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. The aircraft observed ash layers between altitudes of 2–8 km with peak mass concentrations typically between 200–2000 μ g/m 3 , as estimated from a Cloud and Aerosol Spectrometer (CAS). A peak value of 2000–5000 μ 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 μ m) associated with sulphuric acid and/or sulphate, and a coarse mode (0.6–35 μ m) associated with ash. The ash mass was dominated by particles in the size range 1–10 μ m (volume‐equivalent diameter), with a peak typically around 3–5 μ 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 SO 2 (10–100 ppbv), strong aerosol scattering (50–500 × 10 −6 m −1 ), and low Ångstrom 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 (∼1000 km downwind) from this eruption.