Chemistry and oxidation capacity of the nitrate radical in the continental boundary layer near Berlin

Abstract

The nitrate radical is in many situations the most important nighttime oxidizing species, removing, for example, hydrocarbons, which would otherwise be available to daytime ozone formation. In spite of its importance in the night and probably also under certain conditions during the day, our understanding of the NO 3 chemistry and its impact on the oxidation capacity of the atmosphere is still incomplete. Here we present measurements of NO 3 by differential optical absorption spectroscopy (DOAS) and a number of other atmospheric trace gases performed during the Berliner Ozonexperiment (BERLIOZ) campaign at Pabstthum near Berlin, Germany, to quantify the contribution of NO 3 to the atmospheric oxidation rate of volatile organic compounds (VOCs) and NOx removal. The measurements show that only two NO 3 sinks were of importance: (1) About 50–30% (depending on the distance (0.1–3 km) to a near forest) of the NO 3 was lost due to reaction with biogenic hydrocarbons. (2) The major part of the remaining loss probably can be attributed to the indirect loss via the reaction of N 2 O 5 on aerosol surfaces. Assuming that heterogeneous hydrolysis of N 2 O 5 is occurring, the nonphotolytical conversion of NOx to HNO 3 via N 2 O 5 was found to be comparable with daytime conversion by the reaction of OH with NO 2. In combination with measurements of the OH concentration, it was possible for the first time to derive a relative contribution of 28% (24‐hour average) for the NO 3‐initiated oxidation to the total VOC degradation.