Page 103 - Fundamentals of Air Pollution

I. General 73

may be of such magnitude as to harm the environment for a considerable
distance from the volcanic source. Clouds of volcanic particulate matter and
gases have remained airborne for very long periods of time. The eruption of
Mt. St. Helens in the state of Washington is a classic example of volcanic
activity. Figure 6-1 is a photograph of Mt. St. Helens during the destructive
eruption of May 18, 1980.
Accidental fires in forests and on the prairies are usually classified as
natural sources even though they may have been originally ignited by
human activities. In many cases foresters intentionally set fires in forest
lands to burn off the residue, but lightning setting off a fire in a large
section of forest land could only be classed as natural. A large uncontrolled
forest fire, as shown in Fig. 6-2, is a frightening thing to behold. Such a
fire emits large quantities of pollutants in the form of smoke, unburned
hydrocarbons, carbon monoxide, carbon dioxide, oxides of nitrogen, and
ash. Forest fires in the Pacific Northwest of the United States have been
observed to emit a plume which caused reduction in visibility and sunlight
as far away as 350 km from the actual fire.
Dust storms that entrain large amounts of particulate matter are a com-
mon natural source of air pollution in many parts of the world. Even a
relatively small dust storm can result in suspended particulate matter read-

Fig. 6-1. Mt. St. Helens during the eruption of May 1980. Source: Photo by C. Rosenfeld,
Oregon Air National Guard.