Environmental Engineering Reference
In-Depth Information
designated human habitats or ecologically sensitive areas. 3 In this section we shall deal only with
nonreactive pollutants—that is, those that are not transformed in the atmosphere. In subsequent
sections we shall incorporate transformation processes into air-quality models. Air Pollution Meteorology
A basic information necessary for air-quality modeling is the wind statistics for the modeling domain
and the dispersion characteristics of the atmosphere. Winds blow from high- to low-pressure regions
on earth. Because the earth is a rotating body revolving around the sun, any spot on earth receives
constantly changing insolation over day and night and over the seasons. In addition, orographic
effects—that is, mountains and valleys—alter the course of winds, as does surface friction, sea-
land interfaces, street canyons, and so on. Thus, for air-quality modeling, multiyear wind statistics
are necessary for predicting the advection by winds of pollutants from the sources to the receptor.
Meteorological data are available from numerous weather stations operating around the world,
especially in the more developed countries. These weather stations measure and record surface
and upper air winds, atmospheric pressure, humidity, precipitation, insolation, temperature at the
ground, and the temperature gradient in the atmosphere—that is, the temperature variation with
altitude. The measurements are usually rendered twice daily at 0000 and 1200 Greenwich Mean
Time (GMT), so that measurements are synchronized all over the world. Past and present weather
data are available from national repositories—for example, in the United States, the National
Weather Service in Asheville, North Carolina.
In the atmosphere, dispersion occurs mostly by turbulent or eddy diffusion. Such diffusion is
orders of magnitude faster than molecular or laminar diffusion. The cause for turbulent diffusion is
either mechanical or thermal. Mechanical turbulence is due to wind shears in the free atmosphere
(adjacent layers of the atmosphere move in different directions or speeds), or friction experienced
by winds blowing over the ground surface and obstacles, such as tree canopies, mountains, and
buildings. The other cause of turbulence is the thermal gradient in the atmosphere. In the lower
troposphere, the temperature is usually higher near the ground and declines with altitude. In a dry
atmosphere, the gradient amounts to approximately
10 C/km. This is the dry adiabatic lapse
rate. Occasionally, the gradient is steeper, meaning more negative than
10 C/km. In a moist
atmosphere the gradient is less steep, due to the addition of the latent heat of condensation of water
vapor. At night, due to radiative cooling of the surface, the gradient may become positive, with
temperature increasing with altitude. This is called an inversion. Inversions can also occur aloft,
when a negative gradient is interrupted by a positive one. The bottom layer up to the inversion is
called the mixing layer, and the height to the inversion is called the mixing depth. An inversion layer
acts like a lid on the mixing layer. With an inversion, atmospheric conditions are especially prone
to air pollution episodes, because pollutants emitted at the ground are concentrated in the shallow
mixing layer. Later in the day, as the sun rises, the inversion layer may break up, allowing pollutants
to escape aloft and thus alleviating the pollution episode. Valleys and urban areas surrounded
by mountain chains experience frequently inversion layers, and therefore they are plagued with
3 Another type of modeling is called receptor modeling. These models attempt to identify and quantify the
contribution of various sources to the amount and composition of the pollutant concentration at the receptor
by using some characteristics of the sources. For example, receptor modeling may compare the distribution
of trace elements at the receptor to the distribution of trace elements in the emissions of various sources.
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