Page 182 - Fundamentals of Air Pollution

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

to atmospheric haze problems associated with power plants, urban plumes,
and agricultural activities.
Average airport visibilities over the eastern half of the United States have
been determined over a period of approximately 25 years (1948-1974) (6).
Although seasonal variations occur, the long-term trend has been decreased
visual air quality over the time period.

IV. VISIBILITY

Holzworth and Maga (16) developed a technique for examining the trend
in visibility and analyzed data for several California airports. Bakersfield's
visibility deteriorated over the period 1948-1957, and Sacramento's visibility
decreased over the period 1935-1958. Los Angeles had decreasing visibility
from 1932 to 1947, with little change over the period 1948-1959.
Holzworth (17) reported on the frequency of visibility of less than 7 miles
for 28 cities. Two periods of record were compared for each city. There
were increases in low visibility in only 26% of the comparisons from the
early period (around 1930-1940) to a later period (around the mid-1950s).
Miller et al. (18), using analyses for Akron, Ohio; Lexington, Kentucky;
and Memphis, Tennessee, concluded that "summer daytime visibilities
were significantly lower during the period 1966-69 than visibilities for the
preceding 4-year period."
Faulkenberry and Craig (19), in examining the trends at three Oregon
cities, utilized a modification of the Holzworth-Maga technique by which
a single statistic can be calculated for each year, indicating the probability
of observing better visibility at Salem, Oregon, with no trends at Portland
and Eugene, Oregon over the period 1950-1971.
Arizona has traditionally been a large copper-producing state. SO X emis-
sions from copper smelters near Phoenix and Tucson are shown in Fig. 10-
10 (1). Phoenix is located 100 km from the nearest smelter, and Tucson is
60 km from the nearest smelter. The improvement in visibility in the
1967-1968 period was due to a decrease in SO* emissions when there was
9-month shutdown caused by a strike. Improvement in visibility in the
mid-1970 was the result of better control technology and process changes.
Zannetti et al. (20) did an analysis of visual range in the eastern United
States again showing the importance of humidity but also showing the
importance of air mass type, which is usually related to its direction of
origin.
Mathai (21) summarized the specialty conference on atmospheric visibil-
ity. With the exception of water content of particles and the measurement
of organic species, analytical laboratory techniques are readily available
for particle analysis. Regulatory approaches to mitigate existing visibility
impairment and to prevent further impairment are being formulated. A