如何判断PM2.5污染程度 – 是否能见度越差污染就越严重?

能见度很差的污染天气,被老百姓称为“雾霾”天气。实际上,雾(fog)和霾(haze)是两个不同的概念。雾主要是由小水滴组成的,霾则主要是由固态颗粒物组成的。一般来说,相对湿度较低时形成的是霾,相对湿度大于90%时形成的是雾。相同的是,雾和霾都会对能见度造成影响。

通过观察是否有能见度降低的灰霾现象发生来判断PM2.5污染是否严重是一个简单易行的办法。但这只是一种定性的观察,无法做到准确定量,甚至有时也并不能够准确定性。比如,在同样程度的颗粒物污染情况下,空气湿度越高,能见度的降低越大。另外,雾天也会有因雾滴和水汽造成的能见度降低。因此,除了能见度之外,普通公众还可以借助空气湿度判断污染的类型。如果相对湿度超过100%,能见度的下降就主要是由大粒径的雾滴造成的。

当浓雾发生时,虽然能见度急剧降低,PM2.5污染可能并不严重,原因是这些颗粒物被水滴包裹,形成更大的粒径,被暂时“悬浮式湿清除”了,不易进入人体。实际上,在大气污染物较多的大中城市中,很难会出现纯粹无污染的雾天。但是,平流雾条件下,“清洁”雾的发生概率会大些。另外,雾的形成,实际上是对PM2.5的一种清除,因为细粒子吸收水分变成了粒径很大的雾滴。但这种“悬浮式清除”有可能只是暂时的。另外,不论哪种雾,其中的水滴都会加剧污染物的多相氧化反应,形成更多的颗粒物。但是雾滴中的成分最终是否有健康影响,还要看这些成分是否会被有效地清除掉。如果雾滴没有沉降到地表或被风吹散,那么当气温上升、湿度降低后,雾会由于其中的水分蒸发而转变为霾,空气中细粒子浓度显著上升,对健康产生影响。城市中雾和污染的关系很复杂,有待更多的研究。

另外,在我国北方城市的冬春季节,气候干燥,有时大风会将大量地面的沙土卷入空中,形成沙尘暴或者是扬沙和浮尘。在这样的风沙天气,能见度也很差,但这时能见度的下降主要是由空气中粒径较大的沙尘造成的,天空的颜色发黄。沙尘颗粒中的大部分成分来自地壳、土壤,以二氧化硅、三氧化二铝等为主。由于沙尘颗粒较大(其中也有部分较细的飘尘),不像PM2.5那样容易被吸入肺部深处,沙尘颗粒中含有的有毒有害化学物质也不像PM2.5中那么多,而且在风沙天气人们往往会戴上口罩或减少户外活动,所以沙尘暴虽然来势凶猛,但对健康的危害反而不如PM2.5那么大。

要定量判断是否有PM2.5污染,还是需要用精密的科学仪器来进行日常监测。将PM2.5从一定体积的空气中分离出来后,再测定它的重量,就能计算出它在空气中的浓度。测定PM2.5的重量主要有3种方法:重量法、微量振荡天平法和β射线法。重量法,就是将采集到PM2.5的滤膜用天平称重,这是最准确的方法,也是《环境空气质量标准》中建议的标准方法。重量法虽然准确,但也有一个缺点,就是无法进行自动监测。要获取实时数据,可以采用微量振荡天平法和β射线法等自动分析方法。微量震荡天平法利用物体的固有振动频率与其质量之间的关系来测定重量,β射线法则是通过测定β射线穿过滤膜和颗粒物后的衰减来测定重量。要准确测量PM2.5的浓度,必须要由专业人员采用专业的仪器,遵照规范的流程进行操作。实际上,上述3种专业方法的测定结果之间都可能会有20%~30%的差异,因此专业人员在测定时都要做大量的比对、标定和校正工作。现在有一些热心的环保人士,自购手持设备来测定PM2.5的浓度,这种设备一般是利用光散射的原理进行测量的,其测定结果虽然可能有一些定性的参考价值,但是误差会很大。目前,全国有170多个重点城市监测PM2.5的小时浓度并实时公布,这对公众准确判断PM2.5污染程度提供了最直接的科学依据。

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30 comments on “如何判断PM2.5污染程度 – 是否能见度越差污染就越严重?
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