Fertilized soils weigh in as heavy contributor to smog.
A satellite survey has revealed that farmlands may have a bigger impact on air pollution and ground-level ozone than previously thought.
Lyatt Jaeglé from the University of Washington in Seattle and her colleagues measured the amounts of nitrogen dioxide rising from the Earth's surface in 2000. Nitrogen dioxide is known to produce ozone in the lower atmosphere, which can cause respiratory problems and helps to create photochemical smog.
The data were collected from the Global Ozone Monitoring Experiment (GOME) on the European Space Agency's satellite ERS-2. The researchers distinguished emissions of agricultural origin from industrial ones by simply looking at the type of land from which the gas emanated.
The team found that soil contributed 8.9 million tonnes of nitrogen in the form of nitrogen oxides, equivalent to 22% of the Earth's total surface emissions. That is about two-thirds greater than previous estimates.
As expected, the single largest source of nitrogen oxides was fuel combustion, accounting for 64% of the total emissions. Large-scale fires made up the remainder, broadly tallying with past findings.
"The result about soil emissions is a first," says team member Randall Martin, an atmospheric physicist at Dalhousie University in Halifax, Canada. The team's results appear in the journal Faraday Discussions1.
Scientists estimate that human activities have boosted nitrogen oxides emissions by three to six times since pre-industrial days.
Previous estimates of soil emissions relied on studying a small plot of land, and scaling up to the whole world.
"But that's a very difficult thing to do," says Martin. Different soils around the planet exist in a wide variety of conditions that can hugely affect their gas discharges. Satellite monitoring is thought to be a much more reliable method.
The nitrogen oxides originate from microbes in the soil that feed on inorganic forms of nitrogen, such as the ammonium and nitrate found in chemical fertilizers. They release nitrogen monoxide as waste, which rises from the soil to be oxidized into nitrogen dioxide, which in turn creates ozone.
The team found that soil emissions were highest from regions that were heavily fertilized. "That doesn't prove that there's a direct link," cautions Dwayne Heard, an atmospheric chemist from Leeds University.
But Martin thinks it is enough to prompt action to minimize nitrogen oxides emissions. Instead of risking agricultural productivity by using less fertilizer, this could perhaps be done by changing the timing of application, he says.
The team has collected seven years of data from the satellite. That's not sufficient to detect any trends, says Martin, but after another three years it may be possible to understand how nitrogen emissions change over time.
Getting good estimates of nitrogen oxides emissions is important for studies of both health and climate. As well as helping to form ozone, nitrogen dioxide can form aerosols in the atmosphere that affect climate by reflecting or absorbing radiation from the Sun.
JaegléL., SteinbergerL., MartinR. V. & ChanceK.Faraday Discuss., published online, doi: 10.1039/b502128f (2005).