AIR POLLUTION METEOROLOGY 65
layer varies with the size of the city, and depends on how
long the air has moved over the city. In New York, for exam-
ple, the mixed layer is typically 300 m thick; in Johnstown,
Pa., an industrial valley city with just under 100,000 popu-
lation, it is only a little over 100 m.
Figure 6 indicates how the temperature changes shown
in Figure 5 influence the diurnal variation of pollution due
to an elevated source in the country; at night, vertical mixing
is negligible and the air near the ground is clean. Some time
shortly after sunrise, the mixed layer extends to just above
the source, and the elevated polluted layer is mixed with the
ground air, leading to strong pollution (also referred to as
“fumigation”), which may extend many kilometers away
from the source. Later in the morning and early afternoon,
the heating continues and thickens the mixed layer. Also, the
wind speed typically increases, and the pollution decreases.In the city, many sources usually exist in the thin night-
time mixed layer. Since this layer is so thin, and the wind
usually weak, dense pollution occurs at night. Right after
sunrise, the pollution at first increases somewhat, as the efflu-
ent from large, elevated sources is brought to the ground. As
the mixed layer grows, the concentrations diminish, and, in
the early afternoon, they are often less than the nighttime
concentrations (see Figure 7).
Thus, the main difference between air pollution climates
in the city and country is that country air near industrial
sources is usually clean at night, whereas the city air is dirtier
at night than in the middle of the day. These differences are
most pronounced during clear nights and days, and can be
obliterated by diurnal variations of source strengths. Figure 7
shows the characteristic behavior only because the sources of
pollution at Johnstown, Pa., are fairly constant throughout.MIDDAY DAYMORNING (FUMIGATION) MORNING (FUMIGATION)NIGHT NIGHTCOUNTRY CITYMIXED LAYERFIGURE 6 Concentrations of effluent (schematic) as function of time of day, over
city and country.504030(^2005101520)
Time of day
100-T, (%)
FIGURE 7 Concentrations of air pollution (100-T%), as function of time of day, on clear
day (solid line) and cloudy day (dashed line), at Johnstown, Pa.
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