But new research led by a University of Washington atmospheric scientist shows that, in some regions, nitrogen oxides emitted by the soil are much greater than expected and could play a substantially larger role in seasonal air pollution than previously believed.
Nitrogen oxide emissions total more than 40 million metric tons worldwide each year, with 64 percent coming from fossil fuel combustion, 14 percent from burning and a surprising 22 percent from soil, said Lyatt Jaeglé, a UW assistant professor of atmospheric sciences. The new research shows that the component from soil is about 70 percent greater than scientists expected.
Instead of relying on scattered ground-based measurements of burning and combustion and then extrapolating a global total for nitrogen oxide emissions, the new work used actual observations recorded in 2000 by the Global Ozone Monitoring Experiment aboard the European Space Agency's European Remote Sensing 2 satellite.
Nitrogen oxide emissions from fossil fuel combustion are most closely linked to major population centers and show up in the satellite's ozone-monitoring measurements of nitrogen dioxide, part of the nitrogen oxides family. Other satellite instruments can detect large fires and the resulting emissions also can be measured by the ozone-monitoring experiment, Jaeglé said.
But the satellite also picks up other nitrogen oxide signals not attributable to fuel combustion or burning, and those emissions must come from soil, Jaeglé said.
"We were really amazed that we could see it from space, but because the pulse is so big the satellite can see it," she said.
Soil emissions are seen primarily in equatorial Africa at the beginning of the rainy season, especially in a re