WASHINGTON, D.C. -- Large quantities of dust, originating in Africa, are blown across the Atlantic Ocean each summer and constitute up to one half of breathable particles in the air over Miami, Florida, according to a new study. African dust can on certain days push the total number of airborne particles above the limit set by the U.S. Environmental Protection Agency under the Clean Air Act.
The study, by Joseph M. Prospero of the University of Miami'sRosenstiel School of Marine and Atmospheric Sciences, will appear in the July 20 issue of Journal of Geophysical Research, published by the American Geophysical Union. Describing himself as a "champion of dust," Prospero reviewed 23 years of measurements of airborne particles, or aerosols, at a coastal site in Miami. He believes the impact of African dust is comparable throughout the southeastern United States. His study was funded in part by the National Science Foundation.
Prospero emphasizes that there is nothing new about the transoceanic transport of African dust. He says it has been occurring on a geologic time scale, giving the example that soil composition in Bermuda is consistent with African dust sources and shows little evidence of North American inputs.
The EPA's PM (for particulate matter) standard is based on the total mass of particles measuring 2.5 microns or less that may be observed in a 24 hour period. Particles of this size can penetrate deep into the lung, where they are deposited and react with lung tissue. The standard does not distinguish among various types of particles, which include pollution and local dust along with African dust. Prospero says that manmade photochemical pollution in the eastern and southeastern United States is at its highest in summer, the same time that most of the African dust arrives. The dust causes a dense haze, but people who are unaware of the dust might mistake it for local pollution haze, he says.