Atmospheric scientists have long known that air pollution travels vast distances and is a global phenomenon. Now researchers at NASA Goddard Space Flight Center have conducted the first-ever satellite-based measurements of pollution aerosols transported from East Asia to North America.
The researchers looked at four years of satellite data and found the amount of pollution arriving in North America to be equivalent to 15 percent of local emissions of the United States and Canada. It is “a significant number,” says Hongbin Yu, an associate research scientist at the University of Maryland, in Baltimore, who is working at NASA Goddard and led the study.
“This means that any reduction in our emissions may be offset by the pollution aerosols coming from East Asia and other regions,” says Yu. The new study will be published in April in the American Geophysical Union’s Journal of Geophysical Research.
The study was conducted from 2002 to 2005, using measurements from a satellite instrument called the moderate-resolution imaging spectroradiometer (MODIS) onboard NASA’s Terra satellite. The instrument measures the reflective solar radiation and emitted thermal radiation from the earth’s surface and atmosphere.
The satellite-based instrument can look at 36 different wavelengths of the solar-terrestrial spectrum, and it does so with better spatial resolution than previous satellite instruments, says Lorraine Remer, a physical scientist and a member of the MODIS science team at Goddard.
For the study, the researchers measured the reflected solar radiation at seven different wavelengths. Being able to see different colors of the spectrum allows the researchers to differentiate the types of particles more accurately than the older sensors, says Remer.
“Some particulates are absorbing things like black carbon that come out of diesel exhaust, making it a black color,” says Ronald Prinn, a professor of atmospheric sciences and the director of the Center for Global Change Science at MIT. “Particles that are produced from sulfur that comes from the burning of coal are very bright white. You can look at the multiple colors … and get information about composition and density as well.”
The instrument is able to distinguish between man-made pollution and naturally occurring particles based on size. Naturally occurring dust and sea salt are typically larger than aerosol particles emitted from combustion sources, forest fires, automobiles, and industry, says Remer.
The MODIS instrument works by scanning a broad swath of the earth–about 2,300 kilometers–and counting the number of photons it is receiving by turning them into electrical signals. The instrument can measure the entire earth in one day.
MODIS does a better job than aircraft instrumentation does because it can observe the earth all the time, capturing events that only happen occasionally and accumulating them over the whole year, says Richard Honrath, a professor in the department of civil and environmental engineering at Michigan Technological University, in Houghton. “We can only do continuous measurements at ground level, but then you only see events that hit the ground,” he says.
The instrument also gets “spatial and time detail that one would never get from ground-based measurements, and it captures the entire pollution plumes rather than just having a few observing stations looking up,” says Prinn.
NASA researchers drew two virtual lines at 20 degrees north to 60 degrees north, and they measured the optical effect of the particles as they crossed those lines, says Yu. Using software that he made, the researchers culled this data and mapped it to see globally where the pollution is located.
The researchers found that 18 teragrams–almost 40 billion pounds–of pollution is exported from Asia, and that 4.5 teragrams–10 billion pounds, or about 25 percent–reaches North America annually, says Mian Chin, an atmospheric scientist at NASA and a coauthor of the study. But the instrument measures the total atmosphere column and does not have the vertical structure, so it is unknown how many of the pollutants are at surface level, and how many are aloft in the atmosphere, says Chin.
Despite that uncertainty, the scientists say that it is the higher-altitude pollution that is probably most worrisome. “We think the pollution being imported to North America will impact the weather and climate; we don’t expect any big impact on the air quality because particles from East Asia are exported at high altitude,” says Yu.
“It is very difficult to lower pollution levels of man-made pollutants to extremely low levels because pollutants come in the air from other countries that the U.S. Environmental Protection Agency, for example, cannot control,” says Prinn. Agrees Honrath: “You have to consider the future industrial growth of Asia if you develop long-range plans for meeting air-quality goals in the United States.”