Dust is a fact of life for the Dogon people in Mali.

© 1999, David Tenenbaum.

R E L A T E D
W H Y F I L E S

Air pollution
Asthma

Image courtesy of Total Ozone Mapping Spectrometry, NASA Goddard Space Flight Center

It's another hazy, lazy day of summer down in Miami, with lots of fine particles hanging in the air and causing a bit of a glow in the air. Normally, these particles would be blamed on a local coal-burning power plant or giant road-construction project.

People worry about these particles, which are less than 2.5 millionths of a meter in size, because they are small enough to be inhaled and reach the lungs. It's not a theoretical worry: These particles could cause health problems and have been shown to boost death rates.

But Joseph Prospero of the University of Miami's Rosensteil School of Marine and Atmospheric Sciences says you have to look further than a local bulldozer to explain why the Southeast United States has high levels of airborne dust in summer. The true source of distress, he says, is thousands of miles to the east -- in the drylands of northern Africa.

Prospero, who has been tracking the arrival of African dust in Florida for 23 years, says it may supply as much as half of all fine particles in Miami's air during the summer. A recent summary of dust statistics showed the "dust is here every year without fail. It comes according to the calendar, and the concentrations are very high, year after year."

Unfortunately, summer is also the time when photochemical haze -- smog -- is worst in south Florida, he notes.

Twenty-plus years ago, the idea of blaming Florida's haze on imported dust seemed a bit outlandish, but a number of lines of evidence have made it the accepted wisdom.

• Fine dust is most prevalent in June, July and August, when the strongest trade winds blow west from Africa. (These winds also push hurricanes west toward the Caribbean and Southeast United States.)

• The summer dust is unlike the winter dust, when the trade winds are weaker. When summer rains evaporate, they often leave a fine reddish-brown, rouge-like residue; winter rains, which carry mainly local pollution, leave a greyish, gritty residue.

• The mineral composition of dust resembles that of soil particles in Africa.

  There are few local sources of fine dust near the monitoring station in Miami.

• Satellites have tracked the dust as it flows from one continent to another. Check out the daily "aerosol" (dissolved particle) satellite scan by clicking "aerosol."

Although Prospero has concentrated on dust flow to Florida and the Caribbean, he notes that fine dust from Africa also affects National Parks east of the Mississippi which, oddly enough, have more fine soil particles than parks in the arid Southwest.

All told, during the summer in Miami, about half of dust particles with a diameter of less than 2.5 microns (millionths of a meter) comes from Africa, Prospero says. That dust is subject to the Environmental Protection Agency's so-called PM 2.5 standard."

Much of the dust originates in dry lake beds and other lowlands in Africa, where minerals composed of quartz, carbonate and clay have washed. The total output of airborne dust from northern Africa is estimated at several hundred million tons per year.

Everybody talks about the dust...
Knowing the source of this dust is one thing, but doing something about it is something else. North Africa is a vast, arid place, and every time it gets dry enough to kill the vegetation, the dust flies. The prospects for preventing the dust, Prospero says, are "hopeless."

In the meantime, the proof that so much fine dust comes in with the wind has implications for pollution control and air quality regulations. With such a high level of natural airborne particles, regulations leave less room for dust caused by human activity, Prospero notes. It's a tough problem: Should industry in Florida be allowed to emit as much dust as industry elsewhere, if that dust -- when combined with natural dust -- creates particle concentrations that exceed Environmental Protection Agency standards? The EPA knows about the problem, Prospero says, but has not indicated how it will deal with it.

Despite the concern about intercontinental dust, Prospero says it does seem to have some benefits. The huge flow of dust from China to the central North Pacific Ocean supplies the essential nutrient iron to floating plants that form the basis of the food chain. A lack of iron, he points out, limits plant growth even when major nutrients like phosphorus and nitrogen are present. In fact, iron fertilization of the ocean was once proposed as a scheme to increase oceanic plant growth and reduce greenhouse warming!

The dust can have another benefit -- making soil. In Bermuda, Prospero points out, "The soils are largely comprised of African dust. They look very much like African dust -- fine-grained, reddish-brown soils." Furthermore, he adds, a "prime source" of soil in Miami is this same air-borne dust.

--David Tenenbaum

Long-Term Measurements of the Transport of African Mineral Dust to the Southeastern United States: Implications for Regional Air Quality, Joseph Prospero, Journal of Geophysical Research, July 20, 1999, pp. 15-917 to 15-927.