In 2008 Mount Okmok and Mount Kasatochi started erupting on 12 July and 7 August, respectively, in the Aleutians, depositing emissions of trace gases and aerosols as high as 15.2 km into the atmosphere. During an aircraft campaign, conducted over Europe in between 27 October and 2 November 2008, the volcanic aerosol was measured by an Aerodyne aerosol mass spectrometer, capable of particle chemical composition measurements covering a size diameter range between 40 nm and 1 μm. In the volcanic aerosol layer enhanced submicron particulate sulfate concentrations of up to 2.0 μg m-3 standard temperature and pressure (STP) were observed between 8 and 12 km altitude, while background values did not exceed 0.5 μg m-3 (STP). Twenty-one percent of the volcanic aerosol consisted of carbonaceous material that increased by a factor of 1.9 in mass compared to the free troposphere. Enhanced gaseous sulfur dioxide concentrations measured by an ion trap chemical ionization mass spectrometer of up to 1.3 μg m-3 were encountered. An onboard radiation measurement system simultaneously detected an enhanced aerosol signal. Furthermore, two German lidar stations identified an aerosol layer before and after the campaign. Data analysis shows that the aerosol layer was observed mainly in the lowermost stratosphere. Correlation of particulate sulfate concentration and sulfur dioxide mixing ratios indicates that after a 3 month residence time in the stratosphere, not all sulfur dioxide has been converted into sulfate aerosol. The significant fraction of organic material might have implications on heterogeneous chemistry in the stratosphere, which need to be explored more thoroughly.