Effects of Pollution on Visibility, Ultraviolet Radiation, and Colors in the Sky - Air Pollution and Global Warming: History, Science, and Solutions - page 168

Figure 7.23. Deer at Golden, Colorado, in front of Denver's brown cloud, December 14, 1998. Photo by David
Parsons, available from National Renewable Energy Laboratory, U.S. Department of Energy, www.nrel.gov.

at Bryce Canyon, Utah, corresponding to a meteorolog-
ical range of less than 400 km, suggesting that visibility
was limited by gas scattering.
In Table 7.3, the meteorological range due to NO 2 (g)
absorption decreases from 1,590 to 63.6 km when
the NO 2 (g) mixing ratio increases from 0.01 to 0.25
ppmv. Thus, NO 2 (g) absorption reduces visibility more
than does Rayleigh scattering when the NO 2 (g) mix-
ing ratio is high. Results from a project studying
Denver's brown cloud (Figure 7.23) showed that
NO 2 (g) accounted for about 7.6 percent of the total
reduction in visibility, averaged over all sampling
periods, and 37 percent of the total reduction dur-
ing periods of maximum NO 2 (g). Scattering and
absorption by aerosol particles caused most remain-
ing extinction (Groblicki et al., 1981). In sum, NO 2 (g)

attenuates visibility in urban air when its mixing ratios
are high.
Although the effects of Rayleigh scattering and
NO 2 (g) absorption on visibility are nonnegligible in
polluted air, they are less important than are aerosol
particle scattering and absorption. Scattering by aerosol
particles causes between 60 and 95 percent of visibil-
ity reductions in smog, whereas absorption by aerosol
particles causes between 5 and 40 percent of such reduc-
tions (Cass, 1979; Tang et al., 1981; Waggoner et al.,
1981).
Table 7.4 shows meteorological ranges derived from
extinction coefficient measurements for a polluted and
less polluted day in Los Angeles. Particle scattering
dominated light extinction on both days. On the less
polluted day, gas absorption, particle absorption, and

Table 7.4. Meteorological ranges (km) resulting from gas scattering, gas absorption, particle
scattering, particle absorption, and all processes at wavelength of 0.55 monpolluted and

less polluted days in Los Angeles.

Gas Gas Particle Particle

Day scattering absorption scattering absorption All

Polluted (8/25/83) 366 130 9.6 49.7 7.42

Less polluted (4/7/83) 352 326 151 421 67.1

Meteorological ranges derived from extinction coefficients of Larson et al. (1984).