The composition of the upper troposphere/lower stratosphere region (UTLS) is influenced by long-range or regional transport in the troposphere and stratosphere, vertical transport within convective systems and warm conveyor belts, rapid turbulent mixing, as well as photochemical production or loss of species. This results in the formation of the extratropical transition layer, which has been defined by the vertical structure of CO profiles and studied by now mostly by means of trace gas correlations. Here, we extend the analysis to aerosol particles and derive the ozone to sulfate aerosol correlation in Central Europe from aircraft in-situ measurements during the CAFE-EU/BLUESKY mission in May and June 2020. During the campaign two research aircraft, i.e., DLR-HALO (High Altitude and Long Range Research Aircraft) and DLR-Falcon, were deployed covering an altitude range from the planetary boundary layer up to 14 km altitude and thus probing the UTLS during the COVID-19 period with significant reduced anthropogenic emissions. We operated a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS) to measure the chemical composition of non-refractory aerosol particles in the size range from about 40 to 800 nm. In addition to the C-ToF-AMS data, we use trace gas measurements from both HALO and DLR-Falcon.

In our study, we find a correlation between the ozone mixing ratio (O₃) and the sulfate mass concentration in the lower stratosphere, between 10 and 14 km for all flights. The correlation is not constant with time but exhibits some variability over the two-week period of the campaign exceeding the background sulfate to ozone correlation. Especially during one flight, we observed enhanced mixing ratios of sulfate aerosol in the lowermost stratosphere, where the analysis of trace gases, such as CO, SO₂, H₂O, O₃ and HNO₃ show tropospheric influence during this time. Also, back trajectories indicate, that no recent mixing with tropospheric air occurred within the last 10 days. In addition, we analyzed satellite SO₂ retrievals from TROPOMI for volcanic plumes and eruptions. These satellite observations show enhanced volcanic activities in April 2020 on Kamchatka, Russia, with at least one explosive eruption of the Sheveluch volcano reaching the tropopause region and some minor eruptions of different volcanoes into the free troposphere. From these findings, we conclude that gas-to-particle conversion of volcanic SO₂ leads to the observed enhanced sulfate aerosol mixing ratios.