Two-years of NO3 radical observations in the boundary layer over the Eastern Mediterranean

Abstract

This is the first study that investigates the seasonal variability of nitrate (NO3) radicals in the marine boundary layer over the East Mediterranean Sea. An extensive data set of NO2 radical observations on the north coast of Crete for more than two years (June 2001-September 2003) is presented here. NO2 radicals follow a distinct seasonal dependency with the highest seasonally average mixing ratios in summer (5.6±1.2pptv) and the lowest in winter (1.2±1.2pptv). Episodes with high NO2 mixing ratios have been encountered mainly in polluted air masses originating from mainland Greece, Central and East Europe, and Turkey. Ancillary measurements of ozone, nitrogen dioxide (NO2) and meteorological parameters have been conducted and used to reveal possible relationship with the observed NO2 variability. The acquired NNO 2 nighttime observations provide the up-to-date most complete overview of NNO2 temporal variability in the area. The data show correlations of the NO2 nighttime mixing ratios with temperature (positive), relative humidity (negative) and to a lesser extend with O 3 (positive). As inferred from these observations, on average the major sink of NO2 radicals in the area is the heterogeneous reaction of dinitrogen pentoxide (N2O5) on aqueous particles whereas the homogeneous gas phase reactions of NO3 are most important during spring and summer. These observations support a significant contribution of NO2 nighttime chemistry to the oxidizing capacity of the troposphere.