北京地区酸雨特征及影响因素

Characteristics and Impact Factors of Acid Rain in Beijing

利用2003—2008年北京地区3个酸雨观测站(北京市观象台、昌平站、上甸子站)的酸雨观测资料并结合探空及大气成分资料,分析了近年来北京地区的酸雨变化特征,研究了不同气象条件和大气污染物对酸雨的影响。结果表明:2003—2008年降水平均pH值均小于5.6,且近6年来,降水pH值呈波动下降的趋势。北京地区夏、秋两季降水平均pH值及K值较春、冬季节低;pH值及K值随降水量的增大呈下降趋势,而强酸雨频率则随降水量的增大呈上升趋势;在偏南气流影响下,降水酸度增强且酸沉降量大,酸雨污染严重;当连续发生逆温状况时,酸雨出现频率增大;大气污染物SO_2,NO_2,PM_(2.5)的浓度与降水pH值成负相关关系,说明近地层污染物浓度对降水酸度有重要影响。

The annual and seasonal characteristics of acid rain in Beijing during 2003--2008 and their relationships with the meteorological conditions and pollutant concentrations are investigated using data observed at Beijing Weather Observatory, Changping and Shangdianzi acid rain observation stations. The average pH of precipitation is lower than 5.6 from 2003 to 2008, and the variation in the annual average precipitation acidity displays a decreasing trend over the past 6 years. After 2007, the acid rain pollution becomes more serious in Beijing area, when the average pH of precipitation is lower than 4.5 at all the three observation stations. The annual average pH values of precipitation at Beijing Weather Observatory and Changping are higher than that at Shangdianzi Station in the past 6 years except for 2008. It shows that the acidification degree in background area is heavier than that in urban area. Seasonal and monthly variations in precipitation acidity, conductivity and the frequency of acid rain are observed in Beijing area. The average pH and K values of acid rain in summer and autumn are lower than those in spring and winter. The frequency of acid rain with pH〈4.5 and 4.5〈pH〈5.0 is highest in autumn and summer, respectively. The monthly variation of acid deposition show that the heaviest acid rain pollution occurs in summertime, especially in August. The decrease tendency in the pH and K value and the increase tendency in the frequency of acid rain with pH〈4.5 are found with the increase of precipitation intensity, respectively. The higher pH and K value at Beijing Weather Observatory and Changping Station indicate that the process of belowcloud scavenging is more important in urban area, and alkaline coarse particles in the atmosphere plays an important role in neutralization. The wind direction at 1500 m level has significant influence on the precipitation acidity and conductivity value over Beijing area. The lowest pH is observed with south wind, and higher K value and heavier acid deposition are recorded with the southwest and south wind. The transport of pollutants by the southerly flow enhances the acidity of precipitation. The acid rain is also influenced by the temperature inversion. The frequency of acid rain increases when continuous inversion occurs （temperature inversion occurs at rainy day and the day before that）. Temperature inversion occurs most frequently during autumn that leads to the increase of acid rain pollution in this season. Furthermore, pollutants in the atmosphere have important effect on the acidity of precipitation. There is a significant negative correlation between the concentration of atmospheric pollutants, such as SO2, NO2 and PM2.5, and the pH of rain, suggesting that the variation of pollutant concentration in lower atmosphere layer has significant in- fluence on the acid rain. SO2 and NO2 are the important precursors of the acidic compositions and the major contributors of acidity in rainwater. The neutralization effect of fine particles （PM2.5） is very weak. On the contrary, the acid compositions of PM2.5 are partly contributed to the acidity of precipitation when they are scavenged into the rainwater.