Trace gas measurements in the convective outflow from European thunderstorms have been performed with the German research aircraft Falcon during the LINOX (1996), EULINOX (1998), EXPORT (2000), and recently CONTRACE (2003) field experiments. Here we compare the results from all four campaigns with focus on the contribution of thunderstorms to the NOx (=NO+NO2) budget in the upper troposphere. A rough estimate for the global amount of lightning-produced NOx from CONTRACE resulted in ~4 TgN yr -1 . For Europe the amount of lightning-produced NOx was estimated to 0.06 TgN yr -1 , which is less than the contribution from aircraft emissions (0.1 TgN yr -1 ).

Thunderstorms are essential for the production and redistribution of NOX (=NO+NO2) in the atmosphere. Lightning is one of the most important NOX sources in the upper troposphere besides the upward transport of surface emissions due to deep convection / warm conveyor belts, the downward transport of NOX rich air from the stratosphere, and aircraft emissions. However, in-situ measurements of lightning-produced NOX are difficult to perform and therefore rare. Large uncertainties still exist about the strength of this NOX source (2-20 TgN yr-1). It is important to quantify the NOX emissions rates in the upper troposphere (especially the lightning production part) more accurate since a NOX release at these altitudes can increase the O3 production and affect the hydroxyl radical (OH) concentration. During the field experiment EULINOX (The European Lightning Nitrogen Oxides Project), performed in July 1998 over Central Europe, the most important NOX sources in the upper troposphere were investigated: (1) the production by lightning, and (2) the upward transport of polluted air from the boundary layer (BL). For the first time nitrogen oxides measurements were performed inside thunderstorms over Europe. Different thunderstorm situations (isolated cells and imbedded cells in frontal systems) were selected for the investigations with the DLR research aircraft Falcon. Overall, about 10 different thunderstorms were penetrated and trace gas concentrations (NO, NO2, CO, CO2, and O3) and particles (CN, >10nm) were recorded. The extensive EULINOX data set is a suitable base for cloud scale modeling studies (for detailed information see http://www.pa.op.dlr.de/eulinox/).

Airborne in situ measurements of NO, NO2, NOy, CO, CO2, O3, J(NO2), and CN were performed in European thunderstorms during the field experiment EULINOX in July 1998. The measurements in the upper troposphere show enhanced NOx (= NO + NO2) concentrations within thunderstorms and their outflow at horizontal scales from 300 m to several 100 km. The maximum NO mixing ratio measured inside a thundercloud close to lightning (the aircraft was also hit by a small lightning strike) was 25 ppbv. A regional NOx enhancement of 0.5 ppbv over central Europe could be traced back to a thunderstorm event starting ∼24 hours earlier over Spain. The fractions of NOx in thunderclouds which are produced by lightning and convectively transported from the polluted boundary layer are determined by using CO2 and CO as tracers for boundary layer air. The analyses show that on average about 70% of the NOx increase measured in the anvil region was found to result from production by lightning and about 30% from NOx in the boundary layer. Thunderstorms are also strong sources of small particles. The peak CN concentrations measured within thunderstorm outflows (>30,000 particles STP cm−3) were distinctly higher than in the polluted boundary layer. The amount of NOx produced per thunderstorm and NO produced per lightning flash was estimated. The results imply that the annual mean NOx budget in the upper troposphere over Europe is dominated by aircraft emissions (0.1 TgN yr−1) in comparison to lightning production (∼0.03 TgN yr−1). On the global scale, NOx produced by lightning (mean 3 TgN yr−1) prevails over aircraft‐produced NOx (0.6 TgN yr−1).

Total reactive nitrogen (NOy), ozone (O-3), the tracer nitrous oxide (N2O), and carbon monoxide (CO) were measured in situ as part of the Polar Stratosphere Aerosol Experiment (POLSTAR 1 and 2). In total, 14 missions were performed in January and February 1997 and 1998 in the Arctic subvortex region, by using the Deutsches Zentrum fur Luft- und Raumfahrt research aircraft Falcon. During the two campaigns, no signatures of nitrification events in the lowermost stratosphere, due to sedimentation and evaporation of aerosol particles containing HNO3, were found. Compact correlations were observed between NOy and O-3, between NOy and N2O, and between O-3 and NaO. Mean NOy/O-3 ratios observed in the lowermost Arctic stratosphere in 1997 and 1998 were 0.0046 and 0.0030, respectively. During POLSTAR 1, NOy-N2O and O-3-N2O slopes compare quite well with results of previous measurements in the lowermost stratosphere. However, indications for a nonstratospheric NOy source, possibly aircraft emissions, were found. During POLSTAR 2, inferred NOy-N2O and O-3-N2O slopes were significantly lower than those previously observed in this region. This might indicate transport from lower latitudes to the Arctic subvortex region.

Airborne chemical in situ measurements were performed in thunderstorms during the field experiment EULINOX in July 1998. NOX (=NO+NO2) enhancements observed in the upper troposphere, which were caused by lightning or/and convective transport of polluted boundary layer air, were investigated in detail. Tracers for boundary layer air (CO2, CO, and O3) were used to determine the fraction of lightning-produced NOX and convective transported NOX in thunderstorms. The amount of lightning-produced NOX in comparison to aircraft emissions in the upper troposphere was estimated on the European and global scale.

Measurements of gas‐ and condensed‐phase NOy, HNO3, O3, and particle concentration and size were performed on 24 January 1997 during POLSTAR on a flight through an ice cloud. In the ice cloud below the tropopause only 2 to 4 pptv particle NOy were detected. The observed average mixing ratio of total HNO3 is significantly larger than the detected particle NOy. Considering the combined error limits of the NOy and HNO3 measurements, however, it cannot be ruled out that the observed uptake of NOy was limited by the available gas‐phase HNO3. The ratio of total HNO3 to gaseous NOy measured during the cirrus cloud penetration amounts only to 0.2 in contrast to 0.9 in the stratospheric part of the flight.

During the 1997 POLSTAR‐1 winter campaign in northern Sweden a flight was performed across a cold trough of air (≃ 196 K) in the tropopause region. Measurements of total water vapour, nitric acid, particles and reactive nitrogen (NOy) were taken. The particle measurements indicate that about 3% of the particles in the moist tropospheric air were ice particles. Forward and backward facing NOy inlets were used simultaneously to determine condensed phase HNO3. The combined NOy and particle measurements reveal that less than 1% of a monolayer of NOy could have resided on the ice particles. This casts doubt on the hypothesis that sedimenting cirrus particles generally lead to a strong downward flux of NOy. In addition to the NOy measurements, independent HNO3 measurements were used to determine total HNO3. Although quantitative uncertainties do not allow to completely rule out that the NOy uptake on ice was limited by total HNO3, the combined NOy and HNO3 data suggest that there was low uptake of NOy on ice despite abundant HNO3 in the gas phase. Model studies indicate, that the most likely explanation of the measured nitric acid partitioning is given by HNO3 in ternary solution droplets coexisting with almost HNO3 free ice in the same air mass.

This paper investigates the role of lightning in the production of nitrogen oxides (NOx) and their subsequent distribution by thunderstorms. These questions were addressed by the field experiment LINOX (lightning produced NOx), which was performed in southern Germany in July 1996. The structure of thunderstorms was observed by radar and satellite, the lightning activity was recorded by a lightning detection network, and airborne chemical measurements were performed aboard a jet aircraft penetrating the storm anvils. NOx concentrations in the storm anvils were found to typically range from 1 to 4 parts per billion by volume. The NO contribution to the total NOx was found to be dominant in narrow peaks produced by flashes as well as near cloud boundaries, probably because of increased photolysis rates of NO2. Using CO2 as an air mass tracer, the lightning-produced NOx amount was discriminated from the contribution due to transport of air from the boundary layer. It was found from a case study of a large storm anvil that lightning-produced NOx was present in the same order of magnitude as the amount of NOx originating from lower levels; during later stages of cloud development, the content of the former even exceeded the latter one. A simple two-dimensional model of advection and dispersion of the lightning-produced NOx was able to reproduce the general structure of the anvil NOx plume. Some NOx peaks could directly be attributed to flash observations close to the aircraft track.

Aircraft‐based measurements of HNO3, NOy, N2O, and O3 have been performed in the Arctic lower stratosphere in January (POLSTAR I) and March (STREAM III) of 1997. The two projects employed different aircraft platforms. In addition, NOy and O3 were measured using different instruments in the two campaigns. HNO3 and NOy were found strongly correlated with correlation coefficients of 0.84 (POLSTAR I) and 0.69 (STREAM III), respectively. The fraction of HNO3 within NOy decreased from 96% in January to 59% in March. The decrease is consistent with the lifetime of HNO3 due to photolysis after polar sunrise. The relationship of NOy and HNO3 with N2O shows that in January NOy and HNO3 values were markedly higher than expected, which may indicate nitrification by PSC‐II particle sedimentation and evaporation. Contradictory, the ratios NOy/O3 observed in January are only slightly elevated. In March, NOy‐N2O and NOy‐O3 relations agree well with others reported in the literature. The difference between the NOy‐O3 and NOy‐N2O relationships is partly explained by an observed O3 decrease of about 30% between January and March.

Between 1994 and 1996 the research aircraft Falcon of the Deutsches Zentrum für 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 O3 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 NOx 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 O3 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.