Page 335 - Fundamentals of Air Pollution 3E

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Transport and Dispersion

of Air Pollutants

I. WIND VELOCITY

A. Wind Direction
The initial direction of transport of pollutants from their source is deter-
mined by the wind direction at the source. Air pollutant concentrations
from point sources are probably more sensitive to wind direction than any
other parameter. If the wind is blowing directly toward a receptor (a location
receiving transported pollutants), a shift in direction of as little as 5° (the
approximate accuracy of a wind direction measurement) causes concentra-
tions at the receptor to drop about 10% under unstable conditions, about
50% under neutral conditions, and about 90% under stable conditions. The
direction of plume transport is very important in source impact assessment
where there are sensitive receptors or two or more sources and in trying
to assess the performance of a model through comparison of measured air
quality with model estimates.
There is normally considerable wind direction shear (change of direction)
with height, especially near the ground. Although surface friction causes
the wind to shift clockwise (veer) with height near the ground, the hori-

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