PM2.5为什么会影响大气能见度?

与气态空气污染物不同的是,除了直接危害人类以及动植物健康,PM2.5还能通过对阳光的吸收和散射效应降低能见度,也就是造成灰霾现象。正因为这个特点,普通公众也能通过对能见度的感知来“判断”空气污染程度。PM2.5造成的能见度降低会殃及农作物产量以及其他植物的生长。另外值得提及的是,PM2.5的组分之一炭黑对光的吸收还能产生温室效应,被政府间气候变化专门委员会(IPCC)确定为除了二氧化碳和甲烷之外的第三大温室效应贡献物种。因此控制炭黑的排放,既有利于人类健康,还有利于防止气候变暖。

PM2.5颗粒非常细小,人眼是看不到的,那么它为什么会造成灰霾,影响能见度呢?大气颗粒物对能见度的影响主要是通过对光的吸收和散射。颗粒物中对光吸收最强的是炭黑。在煤烟型污染的地区,例如20世纪中叶的“雾都”伦敦,大气中的炭黑颗粒对能见度的影响比较大。随着对燃煤污染的控制和治理,在很多地区,由于颗粒物对光的吸收作用而对能见度产生的影响已不是很重要,而颗粒物对光的散射作用成为影响能见度、形成灰霾的主要因素。

大气(包括空气与颗粒物)散射是一种非常重要的自然现象,只不过我们已对它熟视无睹了。实际上,每天进入我们眼睛的光绝大部分都是散射光。如果没有大气散射,就会像在太空中一样,除了太阳、灯光等光源本身和它们直接照射到的地方以外,都将是一片漆黑。

颗粒物对光的散射作用,与它的粒径有密切关系。当微粒的粒径比光的波长小很多,例如小于波长的1/10时,产生的散射称为瑞利散射(Rayleigh scattering),它是以英国物理学家瑞利(John Rayleigh,1842~1919)的名字命名的。晴朗的天空中,空气分子以及空气密度的涨落都会对阳光形成瑞利散射。瑞利散射光的强度和入射光波长的4次方成反比。因为蓝光波长很短,所以其散射强度很大,这就形成了晴朗时的蓝色天空。

而当颗粒物的粒径与光的波长相当时,发生的散射称为米散射,这是以德国物理学家米(Gustav Mie,1869~1957)的名字命名的。可见光的波长范围在400~700纳米(即0.4~0.7微米)左右,所以粒径在此范围内的颗粒物对可见光的散射主要是米散射,而这个粒径范围恰恰属于PM2.5。因为米散射的散射强度比瑞利散射大得多,故而PM2.5对能见度的影响最大。米散射的强度与入射光波长的二次方成反比,对光的波长不如瑞利散射那样敏感,所以在灰霾天气里,天空的颜色不会发蓝,而是呈灰白色。另外,米散射的强度除了与入射光波长和颗粒物的大小有关外,还与颗粒物的折射率有关,而颗粒物的折射率是由它的化学成分以及含水量决定的。例如,空气湿度大的时候,颗粒物的含水量高,对光的散射作用就强。这就是为什么在污染严重且空气潮湿的情况下,天空最容易变成雾蒙蒙的。

颗粒物中不同化学组分对能见度降低的贡献大小是不一样的。综合考虑对光的吸收和散射作用,在颗粒物浓度相同的情况下,炭黑对能见度的影响最大,其次是有机碳和硫酸铵以及硝酸铵。其中硫酸铵和硝酸铵的贡献与空气的相对湿度(某温度时空气中水的蒸气压与同一温度下水的饱和汽压的百分比)有密切的正相关关系,即它们对能见度的影响随着相对湿度的上升而增大。

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