Effects of Meteorology on Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 145

carbon monoxide emissions from vehicles, and gas-to-
particle conversion rates. In particular, higher tempera-
tures increase ozone and other pollutants due to chem-
ical and physical feedbacks (Section 12.5.6).

were those outside (Howard, 1833). Urban areas are
generally warmer during the day than are vegetated
areas around them because urban surfaces replace veg-
etation, reducing evapotranspiration from it. Evapo-
transpiration normally keeps the surface cool (because
evaporation is a cooling process) (e.g., Oke, 1978,
1988). Urban surfaces also have sufficiently different
properties of surface material (e.g., heat capacities, ther-
mal conductivities, albedos, emissivities) to enhance
urban warming relative to surrounding vegetated areas.
Figure 6.18 shows a satellite-derived image of daytime
surface temperatures in downtown Atlanta and its envi-
rons. Road surfaces and buildings stand out as being
particularly hot.
Increased urban temperatures result in increased mix-
ing depths and faster near-surface winds. Although
faster near-surface winds disperse pollutants, higher
temperatures increase ozone and other pollutants due
to chemical and physical feedbacks (Section 12.5.6).
Increased urban temperatures may also be responsible
for enhanced thunderstorm activity (Bornstein and Lin,
2000).

6.7.2. Soil Liquid Water Content
An important parameter that affects ground temper-
atures, and therefore pollutant concentrations, is soil
liquid water content ( soil moisture ). Increases in soil
moisture cool the ground, reducing convection, decreas-
ing mixing depths, and slowing near-surface winds.
Thinner mixing depths and slower wind speeds enhance
pollutant buildup. Conversely, decreases in soil liquid
water increase convection, increasing mixing depths
and near-surface winds, reducing pollution.
Soil moisture cools the ground in two major ways.
First, evaporation of liquid water in soil cools the soil.
Therefore, the more liquid water a soil has, the greater
the evaporation and cooling of the soil during the day.
Second, liquid water in soil increases the average spe-
cific heat of a soil-air-water mixture. The wetter the
soil, the less the soil can heat up when solar radiation is
added to it.
In a study of the effects of soil liquid water on temper-
atures, winds, and pollution, it was found that increases
in soil water of only 4 percent decreased peak near-
surface air temperatures by up to 6 ◦ C, decreased wind
speeds by up to 1.5 m s − 1 , delayed the times of peak
ozone mixing ratio by up to two hours, and increased the
magnitude of peak particulate concentrations substan-
tially in Los Angeles over a two-day period (Jacobson,
1999a). Such results imply that rainfall, irrigation, and
climate change all affect pollution concentrations.

6.7.4. Local Winds
Another factor that affects air pollution is the local wind.
Winds arise due to pressure gradients. Although large-
scale pressure gradients affect winds, local pressure gra-
dients, resulting from uneven ground heating, variable
topography, and local turbulence, can modify or over-
ride large-scale winds. Important local winds include
sea, lake, bay, land, valley, and mountain breezes.

6.7.4.1. Sea, Lake, and Bay Breezes
Sea, lake, and bay breezes form during the day between
oceans, lakes, or bays, respectively, and land. Figure
6.19 illustrates a basic sea breeze circulation. During
the day, land heats up relative to water because land has
alower specific heat than does water. Rising air over
land forces air aloft to diverge horizontally, decreas-
ing surface air pressures (setting up a shallow thermal
low-pressure system) over land. As a result of the pres-
sure gradient between land and water, air moves from
the water, where the pressure is now relatively high,
toward the land. In the case of ocean water meeting
land, the movement of near-surface air is the sea breeze .
Although the ACoF acts on the sea breeze air, the dis-
tance traveled by the sea breeze is too short (a few
tens of kilometers) for the Coriolis force to turn the air
noticeably.

6.7.3. Urban Heat Island Effect
Land cover affects ground temperatures, which affect
pollutant concentrations. Most of the globe is covered
with water (71.3 percent) or snow/ice (3.3 percent). The
remainder is covered with forests, grassland, cropland,
wetland, barren land, tundra, savanna, shrub land, and
urban areas. Urban surfaces consist primarily of roads,
walkways, rooftops, vegetation cover, and bare soil.
Urban construction material surfaces increase sur-
face temperatures due to the urban heat island effect ,
first recorded in 1807 by English meteorologist Luke
Howard (1772-1864), who is also known for classify-
ing clouds. He measured temperatures at several sites
within and outside London and found that tempera-
tures within the city were consistently warmer than