Abstract
Air quality management commonly aims to mitigate emissions of oxides of nitrogen (NO(x)) from combustion, reducing ozone and particulate matter pollution. Despite such efforts, regulations have recently proven ineffective in rural areas like the Salton Sea Air Basin of Southern California, which routinely violates air quality standards. With $2 billion in annual agricultural sales and low population density, air quality in the region is likely influenced by year-round farming. We conducted NO(x) source apportionment using nitrogen stable isotopes of ambient NO(2), which indicate a substantial contribution of soil-emitted NO(x). The soil source strength was estimated based on the mean δ(15)N-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. Therefore, the impact of soil NO(x) in agricultural regions must be re-evaluated.