Trends in Air Pollution During 1996-2003 and Cross-Border Transport in City Clusters Over the Yangtze River Delta Region of China

12345678901234567890123456789012123456789012 12345678901234567890123456789012123456789012 Air quality data from city clusters in the fast developing Yangtze River 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 Delta (YRD) region of China during 1996 2003 were analyzed, with a 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 cross-border transport study using the Regional Acid Deposition Model 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 System (RegADMS). Investigations show that the annual average concen1234567890123456789012345678901212345678901234 trations of SO2, NO2, PM10 are 12 ~ 64 , 13 ~ 57, and 79 ~ 184 μg m 3 − , 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 respectively. As the primary air pollutants in the target area, surface NO2 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 levels increased by 13% while PM10 levels decreased by 9% from 1996 to 1234567890123456789012345678901212345678901234 2003. The surface SO2 concentration showed fluctuations during the study 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 period, reaching a minimum in 1999 and rising again in 2003. Acid rain 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 still remains an important atmospheric environmental issue. The frequency 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 of acid rain was about 23.5 ~ 36.7%, and the pH value of precipitation 1234567890123456789012345678901212345678901234 ranged from 5.09 to 5.48, with little change in these years. Modeling studies 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 indicated that sulfur deposition and nitrogen deposition were in the ranges 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 0.5 ~ 10 and 0.2 ~ 5 g m yr , respectively; these levels exceed the critical 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 load in some regions. The trans-boundary transport of sulfur deposition 1234567890123456789012345678901212345678901234 and nitrogen deposition due to SO2 and NOx emission among city clusters 123456789012345678901 345678901212345678901234 1234567890123456789012345678901212345678901234 (Shanghai and the other 8 cities in Jiangsu Province, including Nanjing, 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 Wuxi, Changzhou, Suzhou, Nantong, Yangzhou, Zhenjiang, and Taizhou) 1234567890123456789012345678901212345678901234 in the YRD region was significant. The emission from Shanghai contrib1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 utes 5% ~ 29% of sulfur deposition and 3% ~ 30% of nitrogen deposition 1 Department of Atmospheric Science, Nanjing University, Nanjing, China * Corresponding author address: Dr. Ti-Jian Wang, Department of Atmospheric Science, Nanjing University, Nanjing, China; E-mail: tjwang@nju.edu.cn doi: 10.3319/TAO.2007.18.5.995(A) Terr. Atmos. Ocean. Sci., Vol. 18, No. 5, December 2007 996 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 1234567890123456789012345678901212345678901234 in the 8 cities, while the 8 cities contribute 27.5% of sulfur deposition and 12345678901234567890123456 12345678901234567890123456 20.2% of nitrogen deposition in Shanghai. 12345678901234567890123456789012123456789 12345678901234567890123456789012123456789 (

Table 1 shows that the annual average SO 2 concentrations for the 9 cities in 2003 were between 30 and 54 µg m −3 , which do not exceed the national standard Band II for air quality (60 µg m −3 ).The SO 2 concentration in Wuxi is the highest, with the next being Shanghai and Taizhou.The annual average concentrations of NO 2 for the 9 cities range from 19 to 57 µg m −3 , which are below the national standard Band II (80 µg m −3 ).However, NO 2 concentration is much higher in large cities (Shanghai, Nanjing, Suzhou) with heavy traffic compared to other cities, suggesting that the contribution of transportation on NO 2 concentration is more important.The annual concentrations of PM 10 vary from 97 to 132 µg m −3 .Except for Shanghai, PM 10 concentrations in the 8 cities of Jiangsu Province reached or exceeded the national standard Band II (100 µg m −3 ), suggesting more severe particulate pollution in urban areas due to fast 1234567890123456789012345678901 1234567890123456789012345678901 developing industry, transportation and construction.

3.1.2 Acid Rain Frequency and pH Values
In 2003, the annual average pH value of precipitation for the 9 cities was 5.16, ranging from 4.73 to 6.08 (see Table 2).The acid rain frequency averaged over the 9 cities is 34.2%.

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Here, acid rain frequency is defined as the percentage of day when pH is less than 5.6, with pH = 5 6 .being the unpolluted value for rainwater.The lowest pH (4.73) and highest acid rain frequency (62.4%) occurred at Nantong, with the next being Nanjing, Yangzhou, Wuxi and Suzhou, with frequencies above 40%.Monitoring data in recent years show that the pH value of acid rain in the Yangtze River Delta region kept fluctuating.Generally, in this region, acid rain problems are more serious than in other regions of China, especially in Nantong and 1234567 1234567 1234567 Yangzhou.

3.1.3 Chemical Composition of Rainwater
The concentrations of major anions and cations in precipitation are listed in Table 3.It shows that SO 4 2-and NO 3 -are dominant compared to other anions and NH 4 + and Ca 2+ are dominant compared to other cations.The high concentrations of SO 4 2-and NO 3 -correspond with high concentrations of SO 2 and NO 2 , indicating that the precursors of acid rain are SO 2 and NO 2 , which are mainly emitted from industry and transportation.The concentration of SO 4 2-is high in Wuxi and Taizhou, while NO 3 -concentration is high in Suzhou and Nanjing, showing the relative contribution of SO 2 and NO 2 on acid rain formation in different areas.For Wuxi and Taizhou, high SO 4 2-concentration in rain water means the important contribution of industrial sources to acid rain.For Suzhou and Nanjing, the contribution of transportation may also play an important role.The concentrations of Na + , K + , Mg 2+ and Cl -, F -are relatively lower, show- ing that sea salt is not very important in the study region.The NH 4 + and Ca 2+ concentrations are 123456789012345678901234567890121234567890123456 123456789012345678901234567890121234567890123456 123456789012345678901234567890121234567890123456 relatively higher, suggesting contributions from agricultural activities and soil dust.The annual surface PM 10 , NO 2 and SO 2 concentrations averaged over the 9 cities are illus- trated in Fig. 2. In the figure, a significant decrease in PM 10 and stable increase in NO 2 are found, while SO 2 shows a gradual reduction before 1999 and rises again after that.For the 9 cities, the annual average concentration of PM 10 was 79 ~ 184 µg m −3 .PM 10 levels averaged over these cities decreased 39% from 1996 (181 µg m −3 ) to 2003 (111 µg m −3 ).Although the PM 10 level declined in the 8-year period, it still exceeded the China Air Quality Standard (CAQS) Band II (annual average 100 µg m −3 ) throughout most of the period considered, which means that particulate matter is still an important factor in air quality of the YRD region.The significant negative trend in PM 10 concentration during 1996 -2003 suggests that control of 1234567890123456789012345678901 1234567890123456789012345678901 1234567890123456789012345678901 particulate matter in recent years has been successful.
The annual average SO 2 concentration of the 9 cities ranges from 12 to 64 µg m −3 .These values, except for those of Changzhou and Wuxi, were all below the CAQS Band II (60 µg m −3 ) post 2000.SO 2 levels averaged over these cities peaked in 1997 ( 48 The range of annual average NO 2 concentrations for the 9 cities is 13 ~ 57 µg m −3 , which increased 13% from 1996 (29.5 µg m −3 ) to 2003 (33.3 µg m −3 ).The current NO 2 level is below the CAQS Band II (80 µg m −3 ).However, it is apparent that these values are going up steadily because of expansion and urbanization in these cities, which mainly results from in- 12345678901234567890123456789012123456789 12345678901234567890123456789012123456789 creased vehicle usage and associated end gases (NO 2 , VOC, CO etc.).Table 3.Chemical composition of precipitation in 2003 ( µ eq L -1 ).

Wang et al. 1001 12345678901234567890123456 12345678901234567890123456 3.2.2 Acid Rain Frequency and pH Value
Figure 3 shows trends in pH values of precipitation and acid rain frequency, which contin- ued to fluctuate during 1996 ~ 2003.The pH value ranges from 5.09 to 5.48 as an average over the 9 cities.Compared to other cities, the precipitation acidities of Nantong and those cities to the south of the Yangtze River are higher.The acid rain frequency is negatively correlated with the pH value of precipitation.Overall, the frequency of acid rain shows fluctuations in these years, from 36.7% in 1996 to 23.6% in 1999 to 36.4% in 2003.It reached its lowest in 1999 and rose again after that, indicating that acid rain is still an important issue in this area.Although the total SO 2 emission control policy has been partly effective in recent years, NO 2 emission was not controlled effectively, leading to fluctuations in the pH value of precipita- tion and acid rain frequency.Therefore, it is not unusual that high acid rain frequency values and low pH values of precipitation in some cities such as Nantong and Nanjing were observed 12345678 12345678 occasionally.12345678901234567890123456 12345678901234567890123456 12345678901234567890123456 3.

Chemical Composition of Rainwater
The chemical composition of rainwater was further analyzed, which is presented in Fig. 4. Of all the components in rainwater, SO 4 2-and NO 3 -ions are the most important, due to their very high concentrations.The trend in SO 4 2-concentration is similar to that of SO 2 , except for the year 2003.In 2003, the average SO 4 2-concentration of the 9 cities is much higher com- pared to other years.The reason may be that the influence of emissions from other regions outside the area was strong in that year.To understand such issues, more studies are necessary.Terr.Atmos. Ocean. Sci., Vol. 18, No. 5, December 20071002 NO 3 -concentration exhibits significant increases as with NO 2 .The SO 4 2-/ NO 3 -ratio was reduced over this period, which means that NO 3 -ions played an increasingly important role in acidifi- cation of rain water while the role of SO 4 2-declined.These changes resulted mainly from the controlling SO 2 emission from 1996 and the uncontrolled NO x emissions in recent years.For modeling acid deposition in the YRD region, a comprehensive emission inventory is necessary.Here, SO 2 and NO x emissions from power plants and other sources was described in Tables 4a and b.The total of the SO 2 and NO x emissions in each city are obtained from the Environmental Monitoring Stations of Jiangsu Province and Shanghai, while the emissions from power plants are estimated using a routine method based on coal consumption, sulfur content, nitrogen content, burning temperature etc.Total SO 2 emission in the YRD is 1305853 ton, of which 47.6% is from power plants.For Shanghai, Wuxi, Changzhou, Zhenjiang, Nantong, and Yangzhou, the SO 2 emission from power plants in each city accounts for more than 50% µg m −3 ) and reached a minimum in 2000 (32 µg m −3 ).SO 2 concentration remained large in 2001 and 2002 (46 µg m −3 ) 123456789012345678901 123456789012345678901 123456789012345678901 and decreased in 2003 (38 µg m −3 ).
(a) SO 2 emission from different sources in the YRD in 2003 (ton yr -1 ).(b) NO x emission from different sources in the YRD in 2003 (ton yr -1 ).Table 5.(a) Simulated regional S deposition in different cities over the YRD in 2003 (ton yr -1 ).(b) Simulated regional N deposition in different cities over the YRD in 2003 (ton yr -1 ).Table 6.(a) Simulated S deposition in different areas caused by different sources in year 2003 (ton yr -1 ).(b) Simulated N deposition in different areas caused by different sources in year 2003 (ton yr -1 ).