PHILEAS - Measurements with HALO in air masses from the Asian monsoon
The upper troposphere and lower stratosphere (UTLS) are particularly sensitive to changes in chemical composition. This region is very important for the radiation budget of the Earth’s atmosphere, and most long-haul air traffic takes place in it. In order to correctly classify the impact of aviation, it is important to understand the background atmosphere where aviation emissions are released.
The Asian monsoon is generally known as a regional and seasonal weather event that can sometimes cause great destruction through extreme rainfall. Less well known is that the monsoon is an important transport route for trace substances into the upper troposphere and lower stratosphere. Thus, heavily polluted air masses reach the tropopause region and are then distributed over the entire northern hemisphere. The UTLS is a region that is particularly sensitive to changes and is of great importance for the radiation budget of the Earth's atmosphere.
As part of the PHILEAS (Probing High Latitude Export of Air from the Asian Summer Monsoon) campaign, the HALO research aircraft was used to investigate this trace substance transport and its effects on the composition of the UTLS in August and September 2023.
The mission was led by the Forschungszentrum Jülich and the Johannes Gutenberg University Mainz. Other partners are the Leibniz Institute for Tropospheric Research in Leipzig, the Max Planck Institute for Chemistry in Mainz, the Karlsruhe Institute of Technology, the German Aerospace Centre in Oberpfaffenhofen, and the universities of Frankfurt and Wuppertal. The PHILEAS mission is integrated in the collaborative research centre TPChange (funded by the German Research Foundation DFG).
The Institute of Atmospheric Physics of the German Aerospace Centre was involved in the extensive scientific payload with two instruments. On the one hand, nitrogen monoxide and the sum of all reactive nitrogen compounds were measured, and on the other hand, the atmospheric trace gases nitric acid, sulphur dioxide and hydrogen chloride were measured with a mass spectrometer. These trace species are key to understanding ozone chemistry and aerosol formation in the tropopause region, as well as the formation of aerosol particles. In addition to ground sources, aviation emissions also contribute to the concentration of these trace substances in the UTLS.
Vortices containing polluted air are shed by the monsoon anticyclone and move away in different directions. The PHILEAS measurement campaign therefore consisted of three phases with measurement flights from Oberpfaffenhofen and Anchorage.
The aim of the flights from Oberpfaffenhofen in August was to sound the western arm of the monsoon foothills. Two measurement flights successfully surveyed these polluted air masses over the eastern Mediterranean. Two further flights from DLR’s home base were assigned to probe the background atmosphere, i.e. the atmosphere unaffected by the monsoon.
On 21 August, HALO was transferred to Alaska with a stopover in Iceland. Until 22 September, the Anchorage airport was the starting point for further measurement flights. Many of them headed southwest from Anchorage to study the air masses influenced by the eastern arm of the monsoon outflow. To extend the range, one of these flights made a refueling stop on the remote island of Adak in the Aleutians.
The forest fires in Canada, which were particularly severe this year, were another target for measurement flights. A total of eleven flights were carried out from Anchorage before the research aircraft was transferred back to Oberpfaffenhofen on 22 September. The last measurement flight on 27 September, from Oberpfaffenhofen to Svalbard and back, measured the background atmosphere once again.
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