Nitrogen oxides are important to the regulation of ozone throughout the Earth's atmosphere. Of particular interest for regional air quality is photochemical production and nocturnal destruction of O3 in the lower troposphere in high NOx(=NO + NO2) environments. Nocturnal tropospheric odd oxygen (Ox), defined as O3 + NO2 + 2NO3 + 3N2O5, is used to assess the impact of NOx emissions on nocturnal O3 loss. Recent aircraft measurements of the components of Ox and HNO3 yield a detailed accounting of the nocturnal Ox budget in a regionally polluted environment. The analysis demonstrates the role of NO3 and N2O5 reactions in nocturnal O3 destruction and shows that multiple factors, including timing of NOx emissions, hydrocarbon and aerosol loading, seasonality and atmospheric mixing, govern the impact of NOx emissions on regional-scale air quality.