Ice nucleation properties of fine ash particles from the Eyjafjallajökull eruption in April 2010

During the eruption of the Eyjafjallaj ¨ okull vol- cano in the south of Iceland in April/May 2010, about 40 Tg of ash mass were emitted into the atmosphere. It was unclear whether volcanic ash particles with d < 10 µm facilitate the glaciation of clouds. Thus, ice nucleation properties of vol- canic ash particles were investigated in AIDA (Aerosol In- teraction and Dynamics in the Atmosphere) cloud chamber experiments simulating atmospherically relevant conditions. The ash sample that was used for our experiments had been collected at a distance of 58 km from the Eyjafjallaj ¨ okull during the eruption period in April 2010. The temperature range covered by our ice nucleation experiments extended from 219 to 264 K, and both ice nucleation via immersion freezing and deposition nucleation could be observed. Im- mersion freezing was first observed at 252 K, whereas the deposition nucleation onset lay at 242 K and RHice = 126 %. About 0.1 % of the volcanic ash particles were active as im- mersion freezing nuclei at a temperature of 249 K. For de- position nucleation, an ice fraction of 0.1 % was observed at around 233 K and RHice = 116 %. Taking ice-active surface site densities as a measure for the ice nucleation efficiency, volcanic ash particles are similarly efficient ice nuclei in im- mersion freezing mode (ns,imm 10 9 m 2 at 247 K) com- pared to certain mineral dusts. For deposition nucleation, the observed ice-active surface site densities ns,dep were found to be 10 11 m 2 at 224 K and RHice = 116 %. Thus, volcanic ash particles initiate deposition nucleation more efficiently than Asian and Saharan dust but appear to be poorer ice nu- clei than ATD particles. Based on the experimental data, we have derived ice-active surface site densities as a function of temperature for immersion freezing and of relative humidity over ice and temperature for deposition nucleation.