Encyclopedia of Environmental Science and Engineering, Volume I and II

66 AIR POLLUTION METEOROLOGY

Day-to-day Variations in Air Pollution

Equation (5) shows that, other things being equal, the con-

centration of contaminants is inversely proportional to the

wind speed. Figure 8 shows this effect on 24-hr total particu-

late concentration at Johnstown, for cases where the source

strengths were roughly the same, during the fall of 1964.

Conditions of particularly bad air pollution over wide

areas and for extended periods are accompanied not only by

light winds and calms, but also by unusually small mixing

depths ( D ) so that the ventilation factor is usually small.

Such conditions occur within large high-pressure areas

(anticyclones). In such areas, air is sinking. Sinking air is

warmed by compression. Thus, in an anticyclone (high-pres-

sure area), an elevated warm layer forms, below which there

is room only for a relatively thin mixed layer (Figure 9). The

inversion on top of the mixed layer prevents upward spread-

ing of the pollution, and when mountains or hills prevent

sideways spreading the worst possible conditions prevail.

A particularly bad situation arose in the industrial valley

town of Donora, Pa., in which many people were killed by

air pollution in 1948.

Cities in California, like Los Angeles, are under the influ-

ence of a large-scale anticyclone throughout the summer, and

an elevated inversion at a few hundred meters height occurs

there every day; that is why Los Angeles had air pollution

problems as soon as pollutants were put into the atmosphere

to any large extent. In the United States outside the West

Coast, stagnant anticyclones occur only a few times per year,

usually in the fall.

So far, relatively little use has been made in the USA of

forecast changes in air pollution potential from day to day.

As air pollution problems become more severe, more use

will be made of such forecasts. Already, this type of infor-

mation has proved itself in air pollution management in

some European countries.

Not much has been said about the influence of wind

direction on air pollution. When pollution is mainly due to

many, relatively small sources, as it is New York, the pollu-

tion is surprisingly insensitive to changes in wind direction.

Even in Johnstown, Pa., wind direction is unimportant except

for the case of easterly winds, when a single, huge steel plant

adds significantly to the contaminant concentration.

In contrast, wind direction plays a major role when most

of the pollution in a given area is due to a single or a few

major plants or if an industrial city is nearby. Also, there

are special situations, in which wind direction is particularly

important; for example, in Chicago, which has no pollution

sources east of the city, east winds bring clean air.

The main difference between the effects of lapse rate,

mixing depth, and wind speed on the one hand, and wind

R

R

500

400

300

200

100

0

012 34 5 6 7 8 9 10

CONCENTRATION,

g/m

3

Viso. mph

μ

FIGURE 8 Dependence of 24-hour average particle concentrations at Johnstown on wind speed at 150 ft.

R denotes rain.

BEFORE

SINKING

AFTER

SINKING

INVERSION

LAYER

MIXED

D LAYER

T

Z

FIGURE 9 Effect of sinking on vertical temperature

distribution (schematic).

C001_005_r03.indd 66C001_005_r03.indd 66 11/18/2005 10:13:39 AM11/18/2005 10:13:39 AM