When winds blow air pollution out of the Eastern United States, relieved residents may think they are done with it. But unbeknown to them, it can take a circular route and return to haunt them at a later day. Thus successive air pollution events may actually be a single episode revisited.
George T. Wolff, Nelson A. Kelly, and Martin A. Ferman of the General Motors Research Laboratories have found this to be true of several pollution episodes they have studied. It also illustrates the truism that, in meteorology, you often can't understand local or even regional events without knowing what larger-scale weather is doing.
The GM scientists have described several such cases of summertime haze in a report in Nature. In these events, which occurred in 1979, the pollution originated at least partly in the Midwestern and Eastern United States. Air circulation carried it eastward then southward and westward so that it reached the Gulf Coastal regions. From there it worked northward again, eventually reappearing more or less where it had started.
In one instance, Hurricane David diverted polluted air blowing out of the Ohio Valley so that it flowed southwestward around the eastern flank of the storm to cause unexpected haziness along the Gulf Coast. Once there, it was caught up in the clockwise circulation around a high pressure system that had moved down from Canada. Eventually, it reappeared in the Midwest.
Tracing such events through a network of ground sample stations, weather charts, and satellite photos, the GM researchers conclude that such recycled smog is a regular part of Eastern United States weather. They bev lieve that it may be responsible for some of the hazy days that occur upwind of air pollution sources in that region.
This illustrates how widespread and persistent the influence of local or regional air pollution can be. It also points up the fact that air-borne dirt -- whether from natural or human sources -- is a factor to be considered in global weather because it can affect large areas not usually associated with the source.
African dust drifting over the Atlantic is another example. Every year, something like 100 million to 400 million metric tons of it are blown across to the Americas, according to estimates cited by J. M. Prospero R. A. Glaccum, and R. T. Nees of the University of Miami in a paper, also published recently in Nature, in which they describe seasonal shifts in the dust transport.
This transport, they note, is more than a curiosity. Dust concentrations such as those they have measured in French Guiana could heat the middle lower atmosphere while cooling the surface. This could, they say, significantly affect weather over a large area of the tropical and equatorial North Atlantic. Also, many viable fungal spores may be carried with the dust.
All of which shows how small and interconnected world we live in.