Page 180 - Fundamentals of Air Pollution

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

sufficiently low vapor pressure for the gas-phase concentration of the oxi-
dized product to exceed its saturation vapor pressure. When this condition
occurs, nucleation and condensation may proceed, relieving supersatura-
tion. These processes result in the transfer of mass to the condensed phase.
Aerosol growth in size occurs while condensation is proceeding.
Coagulation, i.e., the process by which discrete particles come in contact
with each other in the air and remain joined together by surface forces,
represents another way in which aerosol diameter will increase. However,
it does not alter the mass of material in the coagulated particle.
The clearest example of this working model of homogeneous gas-to-
particle conversion is sulfuric acid aerosol formation. Sulfuric acid (H 2SO 4)
has an extremely low saturation vapor pressure. Oxidation of relatively
small amounts of sulfur dioxide (SO 2) can result in a gas-phase concentra-
tion of H 2SO 4 that exceeds its equilibrium vapor pressure in the ambient
atmosphere, with the subsequent formation of sulfuric acid aerosol. In
contrast, nitric acid (HNO 3) has a much higher saturation vapor pressure.
Therefore, the gas-phase concentration of HNO 3 is not high enough to
permit nucleation of nitric acid aerosol in typical atmospheric systems.
Atmospheric haze can occur over regions of several thousand square
kilometers, caused by the oxidation of widespread SO 2 and NO 2 to sulfate
and nitrate in relatively slow-moving air masses. In the eastern United
States, large air masses associated with slow- moving or stagnating anticy-
clones have become sufficiently contaminated to be called hazy blobs. These
blobs have been tracked by satellites as they develop and move across the
country (15).
The evolution of regional hazy air masses has been documented in several
case studies. The development of one such system is shown in Fig. 10-9.
During a 10-day period in the summer of 1975, a large region of the eastern
United States had decreased visibility associated with the presence of fine
particles in the atmosphere. The phenomenon occurred in association with
a slow-moving high-pressure system. Because it seldom rains during the
passage of these systems, the fine particles may have stayed airborne for
a longer period of time than usual.

III. EFFECTS OF ATMOSPHERIC HAZE

The United States Clean Air Act of 1977 set as a national goal the preven-
tion of any future degradation and the reduction of any existing impairment
of visibility in mandatory class I federal areas caused by anthropogenic air
pollution. The Clean Air Act Amendments of 1990 reinforce the support
of these goals. (See Chapter 22 for a discussion of federal classes of areas.)
These areas include most of the major national parks, such as the Grand
Canyon, Yosemite, and Zion Park. This portion of the Clean Air Act ad-