Nitrate radicals are known to act as important oxidizing agents for many volatile organic compounds (VOCs) (especially terpenes and dimethyl sulfide (DMS)) and NOx in the nighttime boundary layer. The quantification of these oxidation rates so far ideally implied the measurement of all species reacting with this radical and was therefore restricted to short-term intensive campaigns. In this paper we present a new method to quantify the seasonal variation of the contribution of NO3 to the atmospheric oxidation capacity and the removal of nitrogen oxides. The indicator is based on the different temperature dependences of the NO3 loss processes, which arise from the temperature dependences of kinetic constants and of the monoterpene (or DMS) emission rate. The application of this indicator is simple: It is sufficient to measure ambient concentration time series of NO3, NO2, O3, and temperature (the data should cover a time period of preferably several months). The three concentrations can be readily measured, e.g., using a Differential Optical Absorption Spectroscopy system. The new indicator was applied to data of long-term observations at a maritime (R¿gen, in 1994) and a continental site (Lindenberg, in 1998) in Germany. At the latter site the seasonal variation of the VOC oxidation and NOx removal (conversion rate of NOx to HNO3) rate was studied: During August a diurnal average of the NO3-induced monoterpene oxidation rate of 9 ¿ 105 molecules cm-3 s-1 was determined, which is 3 times higher than during March. The production rate of HNO3/NO3- was found to have its minimum during the summer time, while it is a factor of 10 higher during March, averaging 6.3 ¿ 105 cm-3 s-1 from March to September 1998 during a full diurnal cycle.