Evaluating the effect of soil dust particles from semi-arid areas on clouds and climate

Primary ice production in mixed-phase clouds predominantly takes place by heterogeneous freezing of mineral dust particles. Therefore, mineral dust has a large impact on cloud properties. Organic matter attached to mineral dust particles can expand their already good freezing ability further to warmer subzero temperatures. These dust particles are called "soil dust". Dusts emitted from deserts contribute most to the total dust concentration in the atmosphere and they can be transported over long distances. Soil dust is emitted from semi-arid regions, e.g. agricultural areas. Besides wind erosion, human activities like tillage or harvest might be a large source for soil dust release into the atmosphere. In this study, we analyze the influence of soil dust particles on clouds with the Norwegian Earth System Model (NorESM; Bentsen et al., 2013: GMD). The parameterization of immersion freezing on soil dust is based on findings from the AIDA cloud chamber (Steinke et al., in prep.). Contact angle and activation energy for soil dust are estimated in order to be used in the dust immersion freezing scheme of the model, which is based on classical nucleation theory. Our first results highlight the importance of soil dust for ice nucleation on a global scale. Its influence is expected to be highest in the northern hemisphere due to its higher area for soil dust emission. The immersion freezing rates due to additional soil dust can on average increase by a factor of 1.2 compared to a mineral dust-only simulation. Using a budget tool for NorESM, influences of soil dust ice nuclei on single tendencies of the cloud microphysics can be identified. For example, accretion to snow is sensitive to adding soil dust ice nuclei. This can result in changes e.g. in the ice water path and cloud radiative properties.