In situ observations of volcanic ash clouds from the FAAM aircraft during the eruption of Eyjafjallajokull 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. The aircraft observed ash layers between altitudes of 2-8 km with peak mass concentrations typically between 200-2000 mu g/m(3), as estimated from a Cloud and Aerosol Spectrometer (CAS). A peak value of 2000-5000 mu g/m(3) was observed over Scotland on 14 May 2010, 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 mu m) associated with sulphuric acid and/or sulphate, and a coarse mode (0.6-35 mu m) associated with ash. The ash mass was dominated by particles in the size range 1-10 mu m (volume-equivalent diameter), with a peak typically around 3-5 mu m. 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 (10-100 ppbv), strong aerosol scattering (50-500 x 10(-6) m(-1)), and low Angstrom exponents (-0.5 to 0.4) from the 3-wavelength nephelometer. Coarse-mode mass specific aerosol extinction coefficients (k(ext)), based on the CAS size distribution varied from 0.45-1.06 m(2)/g. A representative value of 0.6 m(2)/g is suggested for distal ash clouds (similar to 1000 km downwind) from this eruption.