The diurnal variation of NO₃

Nighttime concentration profiles of the nitrate radical (NO 3 ) together with NO 2 and O 3 were observed spectroscopically. Kinetic analysis of the NO 3 formation, the early morning photolysis of NO 3 , and stoichiometric considerations allowed three independent estimates of the equilibrium constant K eq = [NO 3 ] · [NO 2 ]/[N 2 O 5 ]. From those observations an average K eq of (2.0±l) × 10 10 cm −3 (at 10°C) is derived. Maximum concentrations of NO 3 were found to be highly variable. Lifetimes for NO 3 shorter than l min in the presence of fog indicate a fast reaction of N 2 O 5 with liquid water droplets. At relative humidities below 50% the maximum NO 3 concentrations were still lower than expected from the NO x –O 3 reaction sequences which must be due to unknown sinks for NO 3 and/or N 2 O 5 or to an inadequacy in the presently accepted reaction scheme.