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Atmospheric Transport
Atmospheric transport of pollutants can occur over long or short distances
depending on atmospheric structure and dynamics. So in fact in some cases the
near environment (1 to 2 km) of a power plant or a factory may be adversely
affected, such as yellowing of needles on pine trees or diminished yield of crops.
On the other hand, long-range transport can carry acidic material ten to 100 km
or much further from the source before they settle out.
Figure 2 shows different meteorological conditions near the earth's surface and the different accompanying atmospheric dispersion conditions. At the left of each plume sketch is a small plot of temperature as a function of height in the lower atmosphere. The dashed line in each case gives the temperature condition for the atmosphere that we call "neutral" , in that such a temperature condition neither promotes or suppresses convective turbulence. If the actual temperature line is more vertical than this dashed line, we say the atmosphere is "stable" and suppresses convection. This condition typically occurs on clear, calm evenings. If, on the other hand, the line is more horizontal than the dashed line, we say the atmosphere is "unstable" and promotes convection. Sunny summer days typically lead to this condition. Combinations of stable conditions near the surface and unstable aloft or vice versa also are shown.
Typical behavior of a plume from an elevated source is shown for each temperature condition. Notice the marked difference in the location nearest the stack at which the plume likely would reach the ground. In some cases a plume may travel hundreds of kilometers under stable conditions at night without coming near the ground, but be brought down to the surface at some remote location the next morning when it is caught by the looping conditions as surface heating creates unstable conditions near the surface.
The more complicated structures, such as an unstable lower atmosphere capped by a stable upper layer (called a temperature inversion, or just inversion), is typical in certain geographical locations such as Los Angeles, Denver, and Houston. These conditions confine pollutants to a shallow layer near the surface creating smog and unhealthy conditions.
Other geographical constraints, like a mountain valley, can reduce natural ventilation and trap pollutant near the surface. In Vail, Colorado most condominiums have fireplaces which, when operating simultaneously under low wind and stable atmospheric conditions, emit enough smoke to accumulate and cause significant air pollution problems.
Coastal areas, sites of most of the world's largest cities, have unusual atmospheric circulation patterns that may lead to air pollution problems. A typical example is the recirculation of air pollutants from the Chicago area due to the Lake Michigan sea breeze. During the day in summer when no large-scale systems are dominating the weather, the sea breeze blows from the lake toward land and carries urban air pollutants further inland. Here they rise and return at high elevations to locations over the lake. The sea breeze on the following day may then recirculate these pollutants into the Chicago area, or, if they have gradually drifted to the northwest, they could (and occasionally do) degrade air quality for smaller cities along the western shore of Lake Michigan.
NEXT: Sulfur and Nitrogen Compounds
