Laboratory studies have established a number of chemical pathways by which nitrogen oxides (NOx) affect atmospheric organic aerosol (OA) production. However, these effects have not been directly observed in ambient OA. We report measurements of particulate organic nitrates in Bakersfield, California, the nighttime formation of which increases with NOx and is suppressed by high concentrations of organic molecules that rapidly react with nitrate radical (NO3)-evidence that multigenerational chemistry is responsible for organic nitrate aerosol production. This class of molecules represents about a third of the nighttime increase in OA, suggesting that most nighttime secondary OA is due to the NO3 product of anthropogenic NOx emissions. Consequently, reductions in NOx emissions should reduce the concentration of organic aerosol in Bakersfield and the surrounding region.