Effects of Meteorology on Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 144

portion of acid deposition problems in Japan. Aerosol
particles and ozone precursors from Asia travel long
distances over the Pacific Ocean to North America
(Prospero and Savoie, 1989; Zhang et al., 1993; Jacob
et al., 1999; Song and Carmichael, 1999). Hydrocar-
bons, ozone, and PAN travel long distances across
Europe (Derwent and Jenkin, 1991) as do pollutants
from Europe to Africa (Kallos et al., 1998). Pollutants
also travel between the United States and Canada,
between the United States and Mexico, and from North
America to Europe.
The long-range transport of pollution from Asia to
North America is generally strongest during March and
April .Duringthese months, winds are particularly fast,
picking up dust from the Gobi Desert, mixing it with air
pollution from populated cities in China and Japan, and
sending it rapidly across the Pacific, counterclockwise
around the Aleutian low-pressure system and clockwise
around the Pacific high-pressure system (e.g., Figure
6.7) to the west coast of North America. During one
such event in April 1998, dust and pollution were trans-
ported across the Pacific in five days and increased PM 10
concentrations in California by 20 to 50

6.6.3. Cloud Cover
Clouds affect pollution in two major ways. First,
they reduce the penetration of UV radiation, there-
fore decreasing rates of photolysis below them. Sec-
ond, rainout and washout remove pollutants from the
air. Thus, rain-forming clouds help cleanse the atmo-
sphere. In some cases, though, the pollutants in rain-
drops are returned to the air upon evaporation of the
drops before they land. Because cloud cover and pre-
cipitation are often greater and mixing depths, higher
in surface low-pressure systems than they are in sur-
face high-pressure systems, photochemical smog con-
centrations are usually lower in the boundary layer of
low-pressure systems than high-pressure systems.

6.7. Effects of Local Meteorology
on Air Pollution
Although large-scale pressure systems control the pre-
vailing meteorology of a region, local factors also affect
meteorology and thus air pollution. Some of these fac-
tors are discussed briefly.

gm − 3 (Husar

et al., 2001).
Similarly, pollution from the east coast of North
America travels rapidly and sinusoidally around the
highs and lows of the Atlantic Ocean to Europe. In
one study, pollution emitted over the east coast of the
United States during November 2001 was tracked by
aircraft. The pollution was first lifted to the midtropo-
sphere, where it was swept across the Atlantic and then
sank over Europe, intercepting the Alps a week later
(Huntrieser et al., 2005).

6.7.1. Ground Temperatures
Ground temperatures affect local meteorology in at least
three ways, and meteorology feeds back to air pollution.
Figure 6.14, which gives insight into the first mecha-
nism, shows that warm ground surfaces produce high
inversion base heights (thick mixing depths), which, in
turn, reduce pollution mixing ratios. Conversely, cold
ground surfaces produce thin mixing depths and high
pollution mixing ratios.
Second, ground temperatures affect pollution by
modifying wind speed. Warm surfaces enhance convec-
tion, causing surface air to mix with air aloft and vice
versa. Because horizontal wind speeds at the ground
are zero and those aloft are faster, the vertical mixing of
horizontal winds increases wind speeds near the surface
and reduces them aloft. Thus, warmer ground surfaces
increase near-surface winds. Faster near-surface winds
increase dispersion of near-surface pollution but may
also increase the resuspension of loose soil dust and
other aerosol particles from the ground. Conversely,
lower ground temperatures have the opposite effect,
slowing down near-surface winds and enhancing near-
surface pollution buildup.
Third, changes in ground temperatures change near-
surface air temperatures, and air temperatures affect
rates of several processes. For example, temperatures
influence the rates of biogenic gas emissions from trees,

Example 6.3
How long would it take pollution emitted at one
point on the globe at 30 ◦ N latitude and 5-km
altitude to traverse the globe back to its original
position if it travelled without changing latitude
or altitude with a wind speed of 50 m s −1 ?

Solution
From Appendix A.1.4, the radius of the Earth is
R e = 6,371 km. Adding z = 5kmtothisgives

30 ◦ N, the distance from
the axis of rotation of the Earth to a point 5 km in
the air is approximately ( R e +

R e + z = 6,376 km. At =

5,528 km,
giving the circumference of the Earth as 2

z )cos =

( R e +

z )cos 34,732 km. Dividing this distance by a

wind speed of 50 m s −1 gives a time of 8.04 days
to travel around the world at 30 ◦ N.

=