Air Quality Guidelines Global Update 2005 - World Health ...

SOURCES OF AIR POLLUTION
in diameter (1 nm = 10 –9 m), while coarse dust and sea salt particles can be as
large as 100 μm (1 μm = 10 –6 m) or 0.1 mm in diameter. However, the very large
particles have a short atmospheric existence, tending to fall out rapidly through
gravity and wind-driven impaction processes. Thus in practice there are few particles
in the atmosphere exceeding 20 μm in diameter, except in areas very close
to sources of emission. Particulate matter can be separated from atmospheric
gases by drawing air through a filter fine enough to retain the particles, or by
accelerating air through a jet that fires them at a fixed plate, onto which the particles
impact and are collected. Particulate air pollutants have very diverse chemical
compositions that are highly dependent on their source. They are also diverse
in terms of particle size. Fig. 1 illustrates the range of sizes (on a logarithmic
scale) together with the ranges where certain important components are typically
encountered. It shows also the PM10, PM2.5 and ultrafine particle fractions, which
are typically those measured within the atmosphere for the purposes of health effects
studies; the first two fractions are also used for compliance monitoring.
Fig. 1. Size range of airborne particles, showing the health-related ultrafine, PM2.5 and
PM10 fractions and the typical size range of some major components
Ultrafine fraction
Diesel smoke
PM2.5
Sulfate
PM10
Nitrate
Soil, road dust
PM2.5−10
0.001 0.01 0.1 1.0
Particle diameter (μm)
10 100
In the context of discussing sources of air pollution, it is important to consider
the geographical location and distribution of sources. Air pollution occurs on a
range of spatial scales linked primarily to the atmospheric lifetime of the specific
pollutants. Typical spatial scales are the following.
Local scale
Some pollutants, by virtue of their source or of having a very short atmospheric
lifetime, are only encountered in appreciable concentrations close to where
they are emitted. Examples are mainly rather esoteric chemicals, emitted from
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