Page 330 - Fundamentals of Air Pollution 3E

286 18. The Meteorological Bases of Atmospheric Pollution

V. REMOVAL MECHANISMS

Except for fine participate matter (0.2 /xm or less), which may remain
airborne for long periods of time, and gases such as carbon monoxide,
which do not react readily, most airborne pollutants are eventually removed
from the atmosphere by sedimentation, reaction, or dry or wet deposition.

A. Sedimentation (Settling by Gravity)
Particles less than about 20 pm are treated as dispersing as gases, and
effects due to their fall velocity are generally ignored. Particles greater than
about 20 ^tm have appreciable settling velocities. The fall velocity of smooth
spheres as a function of particle size has been plotted (Fig. 18-5) by Hanna
et al. (11). Particles in the range 20-100 /im are assumed to disperse approxi-
mately as gases, but with their centroid moving downward in the atmo-
sphere according to the fall velocity. This can be accounted for by sub-
tracting v gt from the effective height of release, where z> g is the gravitational
fall velocity of the particles and i, in seconds, is xlu, where x is downwind
distance from the source in m and u is wind speed. This is called the tilted
plume model. The model may be modified to decrease the strength of the

Fig. 18-5. Fall velocity of spherical particles as a function of particle diameter and density.
Source: Adapted from Hanna et al. (11).