HARVARD GAZETTE ARCHIVES

Daniel Jacob, a professor of atmospheric chemistry and environmental engineering, worked with graduate student T. Duncan Fairlie and research associate Rokjin Park to build a computer model for estimating the impact of dust from Asia. (Staff photo Rose Lincoln/Harvard News Office)

On the dustiest days in the western United States, 40 percent of the grime blows in from Asia. And fine particles can travel all the way around the world from Africa's Sahara Desert. These unwanted visitors show up in a new model of dust imports developed by researchers from Harvard and the National Aeronautics and Space Administration (NASA). The findings have important implications for air pollution and climate change.

The Asian dust invasion is heaviest in the western states in spring. It moves on into the eastern U.S., but in much lower quantities. The traveling grime is mobilized by strong winds blowing over deserts or dry lakes and streambeds. "Most of the dust is from natural sources and falls out close to its source," notes Daniel Jacob, a professor of atmospheric chemistry and environmental engineering at Harvard. "But fine dust can be transported over long distances: from Asia to North America, and from North Africa to Florida, and all the way around the world to Canada and the U.S."

The grit is a health problem. "Both Asian and Saharan dust events in the United States have exceeded the national air quality standards for such particles [less than 10 micrometers in size]," Jacob says.

A study done by other investigators, at the Harvard School of Public Health, concludes that an increase of particulate air pollution increases the risk of early death for people with diabetes, chronic obstructive lung disease, congestive heart failure, and inflammatory ailments like rheumatoid arthritis and lupus. Fine mineral dust is so damaging because it can penetrate much deeper into the lungs than larger particles.

According to Joel Schwartz, a professor of environmental epidemiology who led the study, each time dust levels increase to10 micrograms per cubic meter over two years, the risk of dying goes up by 28 percent for those with chronic obstructive pulmonary disease. Jacob's group found persistent Asian dust levels averaged 1.2 micrograms per cubic meter over the western United States in spring. Fortunately, levels are much lower in summer and fall and over the eastern part of the country.

Spring dustfalls

Jacob worked with graduate student T. Duncan Fairlie and research associate Rokjin Park to build a computer model for estimating the impact of dust from Asia. They tested the model's accuracy with measurements from a NASA aircraft mission over the Pacific led by Jacob in 2001. The results were compared with dust records from Japan, various Pacific Islands, and air quality observing stations in the United States.

The model simulates the highs and lows of dust flow. Following the largest flow in spring, things quiet down in summer. Then a second, less active peak blows dust around in the fall. Winter is quiet.

North African dust imports peak during summer months in Florida and along the Gulf Coast. It adds haze in the Great Smoky Mountains, the Appalachians, and other East Coast locations.

In addition to occasionally exceeding air quality standards, concentrations sometimes surpass air quality standards set by the Environmental Protection Agency (EPA). Dust also spoils the view. It affects what the EPA calls its "Regional Haze Rule," an effort to achieve natural visibility conditions at some national parks, monuments, and other wilderness areas. The agency wants to attain this clear goal by 2064. However, using the Harvard model, research associate Park found that Asian dust imports would render that goal unreachable even if the United States could somehow shut down the sources of pollution.

EPA strategy for improving wilderness visibility has so far focused exclusively on reducing emissions originating in this country. Jacob says that the agency must decide what to do about dust imported from Asia, as well as from Mexico and Canada. "Either EPA views this imported pollution as part of 'natural background' over which it has no control, or it has to engage other countries in the effort to improve visibility," he comments. "Of course, the United States is not solely an importer of pollution, but a large exporter as well."

However, Jacob adds, "the U.S. is the only nation that has set visibility standards, Officially, other countries don't care."

A broader problem concerns how Asian and Saharan dust impacts weather in the United States and the rest of the world. Mineral dust is the most important source of pollution of Earth's atmosphere. The particles heat the planet by absorbing radiation from the sun and trapping heat that would otherwise escape into space. But they also cool the planet by scattering incoming solar rays. "The net effect," Jacob admits, "is highly uncertain and widely debated. What is not disputed is that the effect is potentially large and will likely increase as a result of expanding desertification [due to a warming climate] and erosion of farmland."

Blowin' in the wind

Jacob and his team continue to do what they call "near-real-time simulations of dust and other chemicals" invading our shores. They post their results, along with maps, on the Web only 48 hours after the fact. The project is done in cooperation with the University of Washington and handled at Harvard by graduate student Rynda Hudman and software engineer Bob Yantosca.

Such a simulation, notes Jacob, "was used last year by scientists at the University of Vancouver in Canada to explain a dust event observed in British Columbia. It turned out to be caused not by particles from Asia, but from Saharan dust traveling all the way around the world to cross the Pacific Ocean.