Abstract
Nitrogen oxides (NO (x) = NO + NO(2)) are a primary component of air pollution-a leading cause of premature death in humans and biodiversity declines worldwide. Although regulatory policies in California have successfully limited transportation sources of NO (x) pollution, several of the United States' worst-air quality districts remain in rural regions of the state. Site-based findings suggest that NO (x) emissions from California's agricultural soils could contribute to air quality issues; however, a statewide estimate is hitherto lacking. We show that agricultural soils are a dominant source of NO (x) pollution in California, with especially high soil NO (x) emissions from the state's Central Valley region. We base our conclusion on two independent approaches: (i) a bottom-up spatial model of soil NO (x) emissions and (ii) top-down airborne observations of atmospheric NO (x) concentrations over the San Joaquin Valley. These approaches point to a large, overlooked NO (x) source from cropland soil, which is estimated to increase the NO (x) budget by 20 to 51%. These estimates are consistent with previous studies of point-scale measurements of NO (x) emissions from the soil. Our results highlight opportunities to limit NO (x) emissions from agriculture by investing in management practices that will bring co-benefits to the economy, ecosystems, and human health in rural areas of California.