The ASCCI (Arctic Springtime Chemistry-Climate Investigations) measurement campaign is primarily concerned with the question of how the gases ozone and water vapour in the upper troposphere and lower stratosphere - i.e. seen from the ground at altitudes between around five and 15 kilometres - influence climate change in the Arctic and are influenced by it. The campaign is specifically investigating the processes that take place in spring. These include the depletion of ozone in the stratosphere, which has negative consequences - both for humans and their environment - as more UV radiation reaches the Earth's surface. Particularly in years with a cold stratosphere, it is possible that a significant part of the Arctic ozone layer is destroyed.

"There are warmer and colder winters in the stratosphere, which is a completely normal variability from year to year. However, we are also observing that the stratosphere is getting colder and colder due to the increase in greenhouse gases, while temperatures on the ground and in the troposphere continue to rise," says Professor Björn-Martin Sinnhuber from the Institute of Meteorology and Climate Research at the Karlsruhe Institute of Technology, which is coordinating the campaign together with Goethe University Frankfurt.

Process is still being initiated

"This winter, temperatures in the Arctic stratosphere were colder than ever measured before. And although chlorofluorocarbons and other ozone-depleting substances have not been produced for decades, it will still take decades before they have completely disappeared from the atmosphere," says the expert.

The chlorine compounds are chemically converted at temperatures below minus 78 degrees Celsius in such a way that they attack and partially destroy the ozone layer. At present, there is still so much ozone-damaging chlorine and bromine in the stratosphere that this process is still being triggered.

At the same time, due to the eruption of the underwater volcano Hunga-Tonga three years ago, there is still significantly more water in the stratosphere than normal. The researchers also want to investigate how this affects the ozone layer during ASCCI.

In spring, air pollutants in particular are transported into the Arctic, where they can act as short-lived greenhouse gases. Another aim of the campaign is to better understand these processes through targeted measurements.

HALO as a "flying laboratory" equipped with numerous instruments

The measurement flights take place with the HALO research aircraft, which is stationed in Kiruna in northern Sweden until the beginning of April. A central measuring instrument on board is the GLORIA infrared spectrometer. It is one of the few instruments in the world that can measure high-resolution vertical distributions of a large number of trace substances at high altitudes.

Atmospheric physicists from the University of Wuppertal are contributing the HAGAR-V instrument, which was specially developed for HALO, to answer the scientific questions posed by the campaign. It combines different measurement methods in three measurement modules to measure over 30 relevant trace gases with high time resolution and precision.

"In addition to carbon dioxide and other greenhouse gases, our measurements include a large number of air pollutants and, above all, most of the gases containing chlorine and bromine, which are responsible for the destruction of the ozone layer," explains Prof Michael Volk from the Institute of Atmospheric and Environmental Research at the University of Wuppertal. "In addition, these spatially high-resolution measurements allow conclusions to be drawn about the transport and exchange of trace substances in the troposphere and lower stratosphere, an understanding of which is essential for a more accurate forecast of the future development of the ozone layer, among other things."

The Karlsruhe Institute of Technology, Goethe University Frankfurt, Forschungszentrum Jülich, the German Aerospace Centre (DLR), the University of Wuppertal, Heidelberg University and Johannes Gutenberg University Mainz are all involved in the ASCCI campaign.