Nighttime concentration profiles of the nitrate radical (NO3) together with NO2 and O3 were observed spectroscopically. Kinetic analysis of the NO3 formation, the early morning photolysis of NO3, and stoichiometric considerations allowed three independent estimates of the equilibrium constant Keq = 3>⋅2>/2O5>. From those observations an average Keq of (2.0¿1)¿1010 cm-3 (at 10 ¿C) is derived. Maximum concentrations of NO3 were found to be highly variable. Lifetimes for NO3 shorter than 1 min in the presence of fog indicate a fast reaction of N2O5 with liquid water droplets. At relative humidities below 50% the maximum NO3 concentrations were still lower than expected from the NOx --O3 reaction sequences which must be due to unknown sinks for NO3 and/or N2O5 or to an inadequacy in the presently accepted reaction scheme.