Heterogeneous ice nucleation in the ash plume of Eyjafjoll observed at two central-European EARLINET lidar sites

The eruption of the Eyjafjoll volcano in Iceland in April 2010 and the associated shut-down of air traffic in conjunction with northwesterly winds lead to the advection of free-tropospheric arctic air masses towards central Europe that were polluted only with volcanic ash particles. This scenario was used to study the impact of volcanic ash particles on ice nucleation in clouds that were embedded in the plume.

The investigated observations were performed with two lidars at the German EARLINET (European Aerosol Research Lidar Network) stations of Leipzig and Munich. The observed clouds were categorized as either pure liquid-water or ice-containing clouds according to the measured volume depolarization ratio which is used to identify non-spherical particles. The depolarization ratio is zero only for spherical liquid-water droplets.

Several cloud layers embedded in the ash plume were observed during the main period of ash advection between 16 April and 24 April 2010. Heterogeneous ice formation was observed in altocumulus layers at high cloud top temperatures of between -10 to -15 °C. However, no indications for heterogeneous ice formation were found in the observed cloud layers with top temperatures above -10 °C. In turn, no pure liquid-water clouds were observed at temperatures below -15 °C. This finding implies that the ash particles suspended in the Eyafjoll plume were efficient ice nuclei (IN) below a threshold temperature of -10 to -15 °C. This is in agreement with findings from laboratory studies of Durant et al. [2008] and Fornea et al. [2009].

Besides the observation of ice nucleation in supercooled liquid-water clouds, the formation of a cirrus cloud in the absence of any influence of air traffic was observed above Leipzig on 18 April 2010. This cloud formed in a volcanic ash layer at temperatures of below -40 °C. The laminar, layered structure and the absence of strong virgae which usually immediately develop in evolving upper tropospheric clouds implies an influence of the volcanic ash particles on the formation of the cirrus cloud. One possible explanation would be the formation of a large number of small ice crystals caused by a high deposition-freezing efficiency of the ash particles. The size of these crystals may consequently be too small to form precipitating structures like virgae.

References: Durant, A. J., R. A. Shaw, W. I. Rose, Y. Mi, and G. G. J. Ernst (2008), Ice nucleation and overseeding of ice in volcanic clouds, J. Geophys. Res., 113, D09206, doi:10.1029/2007JD009064.

Fornea, A. P., S. D. Brooks, J. B. Dooley, and A. Saha (2009), Heterogeneous freezing of ice on atmospheric aerosols containing ash, soot, and soil, J. Geophys. Res., 114, D13201, doi:10.1029/2009JD011958.