Agricultural nitrogen oxide emissions higher than previously thought
While California regulates nitrogen oxide emissions from cars and engines, an unexpectedly significant amount of the gases may be coming from an unrecognized source: agricultural lands. A UC Davis study shows that agricultural lands emit between 20 and 32 percent of the total nitrogen oxide gases in the state, far more than previously thought.
“The model we used showed a large and previously undetected source of NOx coming mainly from fertilized soils,” said Maya Almaraz, a postdoctoral fellow in the Department of Land, Air and Water Resources at UC Davis, in an email interview. “While this was surprising at first, after diving into the literature we found that this compared well with estimates of soil NOx in other agricultural regions of the world. This is the first study of its kind to take place in California, so we still have a lot more questions and a lot more to learn about soil NOx in rural areas of the state.”
NOx gases are an air pollutant harmful to human health, which is why they are regulated by various state agencies, including the California Air Resources Board. NOx can also help form fine particulate matter or react with volatile organic compounds in sunlight to form ground-level ozone, two of the components of smog. Some of the worst air quality sites in the nation are found in the Central Valley.
“NOx includes NO and NO2,” said Edith Bai, a professor at the Institute of Applied Ecology at the Chinese Academy of Sciences in an email interview. “NO is usually quickly oxidized to NO2 and so they are usually expressed as NOx. They are air pollutants and have been linked to upper respiratory disease, asthma, cancer, birth defects, cardiovascular disease, and sudden infant death syndrome, and environmental problems such as ozone depletion and acid rain. Many studies have reported high NOx emissions from croplands. Especially, several recent in situ experimental studies suggested that soil sourced NOx emission into the atmosphere may be high. However, in California, there is no state-wide estimation of soil NOx emission.”
Some earlier studies had measured NOx from cropland within 200 kilometers of Sacramento, but study co-authors Almaraz and Bai expanded their scope to include all of California, including some of the most heavily fertilized agricultural lands. They created a model combining a mass-balance and stable isotope model and used multiple data sources to predict the amount of NOx emitted from cropland, which had never been done before. Aircraft from Scientific Aviation flew over the San Joaquin Valley and collected gas to compare the results.
“These methods are both different from more traditional chamber based point measurements that are limited in their range and satellite imagery, which is unable to differentiate between soil and fossil fuel produced NOx,” Almaraz said. “Models like ours have been used elsewhere to estimate NOx emissions, just never before in California.”
Both strategies together suggested higher fluxes than reported by the CARB. In fact, Imperial Valley had some of the highest emissions ever recorded. While the researchers could not sort the collected NOx by source, the process of elimination provided sound evidence that agricultural lands were major contributors of NOx gases.
“The aerial measurements got the total NOx concentration and did not control for NOx transported from roadways or cities,” Bai said. “However, the NOx fluxes from the roadways and cities were estimated by previous studies. When we subtract these fluxes from the total NOx, we can get soil sourced flux.”
The amount of NOx emitted in a particular area was measured by sampling air collected by plane at various heights above the ground. As NOx rises, it mixes with the clean air above, becoming diluted. In the Central Valley, mountains on the east and west sides create barriers for air mixing and dilution. Ian Faloona, a biomicrometeorologist and associate professor in the Department of Land, Air and Water Resources at UC Davis, explained why NOx tends to stick around California.
“One of the things my work is focusing on is trying to really understand the meteorology of how the air moves through [the Central Valley],” Faloona said. “Air flows in and it slams into these mountains and doesn’t have enough energy to go over it. So it just kinda pools up here, and then it circulates and splashes around and slips out here and there, but you have this big mass of air that’s just sitting there. It gets polluted, pollution gets mixed into it, and it just sits there. So there’s definitely a meteorological problem because you’re in this giant bathtub, and so the air just doesn’t ventilate. That was the problem with Los Angeles.”
Los Angeles is now relatively smog-free compared to its early days. The air quality in the Central Valley hasn’t been improving quite as quickly, but thanks to programs focused on vehicle emissions, industry, and other particulate sources, the air quality is improving.
“NOx has been nicely decreasing in the San Joaquin Valley — NOx is the precursor to ozone — and that’s another important point of this,” Faloona said. “The precursor is going down, and so as we clean up our air, more and more things are going to become apparent that weren’t apparent when it was totally dominated by car exhaust.”
Back before the Clean Air Act and pollution reforms, the agricultural emissions seen in the study by Bai and Almaraz would have constituted a small fraction of the overall budget. As mobile emissions grow less and less, other sources emerge as frontrunners.
“But as we steadily improve our air quality, these types of things become relatively more important,” Faloona said. “That’s my justification for why everyone’s missed it so far.”
The results of the study indicate a need for more testing and measurement of NOx from cropland so that the soil pathways can be better understood and accounted for in the state budget. NOx produced by agriculture varies with soil moisture, temperature, and fertilizer application. Some areas may emit more than others. Bai and Almaraz suggest several possible alternatives that could minimize nitrogen emissions while maintaining production efficiency. Slow release fertilizers allow plants to more efficiently take up nitrogen, while cover crops have been shown to lessen NOx emissions.
“We are not pointing the finger at growers — they provide a huge service to society and should be commended for what they do,” Almaraz said. “Rather, it’s critical that we focus on incentives to bring the latest nutrient management technologies to farms, so that growers can produce food more efficiently, increasing their bottom line and improving environmental health.”
Written by: Kira Burnett and George Ugartemendia — science@theaggie.org
