202 CHAPTER 8 Air and Air Pollution
How Weather and Topography
Affect Air Pollution
Changes in temperature throughout the day produce air
circulation patterns that dilute and disperse air pollutants.
During a temperature inversion,
however, polluting gases and partic-
ulate matter remain trapped in high
concentrations close to the ground,
where people live and breathe.
Temperature inversions usually per-
sist for only a few hours before solar
energy warms the air near the ground. Sometimes a stalled
high-pressure air mass allows a temperature inversion to per-
sist for several days, causing atmospheric stagnation.
Certain types of topography (surface features)
increase the likelihood of temperature inversions. Cit-
ies located in valleys, near a coast, or on the leeward sidehydrocarbons; and oxygen in the atmosphere to produce
ozone. This reaction requires solar energy (Figure 8.9).
Ozone, a principal component of photochemical smog,
reacts with other air pollutants, including hydrocarbons,
to form more than 100 different secondary air pollutants
(peroxyacyl nitrates [PANs], for example). These pollut-
ants injure plant tissues, irritate eyes, and aggravate respi-
ratory illnesses in humans.
The main human sources of the ingredients for pho-
tochemical smog are combustion and spilling of petro-
leum products, and in particular automobile exhaust and
gasoline filling stations. Bakeries and dry cleaners are also
significant contributors. When bread is baked, yeast by-
products that are volatile hydrocarbons are released to the
atmosphere, where solar energy powers their interactions
with other gases to form ozone. The volatile fumes from
dry cleaners also contribute to photochemical smog.
xtemperature
inversion A layer of
cold air temporarily
trapped near the
ground by a warmer
upper layer.«ÃÌÊvÊ« ÌV iV>ÊÃ}ÊUÊ}ÕÀiÊn°Ê
Photochemical smog is a mixture that includes ozone, peroxyacyl nitrates (PANs),
nitric acid, and organic compounds such as formaldehyde.