Page 329 - Fundamentals of Air Pollution 3E

IV. Effects of Pollution on the Atmosphere 285

to estimate the aerosol loading of the atmosphere. By assuming a particle
size distribution in the size range 0.1-10 />tm and a particle density, the
total number of particles can be estimated. The mass loading per cubic
meter can also be estimated. Because of the reasonable cost and simplicity
of the sun photometer, it is useful for making comparative measurements
around the world.

B. Precipitation
Depending on its concentration, pollution can have opposite effects on
the precipitation process. Addition of a few particles that act as ice nuclei
can cause ice particles to grow at the expense of supercooled water droplets,
resulting in particles large enough to fall as precipitation. An example of
this is commercial cloud seeding, with silver iodide particles released from
aircraft to induce rain. If too many such particles are added, none of them
will grow sufficiently to cause precipitation. Therefore, the effects of pollu-
tion on precipitation are not at all straightforward.
There have been some indications, although controversial, of increased
precipitation downwind of major metropolitan areas. Urban addition of
nuclei and moisture and urban enhancement of vertical motion due to
increased roughness and the urban heat island effect have been suggested
as possible causes.

C. Fogs
As mentioned in the previous section, the increased number of nuclei
in polluted urban atmospheres can cause dense persistent fogs due to the
many small droplets formed. Fog formation is very dependent on humidity
and, in some situations, humidity is increased by release of moisture from
industrial processes. Low atmospheric moisture content can also occur,
especially in urban areas; two causes are lack of vegetation and rapid runoff
of rainwater through storm sewers. Also, slightly higher temperatures in
urban areas lower the relative humidity.

D. Solar Radiation

In the early part of this century, the loss of ultraviolet light in some
metropolitan areas due to heavy coal smoke was of concern because of the
resulting decrease in the production of natural vitamin D which causes the
disease rickets. Recently, measurements in Los Angeles smog have revealed
much greater decreases in ultraviolet than visible light. This is due to both
absorption by ozone of wavelengths less than 0.32 /xm and absorption by
nitrogen dioxide in the 0.36-0.4 ^m range. Heavy smog has decreased
ultraviolet radiation by as much as 90%.