Page 320 - Fundamentals of Air Pollution 3E

276 18. The Meteorological Bases of Atmospheric Pollution

Fig. 18-1. The atmosphere's role in air pollution.

there can be a buildup of concentration over time even with significant air
motion if there are many sources.
Low- and high-pressure systems have considerably different ventilation
characteristics. Air generally moves toward the center of a low (Fig. 18-2)
in the lower atmosphere, due in part to the frictional turning of the wind
toward low pressure. This convergence causes upward vertical motion near
the center of the low. Although the winds very near the center of the low
are generally light, those away from the center are moderate, resulting in
increased ventilation rates. Note the increased wind in the area to the west
of the low in Fig. 18-2. Low-pressure systems generally cover relatively
small areas (although the low-pressure system shown in Fig. 18-2 covers
an extensive area) and are quite transient seldom remaining the same at a
given area for a significant period of time. Lows are frequently accompanied
by cloudy skies, which may cause precipitation. The cloudy skies minimize
the variation in atmospheric stability from day to night. Primarily because
of moderate horizontal wind speeds and upward vertical motion, ventila-
tion (i. e., total air volume moving past a location) in the vicinity of low-
pressure syterns is quite good.
High-pressure systems are characteristicaly the opposite of lows. Since
the winds flow outward from the high-pressure center, subsiding air from
higher in the atmosphere compensates for the horizontal transport of mass.