Organic nitrates formed from nighttime reaction between anthropogenic nitrate radicals (NO3) and biogenic volatile organic compounds (BVOCs) are an important but highly uncertain source of secondary organic aerosol (SOA). Here we report on the enhanced nighttime biogenic SOA formation observed in a polluted residual layer over Sacramento, California, the morning of 15 June 2010 during the Carbonaceous Aerosols and Radiative Effects Study (CARES). Trajectory analysis showed that the residual layer air, containing isoprene and trace amounts of monoterpenes left over after nighttime oxidation, was influenced by the San Francisco Bay Area emissions the previous evening. The residual layer aerosol was also enriched in nitrate, with about 64% of it estimated to be in the form of organic nitrates. The nitrate:organic mass ratio of the SOA was about 0.47 ± 0.044, which corresponds to the range typically found in isoprene mononitrates. Assuming the SOA was composed of organic mononitrates, its nominal molecular weight was estimated at 186 ± 11 g mol−1, consistent with the highly functionalized isoprene hydroxynitrates that have been observed in the particle phase in the southeast United States. Overall, our findings show that the efficiency of nighttime biogenic SOA formation, expressed as the change in organic aerosol mass relative to carbon monoxide (∆OA/∆CO), equals ~100 μg m−3 ppmv and is comparable to the range previously estimated for enhanced daytime SOA formation from mixed anthropogenic and biogenic emissions during CARES. Assuming the SOA was formed from isoprene oxidation by NO3, we estimated mass yields of up to 0.55, consistent with previous field estimates. Key Points Airborne observations of trace gases and aerosol in a polluted residual layer over Sacramento, California, are analyzed Nighttime secondary organic aerosol formation from mixed biogenic and anthropogenic emissions is as efficient as daytime production The estimated nitrate:organic mass ratio and molecular weight are consistent with those for highly functionalized isoprene hydroxynitrates