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Nitrogen oxides (NO x ) are a key precursor in O 3 formation. Although stringent anthropogenic NO x emission controls have been implemented since the early 2000s in the United States, several rural regions of California still suffer from O 3 pollution. Previous findings suggest that soils are a dominant source of NO x emissions in California; however, a statewide assessment of the impacts of soil NO x emission (SNO x ) on air quality is still lacking. Here we quantified the contribution of SNO x to the NO x budget and the effects of SNO x on surface O 3 in California during summer by using WRF-Chem with an updated SNO x scheme, the Berkeley Dalhousie Iowa Soil NO Parameterization (BDISNP). The model with BDISNP shows a better agreement with TROPOMI NO 2 columns, giving confidence in the SNO x estimates. We estimate that 40.1% of the state's total NO x emissions in July 2018 are from soils, and SNO x could exceed anthropogenic sources over croplands, which accounts for 50.7% of NO x emissions. Such considerable amounts of SNO x enhance the monthly mean NO 2 columns by 34.7% (53.3%) and surface NO 2 concentrations by 176.5% (114.0%), leading to an additional 23.0% (23.2%) of surface O 3 concentration in California (cropland). Our results highlight the cobenefits of limiting SNO x to help improve air quality and human health in rural California.