Page 322 - Fundamentals of Air Pollution 3E

278 18. The Meteorological Bases of Atmospheric Pollution

clone (i.e., of a high). If the high has a warm core, there is likely to be very
little air movement near the center, i.e., stagnation. Under such circum-
stances, winds are very light. Skies are usually cloudless, contributing to
the formation of surface-based radiation inversions at night. Although the
clear skies contribute to instability in the daytime, the depth of the unstable
layer (i.e., mixing height) may be severely limited due to the subsidence
inversion over the high.
The mixing height at a given time may be estimated by use of the morning
radiosonde ascent plotted on a thermodynamic chart. The surface tempera-
ture at the given time is plotted on the diagram. If a dry adiabat is drawn
through this temperature, the height aboveground at the point where this
dry adiabat intersects the morning sounding is the mixing height for that
time. The mixing height for the time of maximum temperature is the maxi-
mum mixing height. Use of this sounding procedure provides an approxi-
mation because it assumes that there has been no significant advection
since the time of the sounding.

III. METEOROLOGICAL CONDITIONS DURING HISTORIC
POLLUTION EPISODES

A. Meuse Valley, Belgium

During the period December 1-5, 1930, an intense fog occupied the
heavily industrialized Meuse Valley between Liege and Huy (about 24 km)
in eastern Belgium (1). Several hundred persons had respiratory attacks
primarily beginning on the 4th and 63 persons died on the 4th and 5th after
a few hours of sickness. On December 6 the fog dissipated; the respiratory
difficulties improved and, in general, rapidly ceased.
The fog began on December 1 under anticyclonic conditions. What little
air motion occurred was from the east, causing air to drift upvalley, moving
smoke from the city of Liege and the large factories southwest of it into
the narrow valley. The valley sides extend to about 100 m, and the width
of the valley is about 1 km. A temperature inversion extended from the
ground to a height of about 90 m, transforming the valley essentially into
a tunnel deeper than the height of the stacks in the valley, which were
generally around 60 m. Much of the particulate matter was in the 2-6 /^m
range. The fog was cooled by radiation from the top and warmed by contact
with the ground. This caused a gentle convection in the "tunnel," mixing
the pollutants uniformly and resulting in nearly uniform temperature with
height.
The symptoms of the first patients began on the afternoon of December
3 and seemed to occur simultaneously along the entire valley. Deaths took
place only on December 4 and 5, with the majority at the Liege end of the