Measuring the mixture of greenhouse gases over the Atlantic

"Tropical storms and hurricanes over the West Atlantic, weather fronts and the Asian monsoon provide very effective transportation of climate-relevant trace gases from the ground to higher atmospheric layers," explains Peter Hoor from the University of Mainz, one of the campaign's coordinators. "Air masses enriched with water vapour and other trace substances are then transported to Europe via the so-called jet stream and mixed so that we can also measure the effects of these weather phenomena in Europe," adds Christiane Voigt from the DLR Institute of Atmospheric Physics. "Through these measurements, we now have a much clearer picture of how these trace gases are distributed across the northern Atlantic and how clouds affect the stability of the atmospheric layers and thus the distribution of trace gases," Voigt continues.

For these measurements, the DLR-operated HALO (High Altitude and Long Range Aircraft) research aircraft flew a total of 17 times over the Atlantic. "The flights generally lasted between nine and 10 hours and went far out over the ocean," says Andreas Minikin, explaining the DLR Flight Operations (FB). "At an altitude of 13 to 14 kilometres, and in close agreement with air-traffic control, HALO generally flew above transatlantic air traffic." The HALO research aircraft is characterised by its unusual range and altitude.

Trace gases heat cold zones

The current measurement campaign confirmed the findings of the Jülich coordinator of the research campaign, Martin Riese, namely that changes in the concentration of the greenhouse gases water vapour and ozone in the relatively cold area of the atmosphere between about five and 20 kilometres above sea level have a particularly strong effect on the temperature on the ground. At the very cold interface between the troposphere and the higher stratosphere in particular, changes in trace gases have a significant effect on our climate.

Using the Differential Absorption Lidar (DIAL), developed by DLR, it has been possible for the first time to simultaneously measure water vapour and ozone, both important greenhouse gases, with a very high degree of accuracy and wide coverage. Furthermore, these measurements also provide information on the properties of high ice clouds and their influence on the exchange process in the upper troposphere and lower stratosphere.

The observation of nitrogen oxides in this boundary region is also of great interest in understanding the dynamics of the exchange processes between the atmospheric layers. "We have been able to measure the concentration of reactive nitrogen compounds in the atmosphere with very high accuracy for over 20 years," explains the atmospheric researcher Helmut Ziereis from the DLR Institute of Atmospheric Physics. "We upgraded this instrument again for HALO." The atmospheric concentration of nitrogen oxides is determined by a number of different sources. In addition to lightning and aircraft emissions, the stratosphere itself is the largest local source of long-lasting nitrogen oxide compounds in the upper troposphere and lower stratosphere. Added to this is, of course, the nitrogen oxide transported upwards from the ground. This is why the measurement of nitrogen oxides is particularly suitable as a marker for characterising the exchange of air masses between the troposphere and stratosphere.

The distribution of trace gases over Europe changes significantly through the intercontinental transport of air masses from Asia and America. To detect air from Asian or American regions, researchers used the AIMS mass spectrometer. This measuring instrument can be used to identify very low concentrations of sulphur compounds and other acidic trace substances, in order to infer the origin of the air. "We would like to find out which meteorological situations have a strong influence on the composition of air masses over Europe, and which processes are reinforced by the intermixing of air masses," adds Christiane Voigt. Only by mixing can the composition of the air in the climate-sensitive upper troposphere and lower stratosphere be changed permanently. "In addition, we want to investigate the influence of ice clouds on this atmospheric layer," continues Voigt. Earlier measurement campaigns using HALO have shown that ice clouds directly alter the radiation balance of the upper troposphere and lower stratosphere, and thus the climate. The WISE scientists are now additionally investigating the extent to which the ice clouds can also change transport and mixing in this climate-sensitive atmospheric layer.

The partners

In addition to the Jülich Research Centre and the University of Mainz, the partners in the WISE (Wave-driven ISentropic Exchange) joint project include the German Aerospace Center (DLR), the Karlsruhe Institute of Technology, the Universities of Heidelberg, Frankfurt and Wuppertal, as well as the National Metrology Institute of Germany (PTB; Physikalisch-Technische Bundesanstalt). The scientific flights are supported by a team of around 90 people.