T. Clauss

Heterogeneous freezing of droplets with immersed mineral dust particles - measurements and parameterization

During the measurement campaign FROST (FReezing Of duST), LACIS (Leipzig Aerosol Cloud Interaction Simulator) was used to investigate the immersion freezing behavior of size selected, coated and uncoated Arizona Test Dust (ATD) particles with a mobility diameter of 300 nm. Particles were coated with succinic acid (C 4H6O4), sulfuric acid (H2SO4) and ammonium sulfate ((NH 4)2SO4). Ice fractions at mixed-phase cloud temperatures ranging from 233.15 K to 239.15 K ( ±0.60 K) were determined for all types of particles. In this temperature range, pure ATD particles and those coated with C 4H6O4 or small amounts of H2SO4 were found to be the most efficient ice nuclei (IN). ATD particles coated with (NH4)2SO4 were the most inefficient IN. Since the supercooled droplets were highly diluted before freezing occurred, a freezing point suppression due to the soluble material on the particles (and therefore in the droplets) cannot explain this observation. Therefore, it is reasonable to assume that the coatings lead to particle surface alterations which cause the differences in the IN abilities. Two different theoretical approaches based on the stochastic and the singular hypotheses were applied to clarify and parameterize the freezing behavior of the particles investigated. Both approaches describe the experimentally determined results, yielding parameters that can subsequently be used to compare our results to those from other studies. HowCorrespondence to: D. Niedermeier (niederm@tropos.de) ever, we cannot clarify at the current state which of the two approaches correctly describes the investigated immersion freezing process. But both approaches confirm the assumption that the coatings lead to particle surface modifications lowering the nucleation efficiency. The stochastic approach interprets the reduction in nucleation rate from coating as primarily due to an increase in the thermodynamic barrier for ice formation (i.e., changes in interfacial free energies). The singular approach interprets the reduction as resulting from a reduced surface density of active sites.

Heterogeneous ice nucleation on biological particles: Bacteria and pollen

In the atmosphere the importance of biological ice nuclei is still not well understood. Therefore we investigated the ice nucleation behavior of Snomax™, used as a model for bacterial ice nucleation, and birch pollen washing water, used as a model for pollen induced ice nucleation. Thereby we quantified the ice nucleation behavior of the INA protein complexes controlling the ice nucleation ability of Pseudomonas syringae, and that of sugar-like macromolecules controlling the ice nucleation ability of birch pollen. The given parameterizations can be used to describe the ice nucleation behavior of the respective ice active bacteria and pollen in atmospheric models.