Effects of Meteorology on Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 147

6.7.4.3. Effects of Sea and Valley Breezes
on Pollution
In the Los Angeles Basin, sea and valley breezes are
instrumental in transferring primary pollutants, emit-
ted mainly from source regions on the west side of
the basin, to receptor regions on the east side of the
basin, where they arrive as secondary pollutants. Figure
4.13 shows an example of the daily variation of NO(g),
NO 2 (g), and O 3 (g) at a source and receptor region in
Los Angeles. The valley breeze, in particular, moves
ozone from Fontana, Riverside, and San Bernardino, in
the eastern Los Angeles Basin, up the San Bernardino
Mountains to Crestline, historically one of the most
polluted locations in the United States in terms of
ozone.

7

6

Day 3

5

4

3

2

Day 1

Day 2

1

0

0 2 4 6 8 0 2

Hour of day

Figure 6.20. Near-surface wind speeds at Hawthorne,
California, August 26-28, 1987.

land breeze ,anear-surface flow of air from land to
water.
Figure 6.19 illustrates that a basic sea breeze circu-
lation cell can be embedded in a large-scale sea breeze
cell. The Los Angeles Basin, for example, is bordered
on its southwestern side by the Pacific Ocean and on
its eastern side by the San Bernardino Mountains. The
Mojave Desert lies to the east of the mountains. The
desert heats up more than does land near the coast dur-
ing the day, creating a thermal low over the desert,
drawing air in from the coast, and creating the circula-
tion pattern shown.
Figure 6.20 shows the variation of sea and land breeze
wind speeds at Hawthorne, California, near the coast
in the Los Angeles Basin, over a three-day period.
Sea breeze wind speeds peak in the afternoon, when
land-ocean temperature differences peak. Similarly, sea
breeze winds peak during summer and are minimum
during winter. Land breezes are weaker than sea breezes
because the land-ocean temperature difference is small
at night.

6.7.4.4. Chimney Effect and Elevated
Pollution Layers
Pollutants in an enclosed basin, such as the Los Angeles
Basin, can escape the basin through mountain passes
and over mountain ridges. A third mechanism of escape
is through the mountain chimney effect (Lu and Turco,
1995; Figure 6.19). Through this effect, a mountain
slope is heated, causing air containing pollutants to rise,
injecting the pollutants from within the mixed layer into
the background troposphere.
Instead of dispersing, gases and particles may build
up in elevated pollution layers . Pollutant concentra-
tions in these layers often exceed those near the ground.
Figure 6.21 shows a brilliant sunset through an elevated
pollution layer in Los Angeles. Elevated layers form in
one of at least four ways.
First, the rising portion of a sea breeze circulation
can lift and inject pollutants into an inversion layer.
Elevated pollution layers formed by this mechanism
have been reported in Tokyo (Wakamatsu et al., 1983),
Athens (Lalas et al., 1983), and near Lake Michigan
(Lyons and Olsson, 1973; Fitzner et al., 1989). Figure
6.22 shows an example of an elevated pollution layer
formed by a sea breeze circulation over Long Beach,
California.
Second, some of the air forced up a mountain slope
by winds may rise into and spread horizontally in an
inversion layer. Of the air that continues up the moun-
tain slope past the inversion, some may circulate back
down into the inversion (e.g., Lu and Turco, 1995).
Elevated pollution layers formed by this mechanism
have been observed adjacent to the San Bernardino and
San Gabriel Mountains in Los Angeles (Wakimoto and
McElroy, 1986).

6.7.4.2. Valley and Mountain Breezes
A valley breeze is a wind that blows from a valley
up a mountain slope and results from the heating of the
mountain slope during the day. Heating causes air on the
mountain slope to rise, drawing air up from the valley to
replace the rising air. Figure 6.19 illustrates that in the
case of a mountain near the coast, a valley breeze can
become integrated into a large-scale sea breeze cell. The
opposite of a valley breeze is a mountain breeze ,which
originates from a mountain slope and travels downward.
Mountain breezes typically occur at night, when moun-
tain faces cool rapidly. As a mountain face cools, air
above the face also cools and drains downslope.