NO3 is the most important night-time radical species in the atmosphere. Knowledge of its concentration is essential for the understanding of night-time chemistry, one of the focus areas of current research. At elevated concentrations NO3 can alter the load and composition of hydrocarbons in an air mass considerably. Another interesting aspect of NO3 chemistry is its role in the removal of NOy from the gas phase. Heterogeneous reactions of N2O5, which is produced by the reaction of NO3 and NO2, form HNO3(s). This process converting gas-phase NOy to particulate NOy has so far not been studied directly in the polluted atmosphere.