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 lowermost stratosphere over the eastern North Atlantic. In situ measurements of NO, NO2, and O-3 were performed during 32 flights. The measurements were carried out during five aircraft campaigns in summer and late autumn, respectively, based from Shannon, Ireland, and Prestwick, Scotland. Most of the flights were conducted in the region of the North Atlantic flight corridor. Main objectives of these measurements included the study of the large-scale distribution of NO and NO, and the development of reliable monthly mean values for different seasons in a region of the atmosphere that is strongly affected by aircraft emissions. Substantial variability of NO and NOx volume mixing ratios was observed in the upper troposphere and lowermost stratosphere. In spite of this variability a significant seasonal dependence was found. 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 across the tropopause. The correlation between NOx and O-3 in the upper troposphere and lowermost stratosphere was only very weak. The present measurements represent a suitable data set for comparison with predictions of the NOx distribution in the upper troposphere over the eastern North Atlantic by three-dimensional models. A comparison with the NOx fields simulated with the European Center for Medium-Range Weather Forecasts. Hamburg Version 3 (ECHAM 3) climate model extended by a simplified NOx chemistry reveals good agreement for summer and autumn conditions.