什么是PM2.5?

我们知道地球上的大气和水同食物一样都是人类赖以生存的物质。人的身体可以耐受一个星期左右不进食,两三天左右不饮水,但是不能离开空气片刻。即使进入睡眠状态,人类也时时刻刻都在呼吸着空气。干净的空气主要由氮气和氧气组成,其他成份非常痕量。可是自工业革命以来,工业化造成每天都有大量有毒有害物质被人类排放进入地球大气。在地球上大多数地方,我们今天呼吸到的空气已经远没有工业化以前清洁,而是充斥了各种空气污染物。在今天的中国,严重的空气污染业已成为人们日常生活的一部分。

空气污染物包括气态污染物和大气颗粒物。气态污染物通常包括二氧化硫、二氧化氮、一氧化碳、臭氧等,它们都会对人体健康、农作物生长造成不利影响。大气颗粒物则可以是完全干燥的固态颗粒,也可以包含水分,它们的化学组成非常复杂,由于其特殊的物理和化学特性,对大气能见度、人体健康和气候等都会产生很大的影响。空气污染物,虽然与空气中的无害成份氮气和氧气的含量相比较起来,仍然是痕量的,但是都是对人类的巨大威胁。

大气颗粒物的形状并不规则,很少呈均匀对称的球体。我们谈到颗粒物的粒径时,指的并不是它们的实际尺寸,而是它们的“空气动力学等效直径”,也就是与颗粒物 在大气中具有相同的沉降速率且密度为1克/立方厘米的球体的直径。研究人员在分离、检测不同粒径的大气颗粒物时,就是利用它们的空气动力学特性。具有不同 空气动力学直径的颗粒物,在一定的气体流速下具有不同的偏转半径,利用这个原理就可以通过以固定流速抽取气体通过不同直径的滤孔从而把不同粒径的颗粒物分 开了。

大气颗粒物的来源、组成和对健康的危害,都与它们的尺寸大小直接相关,因此科学家通常按颗粒物的直径(粒径)将其分类。例如粒径小于等于100微米(1微米是1米的100万分之1)的颗粒物,就称为PM100。PM是颗粒物的英文名称particulate matter的缩写。PM100又称为总悬浮颗粒物,代表大气颗粒物污染的总体水平。比PM100大的颗粒物很难较长时间地悬浮在空气中,会很快沉降到地表。PM10则指粒径小于等于10微米的颗粒物,它们很容易进入人的肺部,故被称为可吸入颗粒物,因为对健康危害较大,一直是科学家关注的对象。近十几年来,科学家发现在可吸入颗粒物中,粒径小于2.5微米的粒子更可怕,它们会深入人的肺泡,其中的超细颗粒甚至会穿过肺泡进入血液,所以又特别提出PM2.5的概念,并专门对此类细颗粒进行了大量的研究和分析。可吸入颗粒物中粒径较大的部分,即PM2.5~10,被称作粗颗粒物,而PM2.5则被称作细颗粒物。

总之,PM2.5是一种颗粒态的空气污染物,是指那些悬浮在大气中的空气动力学粒径小于2.5微米的颗粒物。PM2.5可以被人体直接呼吸进入肺部,甚至直接穿过肺泡进入血液,从而对人体健康造成危害。

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