% of the Chinese territories. At least 50% of the areas in the southern part of the lower and middle reaches of the Yangtze River were registered with a ph value even lower than 4.5, making the region an area hit harder by acid rain in the country. 1.1 Acid rain studies in China During the period from 1986 to 2000, Chinese scientists studied the formation of acid rain and its transport, developed numerical models, and examined its impacts on the ecosystem and associated control measures. During , an acid rain research project was launched to investigate acid rain in the southwest and the southern part of China in the context of the formation of acid rain and associated transport, control methods, and impacts on the ecosystem. Study results indicate that both Chongqing and Guizhou were seriously polluted by acid rain, with a heavy sulphate concentration in rainwater, making the two areas a typical case of sulfuric acid contamination. The temporal distribution shows that the southwestern part of China saw more acid rain in winter and spring. In the case of Sichuan and Chongqing, special meteorological conditions and energy structures were the major contributors to the formation of acid rain. During , acid rain studies started to cover coastal areas in the east and the middle part of the country. Two cities, Qingdao and Xiamen, were selected as the target areas of case studies, in an attempt to understand the cause and source of acid rain contamination, their input and output in the two areas, and their association with inland acid rain contamination (Liu et al., 1997). During , Chinese scientists assessed the impacts of acid deposition on crops and forests and associated economic losses, identified the principles, methodology, and indicators in evaluating and screening the technologies applicable to acid deposition control, and came up with control plans and response strategies based on critical loads of sulfur deposition (Hao et al., 2001). Additionally, some studies were conducted to understand the transport process of atmospheric pollutants, and a sulfides transport model was developed based on the reliable SO 2 emission inventory, in an attempt to calculate the transport of the pollutants across provinces and countries (Wang Zifa et al., 1997). During , efforts were made to work on the areas affected by SO 2 and acid rain and associated control plans. An SO 2 control area was defined in northern China, and an acid rain control area was outlined for the southern part. During , the Ministry of Science and Technology kicked off an initiative to study the formation of acid rain and associated control under the national 973 program. 1.2 Formation of acid rain One cannot understand the formation of acid rain without sorting out its causes in the first place. Analysis of the composition of acid matters in acid rain reveals that the acid rain that occurred in China was mainly caused by unrestricted emission of SO 2 (Fan, 2002). As a result, acid rain was also called soot rain or sulfuric acid rain. In recent years, many large and middle sized cities have witnessed increased NO x emissions due to increased on-road motor vehicle emissions. Accordingly, the composition of acid rain saw an increased concentration of nitric acid (Mei et al., 2005; Liu et al., 2006; Tang et al., 2005). Under increasingly enhanced emissions of acid matters, such as SO 2 and NO x, acid rain was changing from being mainly dominated by sulfuric acid to by both sulfuric acid and nitric acid (Liu et al., 2006; Tang et al., 2005; and Zhu et al., 2006). Meanwhile, suspended particulate matters that can neutralize acid rain have gone down in concentration, which made acid rain control more complicated (Wang and Gao, 2007). Studies show that North China had more acid radical ions in acid rain than the south, though the former had less acid precipitation than the latter, implying that the concentration of atmospheric SO 2 is not positively correlated with the occurrence of acid rain and associated acidity (Wang, 1993, 1994). This means the formation of acid rain has certain precursors, such as SO 2, though it is not a sufficient condition. Sufficient conditions include the combined effects of a range of natural factors (Hua et al., 1998). Natural factors, including ammonia in the atmosphere, soil acidity and

3 NO.2 ZHAO Yanxia and HOU Qing 241 alkalinity, alkaline matter concentration in atmospheric particulates, can make a difference in shaping up the regional features of acid rain (Feng, 2004). Wang (1993, 1994) found that China had a higher concentration of atmospheric particulates, compared with other countries, especially coarse particulate matters, with a significant difference between the north and the south. For example, the northern part of China was high in alkaline matters. As a result, atmospheric particulates had more alkaline matters than acid matters. When coming to precipitation, such atmospheric particulates were able to neutralize acid precipitation. On the contrary, in the south, atmospheric particulates were low in alkaline matters, with a reduced buffering effect, compared with the north. The concentration of alkali metal ions in soil and associated ph value was another major player in the formation of acid rain. Alkaline matters in precipitation were mainly stemmed from soil. In China, the concentration of alkaline particulates in soil went up from the south to the north, which explained why acid rain mainly occurred in the south where low soil alkalinity and ph value dominate. Wang Wenxing et al. (1997) believed that acid rain occurring in the east and southeast coastal areas had a complicated source, as these areas were not only influenced by the continent in the west but also by Japan and the Republic of Korea. In the case of Qingdao, a coastal town that was hit hard by acid rain, acid matters discharged from the contemporary industrial development and from the natural release of marine dimethylsulfide were the major causes for the formation of acid rain. Special features of boundary layer wind and temperature fields above the Qingdao area was another major cause contributing to the formation of acid rain (Liu et al., 1997; Feng, 2004). However, a town located in the southern coastal area could make a different story. For example, Xiamen was not featured with a high concentration of SO 2, NO x, and PM 10, though with a high concentration of ozone and hydroperoxide. Strong oxidation and low concentration of ammonia resulted in reduced alkaline concentration, which in turn contributed to the formation of acid rain there (Zhuang, 1998). Acid rain that occurred in Sichuan and Chongqing were mainly caused by high SO 2 concentration in the atmosphere as a result of high sulfur coal used in the locality, plus special geographic environment and meteorological conditions (Xu and Lan, 2005). Poor weather conditions, such as calm winds and temperature inversion, facilitated the formation of acid rain (Dong et al., 2003; Wu et al., 2004; Huang et al., 2004; Lin et al., 2005). The distribution of rainfall ph values across China has somewhat agreed with the distribution of surface wind fields at the same scale. The Sichuan Basin, Yunnan-Guizhou Plateau, and some areas in Guangxi were the calmest areas with high occurrences of both calm winds and acid rain (Tian et al., 2001). Other meteorological conditions, such as the amount of rainfall, may also pose a major effect on the formation of acid rain (Lin et al., 2005). Generally speaking, the washing-up effects of local precipitation and the medium to long range transport constituted a decisive source for the occurrence of acid rain in most southern areas (Hua et al., 1998; Mao, 1992; Wang et al., 1992). In small- and medium-sized cities and in the rural areas where pollution was relatively light, the acidity of rainfall was mainly determined by the in-cloud process (Huang et al., 1995). 1.3 Damage and associated control measures Acid rain can cause damage in different forms. In addition to its adverse effects on ecological environment, including soil, forest, agriculture, and water body, acid rain may also erode the surface of structures, compromise people s health, and cause huge loss to the national economy (Feng, 1993; Cao et al., 1993; Fan, 2003). Acid rain facilitates the release of nutritional elements, including potassium, sodium, calcium, and magnesium, from soil. The released nutrients would be washed away by rain water, making soil barren and affecting the normal development of plants (Fu and Tian, 2006; Xing, 2002; Chen et al., 2007). Additionally, acid rain would result in more diseases and pests in trees and crops, reduce yield, and deteriorate forests (Liu, 2003; Li et al., 2005). Rivers and lakes affected by acid rain would have reduced alkaline matters in water bodies. In some serious cases,

4 242 ACTA METEOROLOGICA SINICA VOL.24 one could see fish and shrimps dying in the water affected by acid rain, with reduced biospecies of planktons. Acid rain is also a killer to building materials and cultural relics (Zhang et al., 2002). Partial findings derived from the studies conducted during show that acid rain in 11 provinces and autonomous regions, including Jiangxi, Hunan, Guangdong, Guangxi, Sichuan, Guizhou, Hubei, Jiangsu, Zhejiang, Anhui, and Fujian, has caused annual economic and ecological loss worth RMB 45.9 billions (Chen and Chai, 1997). An effective approach to place acid rain under control is to cut down SO 2 and NO x emissions. Industrialized nations, where acid rain started first, have worked on a full-fledged environmental pollution control starting from the late 1970s. Chinese government has paid great attention to acid rain contamination caused by emissions. For example, a policy document on placing acid rain under control was published and implemented in In 1992, efforts were made to charge SO 2 emissions from industrial sources and to curb the development of acid rain in nine municipalities of two provinces (Yang, 2002). In a number of the State policy documents, it is clearly stipulated that the development of acid rain shall be curbed in two major affected regions, and SO 2 contamination shall be placed under control (Liu, 2001). In January 1998, the State Council approved the acid rain and SO 2 contamination regional control plan, in a move to address the increasingly serious contamination problem (Yang, 2002). Unfortunately, in recent years, higher SO 2 emissions outside the two major affected regions have, to some extent, offset the control results within the affected regions, which only account for 11.4% of the national territories. Apparently, the spatial distribution of SO 2 emissions has been changed. In addition, increased NO x emission has resulted in enhanced concentration of acid particulates. Meanwhile, suspended particulate matters that are able to neutralize acid rain saw an annual descending trend. These developments have made acid rain issue more complicated than 20 years ago (Wang and Gao, 2007). In the past 30 years or so, great efforts have been made both at home and abroad to understand the causes and damage of acid rain, to analyze major chemical ions in precipitation, and to work out strategies and measures for curbing the development of acid rain. Impressive findings have been derived from the efforts. Some scientists made a comprehensive summary of acid rain studies (Ding et al., 1997; Xie et al., 2004; Shannon, 1999; Menz and Seip, 2004; Wu et al., 2006). The present study attempts to present a general picture of the acid rain variation in China, through objective analysis of acid rain data collected by more than 80 acid rain monitoring stations of China Meteorological Administration (CMA) in the past 14 years. The study is also accompanied with preliminary analysis and discussion of the causes for the variation. 2. Data The acid rain monitoring network, developed by CMA, is an operational network supported by weather stations across the country. Designed to reflect regional atmospheric acid deposition, the network has covered most provinces, municipalities, and autonomous regions (Hong Kong, Taiwan, and Macao are not included), and is in a position to reflect the spatial and temporal distributions of regional atmospheric acid deposition (Ding et al., 2004). CMA launched its acid rain monitoring operation at 22 stations in In the early 1990s, CMA started to build a nationwide network, with an increasing number of acid rain monitoring stations, from 81 in to 85 in , and further to 87 in During the period of , stations went up to 88 in number. In 2006, 69 national-level stations were built for the purpose, which made the total number of stations with acid rain monitoring reach 157. Furthermore, acid rain monitoring stations established by provincial weather bureaus have all been incorporated into the national network under a unified management. In 2007, there were 294 stations in the CMA acid rain monitoring network. For the sake of continuity and compatibility, 14-yr observational data collected by more than 80 stations starting from 1993 were employed for

5 NO.2 ZHAO Yanxia and HOU Qing 243 events with a ph value lower than 5.6 (5.6 is excluded), before calculating the frequency of acid rain through the following equation: F [<5.6] = N [<5.6] N total 100%, (4) Fig. 1. The acid rain observation network of China Meteorological Administration. Information is as accurate as till June statistical analysis in this study. The distribution of stations is given in Fig Annual mean ph value and frequency of acid rain In this study, 14-yr daily acid rain data collected at more than 80 stations were applied. Annual mean ph value and averaged annual frequency of acid rain were calculated in line with the methodologies defined by CMA Acid Rain Observation Practice (2005) Annual mean ph value Here, the hydrogen ion concentration [H + ] weighted rainfall method was used. In the process, daily rainfall ph value was converted into hydrogen ion concentration, before being multiplied with corresponding daily rainfall. The sum was divided by annual rainfall total to obtain averaged hydrogen ion concentration. Negative logarithm was made to obtain annual mean ph value. It is calculated as follows: [H + ] i = 10 phi, (1) [H + [H + ] i V i ] =, (2) Vi ph = lg[h + ], (3) where ph i represents ph value of daily precipitation, ph is rainfall weighted mean ph value, and [H + ] i is hydrogen ion concentration (mol L 1 ) derived from the calculation of daily precipitation ph value. V i is daily rainfall, and V i is annual rainfall in mm Frequency of acid rain attacks (F ) One has to screen out the occurrences of rainfall where F [<5.6] depicts the frequency of ph value <5.6, and N [<5.6] the times of rainfall events with a ph value lower than 5.6. N total means the number of ph value observations in the year. The frequency of severe acid rain, whose ph value is lower than 4.5, can be calculated similarly. 2.2 Precipitation chemistry At the present stage, only four CMA stations collect precipitation chemistry data. They are: Waliguan Baseline Observatory in Qinghai a global atmospheric baseline station, and three regional atmospheric baseline stations located in Longfengshan, Heilongjiang Province (representing the region of Northeast China), Shangdianzi in Beijing (Beijing-Tianjin-Hebei and adjacent areas), and Lin an in Zhejiang (the Yangtze River Delta area), respectively. Basic facts about these stations are summarized in Table 1. Four stations provide accurate and reliable data, thanks to the fact that they are part of the WMO global atmospheric baseline network operating under a unified international standard. In this study, only the precipitation chemistry data collected by three regional baseline stations from 1999 to 2006 were used, as Waliguan Station sits in an area without acid rain. Averaged annual ion concentration is the weighted average of individual rainfall and ion concentration. Table 1. Information about four stations that observe precipitation chemistry in China Shangdianzi Lin an Longfengshan Waliguan Longitude E E E E Latitude N N N N Elevation (m) SO 2 and NO x data The SO 2 emission data were mainly quoted from China Environment Statistics Bulletin and the Environment Bulletin published by local provinces. The national bulletin data cover the period from 1997 to 2005, while the provincial data from 2000 to 2005.

6 244 ACTA METEOROLOGICA SINICA There is no statistics for NOx emission so far, though energy and environment authorities have published some estimates. The estimates have shown a consistent trend, albeit with different values. 3. Acidity of precipitation In China, acid rain mainly affects the vast region in the south of the Yangtze River, though it also attacks some areas in the north. Averaged annual rainfall ph value shows that more areas have been affected by acid rain in the past 14 years, with increased areas in the north, though the southern part of the country basically remained the same in terms of the affected area size. With regard to the intensity of acid rain, the period of registered acid rain with the largest intensity, though followed by a reduced intensity in During the period of , acid rain became more intense again, with 2006 sitting at the average level of Some areas have seen more acid rain compared with 1998, though VOL.24 a few areas have reduced occurrences of acid rain. Figure 2 indicates that during , the severe acid rain area (annual mean rainfall ph < 4.5) represented by Sichuan and Guizhou provinces had noticeably narrowed down, and the super acid rain area (annual mean rainfall ph < 4.0) was disappearing. Unfortunately, Hunan, Guangdong, and Jiangxi had intensified acid rain. There even appeared some areas in the east of Hunan and west of Jiangxi, which had an annual average rainfall ph value lower than 3.5, implying that heavily polluted areas were shifting from the southwest to the middle part of China and to the middle part of South China. Additionally, some areas in North China have been attacked by higherintensity acid rain. In Fig. 2d, the severe acid rain area was expanding northwards. As a matter of fact, acid rain attacks were eased thanks to the control policies implemented by the end of the 1990s. As of the end of 1999, 98 of 175 cities affected by acid rain had met the SO2 emission standard ( Meanwhile, CMA acid rain Fig. 2. Average ph values of precipitation over (a) , (b) , (c) , and (d) the annual average ph value of 2006.

7 NO.2 ZHAO Yanxia and HOU Qing 245 monitoring network made the period enjoying the least acid rain contamination in the past 14 years. Unfortunately, the later bouncing back of emissions (see Section 5) worsened the acid rain contamination. Therefore, the current intensity of acid rain is only weaker than the worst period, when viewing the country s situation as a whole. 4. Frequency of acid rain attacks As far as the frequency of acid rain attacks is concerned, the South China saw a different story from the north. In the past 14 years, the eastern part of Southwest China (Yunnan, Guizhou, and Sichuan) had less acid rain, with more acid rain in the middle part of the country and in the middle part of South China (Hunan, Jiangxi, and Guangdong). The southern part of East China (Zhejiang and Fujian) had kept a basically unchanged status. In Fig. 3, during the period of , the areas hit hard by acid rain with a frequency larger than 80% were located in Guizhou, Chongqing, the northeast of Hunan, west of Jiangxi, the middle and southern parts of Guangdong, and some areas in Zhejiang. The areas hit hard by acid rain started to narrow down with a reduced intensity in the period of , though the west of Jiangxi failed to see an eased situation. After 2003, Guizhou and the western Guangxi started to enjoy a noticeably reduced attack of acid rain, while the northeast of Hunan, Jiangxi, and Guangdong merged into a large area with more acid rain. Changsha and Ganzhou registered a frequency as high as 90% or more, so did Guangzhou at 97% or more during In addition, Chongqing, a traditional area hit hard by acid rain, had a frequency of attack at 97% or more. Both Zhejiang and Fujian basically sat around 80%, with a few areas, such as Lin an in Zhejiang, reaching 90% or more marked a year with the largest number of areas suffering 80% or more frequency of acid rain attacks in the history. The northern part of China secured less attacks of acid rain during the period of except The Beijing-Tianjin-Hebei area and some parts of Henan had increased attacks of acid rain by 20% 50% from Shandong also had more areas hit by acid rain at a frequency of 50% or more. During the period of , a few areas in northwest Xinjiang and the southern Heilongjiang had recorded a frequency of acid rain attack larger than 50%. For acid rain attacks with a ph value less than 4.5, the southern China had seen a trend similar to the above-mentioned variation, with a frequency ranging from 20% to 50%. A few areas in Guizhou, Hunan, Jiangxi, and Guangdong had recorded a frequency exceeding 80%. The northern part of China saw no large change in the occurrence of intense acid rain, mostly at a frequency of 20% or less. 5. Acid rain variation patterns and associated causes Many factors contribute to the formation of acid rain. Local small-scale acid rain is not only associated with local emissions, but also manipulated by a range of meteorological conditions, including wind direction, wind speed, and temperature inversion. However, when taking China as a whole, acid gas released from human activities, such as SO 2 and NO x, makes a major contributor to the formation of acid rain. The conclusion is confirmed by the agreement between SO 2 emissions and acid rain distribution. In addition, precipitation chemistry data collected by atmospheric baseline stations also make it a strong case. China has so far produced no quantitative data of NO x emissions. However, the estimates published by the State Development and Reform Commission and the former State Environmental Protection Administration have shown a marked ascending trend for NO x in recent years. 5.1 SO 2 emissions in China China s total SO 2 emission registered for the period of (Fig. 4) shows that the period of kept a lower emission level which was confirmed by observational results. Emission started to rise in a steady manner after As a result, acid rain monitoring stations recorded a sustained decrease of rainfall ph value, though with enhanced acidity and increased frequency of acid rain attacks. Apparently,

8 246 ACTA METEOROLOGICA SINICA VOL.24 Fig. 3. Average acid rain frequency over (a) , (b) , (c) , and (d) the annual average ph value of place atmospheric pollution including acid rain contamination under control, though the economic development shall not be compromised as a precondition. 5.2 Precipitation chemistry Fig. 4. The SO2 emission in China during the magnitude of acid rain contamination is closely associated with regional SO2 emission. One can see from Fig. 4 that SO2 emission released from daily life was basically unchanged, and the general development trend fully agreed with that of the industrial emission, implying that acid rain control shall be focused on industry by using extensive desulphurization techniques. Efforts shall also be made to define emission standards and indicators for large emission sources, in order to Of the chemical ions contributing to atmospheric + precipitation, a number of ions, including SO2 4, NH4, Ca2+, and NO 3, are highly concentrated, with some others, such as Mg2+, Na+, K+, Cl, H+, and F, in relatively low concentration. Of the said elements, sulfuric and nitric acids are the most important acid elements for the formation of acid rain in China. Figure 5 shows a sustained ascending trend for both SO2 4 and, confirmed by the observational results at three NO 3 regional baseline stations. In Fig. 6, three stations showed a basically descending trend for rainfall ph value (in Figs. 5 and 6, the data collected by Shangdianzi Station in 2002 was poor in quality due to local construction activities), which agreed with the variation of regional rain acidity in China. However, the

9 NO.2 ZHAO Yanxia and HOU Qing 247 data furnished by three regional baseline stations are not in a position to represent the situation in the whole country, especially the severe acid rain areas in Southwest and South China. In these areas, there are no stations designed with precipitation chemistry measurement. Monitoring results of precipitation chemistry provided by environmental protection authorities and research projects show that in recent years both the eastern part of Southwest China and South China have seen a basically ascending trend for SO 2 4 and NO 3, with different magnitudes in different areas. Furthermore, NO 3 has played an increasingly large role in shaping up acidity, though SO 2 4 remains a key player (Mei et al., 2005; Liu et al., 2006; Zhang, 2007). 6. Case study: acid rain in North China As it was analyzed in the above, acid rain contamination has become appreciably enhanced in the northern part of China. To illustrate the fact in a more objective manner, some provinces and municipalities hit but not seriously by acid rain, including Beijing, Tianjin, Hebei, Henan, Shandong, Shanxi, and Shaanxi, were selected as the targets for statistical analysis. Figure 7 shows that even in the period of featured with rampant acid rain attacks in the south, both North China and the Yangtze-Huai River Valley saw light attacks of acid rain in terms of both acidity and frequency. Unfortunately, during , the same regions witnessed a rainfall ph value that quickly dwindled, with a pronouncedly enhanced intensity, and increased frequency for both regular and intensified acid rain events. In 2006, the worst acid rain contamination in the past 14 years occurred. Sixteen monitoring stations in 7 provinces and municipalities have recorded an averaged annual rainfall ph value of 4.57 with a frequency of acid rain attacks at 45%, and severe acid rain attacks at 23%. The data collected by the Shangdianzi Station (covering Beijing- Tianjin-Hebei area) showed an averaged annual rainfall ph value of 4.76, 4.43, 4.52, and 4.31, respectively, for the period of , with a corresponding frequency of acid rain attacks at 40%, 47%, 51%, and 67%. The above variation is associated with the Fig. 5. SO 2 4 and NO 3 concentration changes at three regional baseline stations during Fig. 6. Annual average ph changes for three baseline stations during Fig. 7. Rain water acidification of seven provinces in northern China in the period of

10 248 ACTA METEOROLOGICA SINICA VOL.24 enhanced emissions of SO 2 and NO x, a result of economic development. Both Henan and Shandong are good examples (Fig. 8) for illustrating the increasing SO 2 emission. In 2005, Henan had a SO 2 emission that was twice the amount in Shandong has sustained an increasingly large SO 2 emission in recent years. In fact, Heilongjiang, Jilin, and Liaoning provinces of Northeast China have also witnessed an acid rain contamination that has become more severe. No discernable improvement in the south and more acid rain in the north characterized the variation of acid rain in China. 7. Conclusion and discussion Overall speaking, China has its acid rain mainly over a vast area in the south of the Yangtze River. Chongqing, Hunan, Jiangxi, and Guangdong are the areas hit hard by acid rain in the country. In addition, the northern part of the country has some areas affected by acid rain that are by no means small, mainly over the Beijing-Tianjin-Hebei area and some areas in Shandong. When viewing the country as a whole, more areas have been attacked by acid rain in the past 14 years, though with a slightly weakened intensity. The northern part of the country, in particular, has seen more areas hit by acid rain with an enhanced intensity, though the southern part of the country basically remained the same in terms of the area size affected by acid rain. It is necessary to adopt effective measures to control the acid rain contamination situation. The acid rain in China is mainly caused by acid gases released from human activities, such as SO 2 and NO x, mostly from industrial sources. The conclusion is supported by the agreement between the variation of SO 2 emission and that of the acid rain and its distribution. In addition, the ascending trend of SO 2 4 and NO 3 in precipitation recorded by the regional atmospheric baseline stations also supported this conclusion. This study discusses the variation of acid rain in China as a whole. Further studies and precipitation chemistry analysis are needed to understand the impacts of on-road motor vehicle gas emission on the development of acid rain in medium- and large-sized cities. Unfortunately, limited precipitation chemistry data make the investigation of the causes of acid rain at the local or smaller scale impossible. This hampers us from working out rational policies and measures to deal with acid rain. It is suggested that more precipitation chemistry observation activities be initiated in the future. Acknowledgements. The authors would like to thank Wang Yaqian for preparing Fig. 1. Thanks also go to Ding Guoan for his kind review and comments. Fig. 8. The SO 2 emission in (a) Henan and (b) Shandong provinces during

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