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 NO₃ radical observations on the north coast of Crete for more than two years (June 2001–September 2003) is presented here. NO₃ radicals follow a distinct seasonal dependency with the highest seasonally average mixing ratios in summer (5.6±1.2 pptv) and the lowest in winter (1.2±1.2 pptv). Episodes with high NO₃ 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 NO₃ variability. The acquired NO₂ nighttime observations provide the up-to-date most complete overview of NO₂ temporal variability in the area. The data show correlations of the NO₃ nighttime mixing ratios with temperature (positive), relative humidity (negative) and to a lesser extend with O₃ (positive). As inferred from these observations, on average the major sink of NO₃ 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 NO₃ nighttime chemistry to the oxidizing capacity of the troposphere.