Page 60 - Fundamentals of Air Pollution

36 3. Scales of the Air Pollution Problem

dally under unstable meteorological conditions that cause portions of the
plume to reach the ground in high concentrations.
There are many releases of pollutants from relatively short stacks or vents
on the top of one- or two-story buildings. Under most conditions such
releases are caught within the turbulent down wash downwind of the build-
ing. This allows high concentration to be brought to the ground. Many
different pollutants can be released in this manner, including compounds
and mixtures that can cause odors. The modeling of the transport and
dispersion of pollutants on this scale is discussed in Chapter 20.
Usually the effects of accidental releases are confined to the local scale.

II. URBAN

There are two different types of air pollution problems in urban areas.
One is the release of primary pollutants (those released directly from
sources). The other is the formation of secondary pollutants (those that are
formed through chemical reactions of the primary pollutants).
Air pollution problems can be caused by individual sources on the urban
scale as well as the local scale. For pollutants that are relatively nonreactive,
such as carbon monoxide and particulate matter, or relatively slowly reac-
tive, such as sulfur dioxide, the contributions from individual sources add
together to yield high concentrations. Since a major source of carbon monox-
ide is motor vehicles, "hot spots" of high concentration can occur especially
near multilane intersections. The emissions are especially high from idling
vehicles, and if high buildings surround the intersection, the volume in
which the pollution is contained is severely restricted. The combination of
these factors results in high concentrations.
Urban problems result from the formation of secondary pollutants. A
major problem of many large metropolitan areas is the formation of ozone
from photochemical reactions of oxides of nitrogen and various species
of hydrocarbons. These reactions are catalyzed by the ultraviolet light in
sunlight and are therefore called photochemical reactions. Many metropoli-
tan areas are in nonattainment for ozone; that is, they are not meeting the
air quality standards. The Clean Air Act Amendments (CAAA) of 1990
recognize this as a major problem and have classified the various metropoli-
tan areas that are in nonattainment according to the severity of the problem
for that area. The CAAA sets timetables for the various classifications
for reaching attainment with the National Ambient Air Quality Standards
(NAAQS). Oxides of nitrogen, principally nitric oxide, NO, but also nitro-
gen dioxide, NO 2, are emitted from automobiles and from combustion
processes. Hydrocarbons are emitted from many different sources. The
various species have widely varying reactivities. Determining the emissions
of the various species from the many sources in order to conduct control