Nitrogen isotopes reveal high NOx emissions from arid agricultural soils in the Salton Sea Air Basin

Air quality management commonly aims to mitigate nitrogen oxide (NO_x) emissions from combustion, reducing ozone (O₃) and particulate matter (PM) pollution. Despite such ongoing efforts, regulations have recently proven ineffective in rural areas like the Salton Sea Air Basin of Southern California, which routinely violates O₃ and PM air quality standards. With over $2 billion in annual agricultural sales and low population density, air quality in the region is likely influenced by the year-round farming activity. We conducted a source apportionment of NO_x (an important precursor to both O₃ and PM) using nitrogen stable isotopes of ambient NO₂, which revealed a significant contribution from soil-emitted NO_x to the regional budget. The soil source strength was estimated based on the mean δ¹⁵N-NO_x from each emission category in the California Air Resources Board's NO_x inventory. Our annual average soil emission estimate for the air basin was 11.4 ± 4 tons/d, representing ~ 30% of the extant NO_x inventory, 10× larger than the state's inventory for soil emissions. Unconstrained environmental factors such as nutrient availability, soil moisture, and temperature have a first-order impact on soil NO_x production in this agriculturally intensive region, with fertilization and irrigation practices likely driving most of the emissions variability. Without spatially and temporally accurate data on fertilizer application rates and irrigation schedules, it is difficult to determine the direct impacts that these variations have on our observations. Nevertheless, comparative analysis with previous studies indicates that soil NO_x emissions in the Imperial Valley are likely underrepresented in current inventories, highlighting the need for more detailed and localized observational data to constrain the sizeable and variable emissions from these arid, agricultural soils.