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形态:
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Particulate matter (PM) is
not a specific chemical entity but a mixture of
particles of different sizes, compositions, and
properties. Examples include combustiongenerated
particles such as diesel soot or fly
ash; photochemically produced particles such
as those found in urban haze; salt particles
formed from sea spray; and soil-like particles
from resuspended dust. Some particles are
liquid, and some are solid. Others contain a
solid core surrounded by liquid. Atmospheric
particles contain inorganic ions, metallic compounds,
elemental carbon, and organic compounds.
The organic fraction is especially
complex, containing hundreds of organic compounds.
PM is called primary if it exists in the
same chemical form in which it was emitted
or generated. Secondary particles are formed
from gases through chemical reactions in
the atmosphere, involving atmospheric oxygen
and water vapor; reactive species such as
ozone (O3); radicals such as the hydroxyl (·OH)
and nitrate (·NO3) radicals; and pollutants such
as sulfur dioxide (SO2) and nitrogen oxides
(NOx).
Particle size of PM is of considerable importance in relating ambient air concentrations
to population morbidity and mortality.
Routing ambient air monitoring studies before
1999 generally measured “thoracic” PM,
namely, PM10 (upper size limited by a 50% cut
at 10-mm aerodynamic diameter). Research
and monitoring studies since 1999 have
measured other fractions, but one of considerable
significance is termed “fine” PM, namely,
PM2.5 (upper size limited by a 50% cut point
at 2.5-mm aerodynamic diameter.) An emerging
measurement of importance is “ultrafine”
particles, namely, PM0.1 (upper size limited
by a 50% cut point at 0.1-mm aerodynamic
diameter).-
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