Science Visualized Environment

Ups and downs in the quest for clean air

BREATHING EASIER  Newly released maps reveal that U.S. air quality has markedly improved over the last decade.

NASA Goddard Scientific Visualization Studio

By Andrew Grant

Newly released maps reveal that U.S. air quality has markedly improved over the last decade. The evidence comes from measurements of nitrogen dioxide (NO2 ), a pollutant emitted by vehicles and coal power plants. Yet as the United States cleans up, rapidly industrializing cities in Asia, the Middle East and Africa are spewing more and more of the yellow-brown gas.

NO2 may not have the notoriety of carbon monoxide or lead, but it causes respiratory problems and combines with other pollutants to form ground-level ozone, a major part of smog. “You wouldn’t get unhealthy levels of ozone in cities without nitrogen dioxide,” says Bryan Duncan, an atmospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

The maps, based on data from NASA’s Aura satellite, show a clear decrease in NO2 nationwide from 2005 to 2011. Duncan attributes much of the change to curbing power plant emissions, spurred by a 2005 Environmental Protection Agency policy that protects states downwind of major polluters.

However, the promising air quality trend doesn’t extend to the developing world. Beijing headlines a host of Chinese cities with steadily rising NO2 emissions, joined by cities in emerging markets such as India, Bangladesh, Pakistan, Iran and Nigeria.

Mixed results

While cities in the United States, Western Europe and Japan are cleaning up their acts (top chart), air pollution is rising sharply in rapidly industrializing cities (bottom chart). Figures are in quadrillions (1015) of NO2 molecules per square centimeter.

Source: A. Hilboll/Univ. of Bremen

The amount of nitrogen dioxide hovering over major U.S. cities dropped substantially from 2005 to 2011, according to observations by NASA’s Aura satellite. Regions shaded red have the most nitrogen dioxide molecules in the column of air above, while blue areas have the least. NASA Goddard Scientific Visualization Studio
The nationwide decrease in the amount of nitrogen dioxide from 2005 to 2011 is attributed to reductions in power plant emissions. NASA Goddard Scientific Visualization Studio
The air above New York, Philadelphia and Washington, D.C., has become a lot cleaner since 2005. New York’s nitrogen dioxide levels fell 32 percent between 2005–2007 (left) and 2009–2011 (right), while Philadelphia’s dropped 26 percent. NASA Goddard Scientific Visualization Studio
Chicagoans aren’t the only ones appreciating the city’s 43 percent decrease in nitrogen dioxide levels from 2005–2007 (left) to 2009–2011 (right). Northern breezes in the summer often push Chicago air over Lake Michigan, meaning that cities around the lake can inherit the Windy City’s smog. NASA Goddard Scientific Visualization Studio
Although known for its smog, Los Angeles has seen its air quality improve. Levels of nitrogen dioxide, a key ingredient for smog, decreased by 40 percent from 2005-2007 (left) to 2009-2011 (right). NASA Goddard Scientific Visualization Studio

A version of this article appears in the August 23, 2014 issue of Science News.

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