We report the first simultaneous in situ observation of a suite of compounds important in nocturnal nitrogen oxide chemistry. Measurements took place at a ground site near Boulder, Colorado, during the fall of 2001. Chemical measurements included NO3, N2O5, NO, NO2 and O3; meteorological data were also available. The concentrations of NO3 and N2O5 showed large dynamic ranges that were consistent with variations in NO2 and NO and with shifts in meteorological conditions at this site. The observed ratio of N2O5 to NO3 agreed with the ratio calculated from the measured NO2 concentration and the temperature-dependent equilibrium constant. In addition, NO3 and N2O5 showed large short-term variability that may indicate inhomogeneously mixed source and sink compounds and/or deposition at this ground-based measurement site. Finally, N2O5 reached a peak concentration of nearly 3 ppbv under polluted conditions and accounted for an appreciable fraction of the total concentration of measured nitrogen oxide species.