PHYSICS AND CHEMISTRY OF
Atmospheric particles originate from a variety of sources and possess a range of
morphological, chemical, physical, and thermodynamic properties.
combustion-generated 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, some are solid; others contain a solid
core surrounded by liquid.
Atmospheric particles contain inorganic ions and elements, elemental
carbon, organic compounds, and crustal compounds.
Some atmospheric particles are
hygroscopic and contain particle-bound water.
The organic fraction is especially complex,
containing hundreds of organic compounds.
Particle diameters span more than four orders of magnitude, from a few nanometers to one
Combustion-generated particles, such as those from power generation,
from automobiles, and in tobacco smoke, can be as small as 0.003 μm and as large as 1 μm.
Particles produced in the atmosphere by photochemical processes range in diameter
0.003 to 2 μm.
Fly ash produced by coal combustion ranges from 0.1 to 50 μm or more.
blown dust, pollens, plant fragments, and cement dusts are generally above 2 μm in diameter.
Particles as small as a few nanometers (Covert et al., 1992; Clarke, 1992) and as large as 100 μm
have been measured in the atmosphere (Lin et al., 1993).
Particles are ubiquitous in the atmosphere.
The lowest concentrations are found in
background marine environments, where particle number concentrations range from 100/cm to
In background continental environments, particle concentrations vary from 100/cm to
5,000/cm ; while in urban areas of the United States concentrations may be as high as
4,000,000/cm (Willeke and Whitby, 1975; Whitby and Sverdrup, 1980).
Particles account for a
mass of a few μg/m near the surface over dry continental areas to several hundred μg/m in
polluted urban areas.