Page 360 - Fundamentals of Air Pollution 3E

VI. Concentration Variation with Averaging Time 315

experiments, even if conducted over a relatively large number of hours, in
relation to the conditions that cause the second highest concentration once
a year is extremely difficult, if not impossible.

VI. CONCENTRATION VARIATION WITH AVERAGING TIME

If emission and meteorological conditions remained unchanged hour
after hour, concentrations at various locations downwind would remain
the same. However, since such conditions are ever-changing, concentra-
tions vary with time. Even under fairly steady meteorological conditions,
with the mean wind direction remaining nearly the same over a period of
some hours, as the averaging time increases, greater departures in wind
direction from the mean are experienced, thus spreading the time-averaging
plume more and reducing the longer averaging time concentration com-
pared with that experienced for shorter averaging times at the location of
the highest concentrations. This effect is more pronounced for receptors
influenced by single point sources than for those influenced by a number
of point sources or by a combination of point and area sources, because
there will be many hours when the wind is not blowing from the source
to the receptor.
Figure 19-12 shows the maximum sulfur dioxide concentrations for eight
averaging times over a 1-year period (1976) for two air monitoring stations
in the Regional Air Monitoring (RAM) network in St. Louis. These two
monitoring stations, 104 and 113, have the highest and lowest maximum

Fig. 19-12. Variation of St. Louis SO 2 maximum concentrations with sampling time for
locations with highest (station 104) and lowest (station 113) maximum 1-hr concentrations.