PM2.5来自哪里?

大气颗粒物的来源很复杂,既有人为源,也有天然源;既来自直接排放,也可以通过气态物质在大气中发生化学反应而间接生成。直接排放到大气中的颗粒物称为“一次颗粒物”,间接生成的颗粒物称为“二次颗粒物”。

一般而言,粒径为2.5~10微米的粗粒子主要是由机械过程直接产生的,例如来自建筑和道路扬尘、风沙、工农业活动等。另外,还可能含有一些生物质颗粒,如花粉、真菌孢子等,在沿海地区则还会含有由于海浪飞沫而产生的海盐颗粒。粗粒子基本上都是一次颗粒物。

而PM2.5中既有一次颗粒物,也有二次颗粒物。PM2.5的一次来源主要是化石燃料燃烧(如机动车、燃煤电厂,燃煤、燃油锅炉)和生物质燃烧(如秸秆焚烧、森林野火等)产生的飞灰以及多种无机和有机化合物,也有一部分来自扬尘风沙和工农业活动等经由机械过程产生的颗粒物中较细的部分。PM2.5的二次来源是大气中的气态物质(二氧化硫、氮氧化物、氨以及挥发性有机物等)通过发生化学反应而生成的颗粒物(如硫酸铵颗粒、硝酸铵颗粒、有机物颗粒等)。在世界上的大多数地区, PM2.5的二次来源比例往往高于一次来源。

研究表明,燃煤电厂和燃煤锅炉是PM2.5的最重要人为来源之一。除了直接排放硫酸盐、硝酸盐及其他颗粒物成分外,燃煤过程排放的气体,如二氧化硫和氮氧化物等,还会通过化学反应生成更为大量的硫酸盐和硝酸盐二次颗粒物。汽车排放(包括尾气排放以及汽油挥发)也被发现是PM2.5的另一个最重要的人为来源之一,不论在发达国家,还是在中国的大都市均是如此。汽车排放对空气中PM2.5的贡献既有其直接排放,比如随尾气排出的炭黑和有机颗粒物,轮胎和刹车系统磨损产生的细小粒子,也有二次生成的贡献,比如尾气排放的氮氧化物和碳氢化合物以及汽油挥发排放的挥发性有机物,可经过复杂的光化学氧化(气态物质在阳光的照射下发生的一系列氧化反应),生成有机颗粒物、硝酸盐颗粒物等。20世纪40年代,美国洛杉矶曾发生过著名的洛杉矶光化学烟雾事件,主要就是由于汽车尾气排放的化学物质在强烈的日光照射下发生光化学反应,一次和二次的气态和颗粒污染物混合在一起,形成了氧化性很强的淡蓝色烟雾,对人的眼睛和喉咙产生强烈刺激,使人头痛、呼吸道疾病加重,严重者出现心肺功能衰竭,以至死亡。70年代,我国兰州西固地区也出现过不明烟雾污染事件,导致人流泪、植物枯萎,北京大学的唐孝炎教授带领研究组研究证实这是和洛杉矶类似的光化学烟雾污染。

值得注意的是,主要由牲畜蓄养以及农业施肥排放的氨气对PM2.5的二次生成贡献往往被忽视。但是有研究表明,氨的存在会加速二氧化硫和氮氧化物向颗粒物的转化,生成硫酸铵、硝酸铵等颗粒物成分。另外一个有趣的事实是,森林树木也对PM2.5中的有机成分有显著贡献。在植被覆盖率高的地区,如果同时人类活动频繁,森林对PM2.5有机成分的贡献往往会超过人为来源。原因是树木的叶片会随着植物生理过程排放异戊二烯、单萜烯和多萜烯等多种烃类化合物,这些天然的挥发性有机化合物遇到人类活动排放的氮氧化物,在强光照射下会快速发生光化学氧化,生成挥发性显著降低的半挥发性、甚至不挥发性的有机物,这类有机物很容易在空气中达到饱和,从而直接转化成有机颗粒物。另一方面,树木叶片排放的分子量较大的有机物,例如某些多萜烯,本身就具有半挥发性的特点,很容易在空气中直接达到饱和,从而成为有机颗粒物。还有一些植物叶片分泌的“难挥发”有机物首先集聚在叶片表面形成固态的蜡膜,当温度升高,这些有机物挥发成气态从而脱离叶片进入空气,但因其低挥发性又很快在空气中达到饱和,形成悬浮于空气中的有机颗粒物。

PM2.5的化学成分复杂,而且会随着地点和季节而改变。世界各地的科学研究表明,PM2.5的主要化学成分有硫酸盐,硝酸盐,铵盐,炭黑,有机碳化合物,微量金属和地壳矿物质比如锌、钾、钠、钙、镁、硅等。在不同的时间和地点,PM2.5里面的这些化学成分的比例会有显著差异。比如冬季燃煤取暖较多的北方城市,PM2.5中的硫酸盐比例会偏高,而机动车密集的都市地区,PM2.5中黑炭以及有机碳化合物的比例会偏高。

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