Aerosol Particles in the Polluted and Global Atmosphere - Air Pollution and Global Warming: History, Science, and Solutions - page 101

Chapter 5

Aerosol Particles in the Polluted
and Global Atmosphere

Although air pollution regulations have historically
focused on gases, aerosol particles cause more health
problems and visibility degradation than do gases. Par-
ticles smaller than 2.5

Aerosol particles smaller than 100 nm in diameter
are nanoparticles ,alsoknown as ultrafine particles .
Those between 100 and 2,500 nm in diameter are fine
particles .Particles larger than 2,500 nm are coarse
particles .
The sum of all aerosol particles smaller than 2.5

mindiameter cause the most
severe health problems. They enter the atmosphere
by emissions and nucleation. In the air, their number
concentrations and sizes change by coagulation, con-
densation, chemistry, water uptake, rainout, washout,
sedimentation, dry deposition, and transport. Particle
concentration, size, and morphology affect the radiative
energy balance in both urban air and the global atmo-
sphere. In this chapter, compositions, concentrations,
sources, transformation processes, sinks, and health
effects of aerosol particles are discussed. The effects
of aerosol particles on visibility are described in Chap-
ter 7, and regulations related to particles are given in
Chapter 8.

m
(2,500 nm) in diameter is referred to as PM 2.5 .The
sum of all particles smaller than 10 mindiameter is

referred to as PM 10 .Assuch, PM 10 includes the contri-
bution of PM 2.5 . PM 1 is the sum of all particles smaller
than 1

mindiameter. Such particles are also referred
to as submicron particles .Conversely, supermicron
particles are those larger than 1

mindiameter.
Hydrometeor particles generally range from 5 mto

8mmindiameter, although hail can reach 115 mm in
diameter.
Table 5.1 compares typical number concentrations
and mass concentrations in different size ranges of
gases, aerosol particles, and hydrometeor particles
under lower tropospheric conditions. It indicates that
number and mass concentrations of gas molecules are
much larger than are those of particles. The number con-
centration of aerosol particles decreases with increasing
particle size. The number concentration of hydrome-
teor particles is typically less than is that of aerosol
particles, but the mass concentration of hydrometeor
particles always exceeds that of aerosol particles.
Aerosol particle size distributions can be divided
into modes ,which are regions of the size spectrum
space in which distinct peaks in concentration occur.
Usually, each mode is described analytically with a

5.1. Size Distributions
Aerosol and hydrometeor particles (defined in Section
1.1.2) are characterized by their size distribution and
composition. A size distribution is the variation in par-
ticle number concentration or mass concentration, for
example, with particle size. Number concentration is
the number of particles of a given size per unit volume
of air, whereas mass (or volume ) concentration is the
mass (or volume) of particles of a given size per unit
volume of air.
Aerosol particle sizes range from 1 nm to 10 mm
in diameter, and thus span seven orders of magnitude.