In situ measurements of the NOx distribution and variability over the eastern North Atlantic

Between 1994 and 1996 the research aircraft Falcon of the Deutsches Zentrum fur Luft- und Raumfahrt was used to probe the upper troposphere and lowerm ost stratosphere over the eastern North Atlantic. In situ measurements of N O, NO2, and O-3 were performed during 32 flights. The measurements were car ried out during five aircraft campaigns in summer and late autumn, respecti vely, based from Shannon, Ireland, and Prestwick, Scotland. Most of the fli ghts were conducted in the region of the North Atlantic flight corridor. Ma in objectives of these measurements included the study of the large-scale d istribution of NO and NO, and the development of reliable monthly mean valu es for different seasons in a region of the atmosphere that is strongly aff ected by aircraft emissions. Substantial variability of NO and NOx volume m ixing ratios was observed in the upper troposphere and lowermost stratosphe re. In spite of this variability a significant seasonal dependence was foun d. NO mean values (averages over all measurements made during one campaign) at altitudes between 10,500 and 11,500 m, where most of the data have been obtained, ranged between about 0.1 and 0.14 ppbv in summer and 0.03 and 0. 10 ppbv in late autumn. NO and NOx did not show a significant gradient acro ss the tropopause. The correlation between NOx and O-3 in the upper troposp here and lowermost stratosphere was only very weak. The present measurement s represent a suitable data set for comparison with predictions of the NOx distribution in the upper troposphere over the eastern North Atlantic by th ree-dimensional models. A comparison with the NOx fields simulated with the European Center for Medium-Range Weather Forecasts. Hamburg Version 3 (ECH AM 3) climate model extended by a simplified NOx chemistry reveals good agr eement for summer and autumn conditions.