Page 178 - Fundamentals of Air Pollution

144 10. Effects on the Atmosphere, Soil, and Water Bodies

surements at a wavelength of 0.5 /im. The ratio of the incident solar trans-
missivity to the extraterrestial solar intensity can be as high as 0.5 in clean
atmospheres but can drop to 0.2-0.3 in polluted areas, indicating a decrease
of 50% in ground-level solar intensity. The turbidity coefficient can also be
derived from these measurements and used to approximate the aerosol
loading of the atmosphere. By assuming a particle size distribution in the
size range 0.1-10.0 /am and a particle density, the total number of particles
can be estimated. The mass loading per cubic meter can also be approxi-
mated. Because of the reasonable cost and simplicity of the sun photometer,
it is useful for making comparative measurements around the world,

C. Precipitation
Pollution can cause opposite effects in relation to precipitation. Addition
of a few particles that act as ice nuclei can cause ice particles to grow at the
expense of supercooled water droplets, producing 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
particles are added, none of them grow sufficiently to cause precipitation.
Therefore, the effects of pollution on precipitation are complex.

II. FORMATION OF ATMOSPHERIC HAZE

Atmospheric haze is the condition of reduced visibility caused by the
presence of fine particles or NO 2 in the atmosphere. The particles must be
0.1-1.0 /u,m in diameter, the size range in which light scattering occurs.
The source of these particles may be natural or anthropogenic.
Atmospheric haze has been observed in both the western and eastern
portions of the United States. Typical visual ranges in the East are <15
miles and in the Southwest >50 miles. The desire to protect visual air
quality in the United States is focused on the national parks in the West.
The ability to see vistas over 50-100 km in these locations makes them
particularly vulnerable to atmospheric haze. This phenomenon is generally
associated with diffuse or widespread atmospheric degradation as opposed
to individual plumes.
The major component of atmospheric haze is sulfate particulate Tnatter
(particularly ammonium sulfate), along with varying amounts of nitrate
particulate matter, which in some areas can equal the sulfate. Other compo-
nents include graphitic material, fine fly ash, and organic aerosols.
The sources of particulate matter in the atmosphere can be primary,
directly injected into the atmosphere, or secondary, formed in the atmo-
sphere by gas-to-partide conversion processes (13). The primary sources
of fine particles are combustion processes, e.g., power plants and diesel