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Abstract:
The composition of the upper troposphere/lower stratosphere region (UTLS) is influenced by horizontal transport, 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 (ExTL), 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, 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 our study, we find a correlation between the ozone mixing ratio (O3) and the sulfate mass concentration in the lower stratosphere. The correlation exhibits some variability over the measurement period 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 shows tropospheric influence. 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. From these analyses, we conclude that gas-to-particle conversion of volcanic SO2 leads to the observed enhanced sulfate aerosol mixing ratios.
