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At least one-quarter of all nitrogen oxide (NO_x) emissions in California's Salton Sea air basin come from soil, according to a study from the University of California, Davis.

Using isotopic analysis, the study found that annual total soil emissions for the basin were about 11 tons per day on average, which is 10 times larger than the state's current inventory for soil NO_x emissions in the region. The work was in Scientific Reports.

The study highlights the need to better understand and account for emissions from agricultural soils in order to comply with state and federal air quality regulations and to improve air quality in the rural Imperial Valley and Coachella Valley and other agriculturally active regions in warm climates.

Underestimated sources

In recent decades, air quality has made tremendous gains in metropolitan areas like Los Angeles, primarily due to reduced fossil fuel emissions of precursors like NO_x from industry and transportation. Yet NO_x emissions appear to be worsening in many rural areas, in part due to wildfires and increasing emissions from agricultural soils.

"Wildfire and soil emissions are the sources I see coming to dominate air pollution this century, and we need to adjust our air quality regulatory thinking to reflect that, especially with respect to its impacts on exurban and rural communities," said corresponding author Ian Faloona, a UC Davis professor in the Department of Land, Air and Water Resources and the Air Quality Research Center.

"We've reached a point where air quality stopped improving about eight years ago in many places across California, and I suspect it has a lot to do with these two underestimated sources."

Fertilizers, irrigation and climate

The Salton Sea is among the most polluted air basins in the country. It runs from Palm Springs, through the Coachella Valley and into the Imperial Valley before ending at the Mexican border in Calexico. The region produces more than $2 billion in annual agricultural sales, with crops grown year-round. Its primarily Latino residents face high rates of poverty, asthma and respiratory disease.

NO_x is a central air pollutant serving as a precursor to the atmosphere's production of both ozone and particulate matter (PM).

Heavy use of fertilizers, irrigation and rising temperatures are known to increase NO_x soil emissions—all of which occur in the study area. California experiences some of the fastest-rising warm season temperatures of any state in the nation. Higher temperatures are known to increase the risk of wildfires and promote greater production by microbes in the soil. Meanwhile, fertilizer sales for the sandy soils of Imperial County increased by 137% since 1991, the study noted.

On-site sampling, lab analysis

To better understand these factors, the authors conducted field sampling between June 2022 and April 2023 at an Imperial Valley high school and an air station on Torres Martinez Desert Cahuilla Indian land in the Coachella Valley, in partnership with community nonprofit Comité Cívico del Valle.

The samples were then extracted and taken to the UC Davis Stable Isotope Facility. Isotopic signatures were used in a Bayesian Mixing Model to estimate the contribution of soils to the overall isotopic signature. This analysis showed that soil sources are currently underestimated by a factor of 10. Nutrient availability, soil moisture and temperature all affect soil NO_x production.

The authors note that additional data on fertilizer application rates and irrigation schedules could further clarify the direct impacts of these variations, enabling regulators to effectively strengthen air quality regulations in the region.

"People call soil emissions and wildfire smoke 'background' pollutants, but they are actually rising and beginning to dominate in many parts of the state," said Faloona. "We are presently failing to account for significant components of our pollution problems."

The study was led by Heather Lieb, who recently earned her Ph.D. under Faloona's mentorship at UC Davis.