除风速外,能见度的降低不仅与大气气溶胶尤其是细颗粒物的质量浓度有关,还与气溶胶中化学组分的本性密切有关。根据对2004年春季北京、上海的能见度与大量PM 2.5样品的各种离子相应浓度数据所进行的统计分析,在北京,能见度与NH+4、SO 2-4、TWSI、PM 2.5的相关系数分别为-0.726、-0.696、-0.678、-0.520;在上海,分别为-0.637、-0.484、-0.427、-0.534。结果表明,能见度与铵盐、硫酸盐、可溶性离子总浓度和气溶胶浓度,均有某种程度的负相关。无论在北京还是在上海,铵盐均是上述4种主要组分(铵盐、硫酸盐、可溶性离子总浓度和气溶胶细颗粒物浓度)中与能见度相关程度最高的组分。铵盐在乌鲁木齐2007年2—3月间大气气溶胶PM 2.5和TSP中的浓度,分别为94.0和103.7μg·m-3,占PM 2.5和TSP总质量的比例高达49.5%和31.0%。显然,高浓度的铵盐,是导致乌鲁木齐冬季严重霾天气的主要因素之一。
综上所述,一种新型的天气模式——霾,每年困扰中国典型的内陆城市乌鲁木齐达140多天,且有细颗粒物浓度高、形成迅速、发生频繁的显著特点。乌鲁木齐已成为中国甚至世界上污染最严重的城市之一。城区人为污染源与外来沙尘的相互混合,SO 2转化形成的硫酸盐和来自准噶尔盆地南缘农业带以及周边盐湖地区含有高浓度硫酸盐的土壤沙尘传送到城区,这是乌市高浓度硫酸盐的主要来源。可溶性离子和具有强吸水性的铵盐分别占PM 2.5中总质量的57.8%和51.0%,高浓度铵盐[主要是(NH 4)2 SO 4]的强吸水性是形成乌鲁木齐重霾的主要机制。
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