Cloud Ice Formation by Arable Soil Dust Aerosol

Dust emission from arable land is mainly caused by wind erosion, overgrazing and tillage activities. The amount of material emitted to the atmosphere strongly depends on season and on soil chemical and physical properties and is thus not easy to quantify. Wind erosion is estimated to lead to average soil losses from 40 to 100 t/ha, while dust emission by tillage activities can be even higher. Also, due to different agglomerate composition, particle density and an enrichment of organic carbon in the top layer of agricultural soil, these processes lead to emitted dust with a higher concentration in organic matter than the remaining soil.

It was shown that biological material can enhance the freezing possibility of supercooled droplets and may thus contribute to ice formation in clouds. Therefore, the investigation and prediction of aerosol-cloud interactions of soil dust, being a complex mixture of mineral, organic and biological compounds, is of high meteorological interest. First published studies indicated a freezing ability that is (depending on material and on temperature) comparable or slightly enhanced to that of arid dust. These studies also implicated the importance of the soil organic matter for initialization of the freezing process.

With help of the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud simulation chamber we have studied the immersion freezing ability of four different arable soil dusts, sampled in Germany, Mongolia and Argentina. In comparison to desert dust aerosol, we found significantly enhanced ice formation for temperatures higher than - 20°C. For use in meteorological models, we provide a suitable parameterization to describe the ice formation induced by arable soil dust aerosol.

We are planning to extend the AIDA dataset, also by fast scanning of available and new soil dust material with the new continuous flow diffusion chamber INKA (Ice Nucleation instrument of the KArlsruhe Institute of Technology). The samples will be also characterized for the mineralogical composition and biological content with the aim to identify the critical factors for the cloud interaction. In addition, we will investigate the role of the aerosol particle size on the nucleation behavior.