In-situ observations of volcanic ash clouds from the FAAM aircraft during the eruption of Eyjafjallajökull in 2010

During April-May 2010 the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft flew 12 flights targeting volcanic ash clouds around the UK. Ash clouds were observed on every flight at altitudes between 2 – 8km. Due to safety considerations the aircraft only penetrated ash clouds forecast to have concentrations less than 2000μg/m. Cloud and Aerosol Spectrometer (CAS) data showed peak ash concentrations in the range 200-2000 μg/m on most flights during the May period. A peak value of 2000-5000μg/m was observed over Scotland on 14 May, although with considerable uncertainty due to the possible contamination by ice. Aerosol size distributions within ash clouds showed a fine mode (0.1 – 0.6 μm) associated with sulphuric acid and/or sulphate, and a coarse mode (0.6 – 35μm) associated with ash. The ash mass was dominated by particles in the size range 1 – 10μm (volume-equivalent diameter), with a peak typically around 3 – 5μm. Electron-microscope images and scattering patterns from the SID-2H (Small Ice Detector) probe showed the highly irregular shape of the ash particles. Ash clouds were also accompanied by elevated levels of SO2 (10 100ppbv), strong aerosol scattering (50 500 x 10 m), and low Ångstrom exponents (-0.5 to 0.4) from the 3-wavelength nephelometer. Mass specific aerosol extinction coefficients (kext), based on the CAS size distribution varied from 0.45 – 1.06 m/g, depending on ash concentration and distance downwind. A representative value of 0.6 m/g is suggested for distal ash clouds (~1000km downwind) from the May period of this eruption.