中国污染源气溶胶的上述空间分布状况,揭示了污染源气溶胶与现代工业化程度以及空气中的相对湿度都有很大关系。采用一个简单的污染源气溶胶指数,可以模拟不同地区污染气溶胶的相对浓度。
污染气溶胶指数I定义为:
上式中PD表示人口密度,Precip表示降水量,WS表示风速。由于人口密度与工业化程度常常呈正相关,而污染源气溶胶浓度又与工业化程度呈正相关,因此用人口密度来表征工业化程度。平均降水量用于近似表述某地区的年平均相对湿度。风速和降水量(相对湿度)作为该地区污染气溶胶的稀释因素,应与污染源气溶胶浓度成反比。本章所使用的人口密度数据来自Atlas of China[30],代表采样点及其周围区域的人口密度值,而不单单是采样点的人口密度。降水量和风速取自H.Arakawa的文献[31(]若该采样点的降水量和风速数据不可查,我们取其周边地区的降水量和风速数据)。表2-3列出了15个中国城市的污染源气溶胶的模拟指数值。
表2-3 15个中国城市的污染气溶胶指数值
图2-7显示了各采样点大气气溶胶中污染元素(Zn、Se)的浓度,以及在各个采样点相应的污染气溶胶模拟指数。由图2-7显而易见,Zn和Se有着与污染源气溶胶指数类似的空间分布规律,即最大值/最小值都出现在相同的地区。图中各采样点基本按照由西北到东南的顺序排列。污染源气溶胶指数在西北地区采样点较低,在内陆地区经过一个广泛的高值区(兰州到北京),而后在沿海地区下降到与西北地区相近似的污染源气溶胶指数值(株洲到临安)。上述模拟指数与实际观测值的变化趋势基本吻合,说明了某一区域的工业发展水平、风速以及降水量,是决定某地污染源气溶胶浓度之主要因素。
综上所述,中国的沙尘源气溶胶主要分布于中国北部和西北地区,采用大气气溶胶中的Ca/Al比值可以简便地判别大气气溶胶中的沙尘来源。海盐气溶胶主要集中于沿海地区,而污染源气溶胶则几乎随处可见,并在中国的中东部地区占据主导地位。
图2-7 各采样点的污染源气溶胶指数以及污染元素(Se、Zn)的浓度(ng·m-3)(彩图见下载文件包,网址见14页脚注)
污染源气溶胶的浓度,首先取决于人为污染源的排放,也与风速、降水等气象条件密切相关。降水不仅能够清除大气中的污染物,还决定着表层土壤的干燥程度,这是产生风沙的一个重要因素。风对气溶胶的稀释起很大作用,但是风速大又有利于沙尘气溶胶的产生。
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