Geoscience Reference
In-Depth Information
increased risk of death due to short-term exposure to
ozone is
particles from the atmosphere are rainout, washout,
sedimentation, and dry deposition. Aerosol particles
are responsible for a variety of health problems that
persist worldwide.
0.0004 per ppbv above the threshold (Ostro
et al., 2006). Although health effects of PM
2.5
vary for
different chemical components within PM
2.5
,almost
all epidemiological studies correlating particle changes
with health use ambient PM
2.5
measurements to derive
such correlations. The increased risk of mortality due
to long-term exposure to PM
2.5
may be
∼
5.9. Problems
5.1. Why do accumulation mode aerosol particles not
grow readily into the coarse mode?
5.2. Why do accumulation mode particles generally
contain more sulfate than do coarse mode particles?
∼
0.004 per
gm
−
3
(Pope et al., 2002).
Example 5.5
Calculate the number of United States deaths
per year due to short-term ozone exposure if the
entire population of 300 million were exposed to
40 ppbv.
5.3. On a global scale, why is most chloride observed
in the coarse mode?
5.4. Why is most ammonium found in accumulation
mode particles?
5.5. Write an equilibrium reaction showing nitric acid
gasreacting with magnesite, a solid. In what particle
size mode should this reaction most likely occur?
Solution
Substituting
y
0
=
0.00833,
P
=
300,000,000,
=
0.0004 per ppbv,
x
=
40 ppbv, and
x
th
=
35
ppbv into Equation 5.13 gives
y
≈
5,000 addi-
5.6. Why is the carbonate ion not abundant in aerosol
particles?
tional deaths/yr.
5.7. Why might a sea spray drop over midocean lose
all its chloride when it reaches the coast?
5.8. Why is more nitrate than sulfate generally
observed in particles containing soil minerals?
5.8. Summary
Aerosol particles appear in a variety of shapes and com-
positions and vary in size from a few gas molecules to
the size of a raindrop. Natural sources of aerosol par-
ticles include sea spray uplift, soil dust uplift, volcanic
eruptions, natural biomass burning, meteoric debris,
and wind-driven emissions of pollen, spores, bacte-
ria, viruses, and plant debris. Anthropogenic sources
include fugitive dust emissions; biomass, biofuel and
fossil fuel combustion; and industrial sources. Aerosol
particle size distributions generally contain three to five
modes, including one or two subnucleation modes, one
or two subaccumulation modes, and a coarse particle
mode. Homogeneous nucleation and emissions from
fossil fuel, biofuel, and biomass burning dominate the
nucleation mode. Emissions of sea spray, natural soil
dust, fugitive soil dust, pollen, spores, and bacteria
dominate the coarse mode. Aerosol particles coagu-
late and grow by condensation or dissolution from the
nucleation mode to the accumulation mode. Chemistry
within aerosol particles and between gases and aerosol
particles affects growth. Growth does not move accu-
mulation mode particles to the coarse mode, except
when water vapor grows onto aerosol particles to form
cloud drops. The main removal processes of aerosol
5.9. Why is coagulation not an important process for
moving particle mass from the lower to the upper accu-
mulation mode?
5.10. Estimate the concentration of 0.01-
m-diameter
particles after 10 s of coagulation if 10
6
particles cm
−
3
of this size exist initially, no other particles exist, and
the coagulation rate coefficient is 10
−
8
cm
3
particle
−
1
s
−
1
.What is the resulting concentration of new parti-
cles formed assuming that they are formed only from
the 0.01-
m particles and none of the new particles
is lost?
5.11. Calculate the percent change in the growth rate
of single-particle volume from Equation 5.3 if the dif-
fusion coefficient is cut in half,
p
s
10
−
6
hPa, the
partial pressure (
p
)isincreased from 2
p
s
to 4
p
s
, parti-
cle density is decreased by a factor of two, and all other
parameters remain constant.
Hint
:Forthis small value
of
p
s
,the left side of the denominator in Equation 5.3
can be assumed to be zero relative to the right side.
=
5.12. Visibility is affected primarily by particles with
diameter
close to the wavelength
of visible light,
0.5
m. Which particle mode does this correspond to,
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