Page 328 - Fundamentals of Air Pollution 3E

284 18. The Meteorological Bases of Atmospheric Pollution

including wet and dry removal processed. These attempts were considered
quite successful, as radioactivity measurements provided some confirma-
tion of the regions affected. Elevated levels of radioactivity were measured
throughout the Northern Hemisphere. Because of the half-life of about 30
years for Cs-137, the contamination will endure.
In such emergencies, it is most important to know the local wind direction
at the accident site, so that the area that should be immediately evacuated
can be determined. The next important factor is the wind speed, so that
the travel time to various areas can be determined, again primarily for
evacuation purposes. Both of these can be estimated on-site by simple
means such as watching the drift of cigarette smoke. It would be well to
keep in mind that wind speeds are higher above ground and that wind
direction is usually different.
As evacuation is taking place, it is important to determine whether meteo-
rological events will cause a wind direction shift later on, requiring a change
in the evacuation scenario. Particularly in coastal areas, or areas of signifi-
cant terrain, authorities should be alert to a possible change in wind direc-
tion in going from night to day or vice versa. Useful advice may be obtained
from the nearest weather forecaster, although accurate forecasting of wind
direction for specific locations is not easy.
If the situation is one of potential rather than current release, specific
concentrations at various distances and localities may be estimated for
various conditions.

IV. EFFECTS OF POLLUTION ON THE ATMOSPHERE

Pollutant effects on the atmosphere include increased particulate matter,
which decreases visibility and inhibits incoming solar radiation, and in-
creased gaseous pollutant concentrations, which absorb longwave radiation
and increase surface temperatures. For a detailed discussion of visibility
effects, see Chapter 10.

A, Turbidity
The attenuation of solar radiation has been studied by McCormick and
his associates (9, 10) utilizing the Voltz sun photometer, which uses mea-
surements at a wavelength of 0.5 ^tm. The ratio of ground-level solar inten-
sity at 0.5 ^tm to extraterrestrial solar intensity can be as high as 0.5 in
clean atmospheres but can drop to 0.2-0.3 in polluted areas, indicating
that ground-level solar intensity can be decreased as much as 50% by
pollution in the air. By making measurements using aircraft at various
heights, the vertical extent of the polluted air can be determined. The
turbidity coefficient can also be derived from the measurements and used