The chemical removal of NO_x at night in urban areas remains poorly constrained due to uncertainties in the contribution of various loss pathways and the impact of the suppressed nocturnal vertical mixing. Here we present long‐path differential optical absorption spectroscopy observations of nocturnal vertical concentration profiles of O₃, NO₂, and NO₃ in the lower atmosphere (33–556 m above ground level) measured during the CalNex‐LA 2010 study. Positive nocturnal vertical gradients of O₃ and NO₃ and negative gradients of NO₂ were observed during the night. Relatively short lifetime of nocturnal NO₃ (less than 1000 s) and high nighttime steady state N₂O₅ mixing ratios (up to 2 ppb) indicated active nocturnal chemistry during CalNex. Comparison of modeled and observed altitude‐resolved NO₃ loss frequencies shows that hydrolysis of N₂O₅ on aerosols was the dominant loss pathway of NO₃ and NO_x. Based on this argument, the nocturnal loss rates of NO_x, L(NO_x), at different altitudes and averaged over the lowest 550 m of the atmosphere were calculated. The nocturnally averaged L(NO_x) ranged between 0.8 and 1.3 ppb h⁻¹ for the lower atmosphere with the L(NO_x) for the first 8 days at about 1 ppb h⁻¹. This number is close to the one previously determined in Houston in 2009 of ~0.9 ppb h⁻¹. Comparisons between daytime NO_x loss due to the OH + NO₂ reaction and nighttime L(NO_x) show that during CalNex, nocturnal chemistry contributed an average of 60% to the removal of NO_x in a 24 h period in the lower atmosphere.