Nitrogen oxides (NOx=NO+NO2) are a principal component of poor air quality, aiding in the formation of ground-level ozone and ammonium nitrate aerosols that contribute to lung disease, premature death and natural ecosystem damages across the globe. In California, regulatory policies have limited fossil fuel sources of NOx pollution; however, recent findings have suggested that soil NOx emissions from the state’s extensive agricultural regions have been overlooked. We use three approaches to constrain soil NOx emissions for the state of California: 1) an Intergraded Model for the Assessment of the Global Environment (IMAGE) to estimate the magnitude and spatial distribution of soil NOx emissions; 2) airborne observations of atmospheric concentrations from agricultural soils combined with surface emissions estimates; and 3) a meta-analysis of empirical observations among different land use types. We hypothesize that, a) biogenic emissions of NOx are substantially greater than the California Air and Resource Board’s estimate of zero, and that b) soil NOx emissions are an important source of atmospheric NOx, particularly in agricultural regions of the state where large amounts of fertilizer are applied.
The results are largely convergent: NOx emissions from cropland soils represent a significant fraction of California’s total NOx emissions (~26-40%). Emissions of NOx from cropland soils totaled 161,129 tons N y-1. While emissions from natural ecosystems were relatively small (0.10 kg N ha-1 y-1), NOx emissions from cropland soils averaged 4.08 kg N ha-1 y-1, with the highest fluxes in hot and arid regions of Southern California. These estimates are consistent with empirical observations, which typically fall between 1-10 kg N ha-1 y-1. Surface emissions estimates of cropland soils generated summertime NOx concentrations of ~2.5 ppbv, contributing ~15% to the overall NOx concentration measured by aircraft. Our results suggest the need to adopt statewide policies to limit NOx emissions from California agriculture, thus benefiting human and ecosystem health in rural areas.