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California's Farms Are an Even Larger Source of Air Pollution Than We Thought

A new study uncovers an underappreciated source of air pollution in the Golden State: fertilized soil.
A farmworker labors in a field on August 11th, 2004, near the town of Arvin, California.

A farmworker labors in a field on August 11th, 2004, near the town of Arvin, California.

California has the strictest car emissions standards in the country, regulations that have helped clear Los Angeles' notorious smog and likely contributed to southern California children's lung development. But there's one important source of pollution that state regulators have overlooked, a new study finds: fertilized soil on farmland, which emits nitrogen oxides. The upshot is that laws that have helped urban Californians' health may not be reaching rural residents.

"The potential impact this could have on health, especially in rural areas, is definitely on our radar," says Maya Almaraz, a postdoctoral researcher at the University of California–Davis who led the new study.

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Nitrogen oxides are gases that can worsen asthma symptoms. When combined with other pollution in the air, they can form ozone and particulates that are associated with more hospital visits and decreased heart and lung function. By running a mathematical model, Almaraz and her team concluded that California may be emitting as much as 52 percent more nitrogen oxides than regulators had previously thought. The big difference is in farmland, which the model finds contributes 20 percent of the state's nitrogen oxides pollution. Cars are thought to contribute 36 percent; "other mobile sources," such as airplanes and heavy equipment, make up 26 percent. The state Air Resources Board currently considers soil to contribute to less than 4 percent of California's nitrogen oxides emissions, according to a paper Almaraz and her colleagues published in the journal Science Advances. (The Air Resources Board didn't respond to a request for comment.)

The research team—made up of environmental scientists from UC–Davis and China—doesn't yet know if these amounts of nitrogen oxides are enough to harm people. They are working with Davis' medical school to study that next. But, Almaraz says, "Considering that these emissions are now going to rival those of mobile sources and even car sources, it's likely."

Almaraz got curious about nitrogen oxides from soil after seeing another study, which suggested that, in the mid-2000s, nitrogen oxide air pollution decreased much more in urban San Francisco and Sacramento than in agricultural Fresno and Bakersfield. "We see smog pollution decreasing in cities, which is awesome, but we're not really seeing that same decrease in these rural areas," Almaraz says. "So what we're thinking is maybe there's this alternate source." Scientists had long known fertilizer created air pollution, but nobody had carefully quantified its effects in California, even though the state has more than 25 million acres of farm and ranch land and produces two-thirds of the country's fruits and nuts.

Almaraz's team set about modeling soil nitrogen oxides emissions in different California regions, then double-checked their findings by comparing them with nitrogen oxide measurements taken by other scientists and with results from a test flight over the San Joaquin Valley, which includes Fresno and Bakersfield. The numbers were often the same, although they're sometimes quite different, which Almaraz attributes to old measurements and how variable nitrogen-gas readings can be from spot to spot.

Almaraz would like to see California farmers apply fertilizer in a more targeted way, but that doesn't mean farmers have to give it up altogether to help solve the problem. "There's a lot of strategies out there," she says, for example, applying fertilizer to crops' roots rather than spraying it overhead, and applying fertilizer at periods of a plant's growth cycle that are known to be more conducive to nitrogen intake.