The experimental investigations have been carried out mainly utilizing the Leipzig Aerosol Cloud Interaction Simulator“ (LACIS). Since beginning of 2013, a "Spectrometer for Ice Nuclei" (SPIN) is available as well. LACIS is well suited for investigating immersion freezing, while SPIN allows for the consideration of mainly deposition freezing.
Concerning the topic of heterogeneous ice nucleation, intensive laboratory investigations are carried out at TROPOS (a member of the research unit INUIT). Goals of these investigations are:
- a better understanding concerning the fundamental processes and the important parameters and properties controlling the heterogeneous freezing of cloud droplets
- identification of aerosol particles and substances which function as atmospheric ice nuclei (IN)
- quantification of the ice nucleation behavior of atmospherically relevant IN, e.g., in terms of nucleation rates
- provision of models and parameterizations for use in larger scale atmospheric models
During the past years, the ice nucleation behavior of especially mineral dust and biological particles has been investigated and quantified. We examined size segregated pure particles as well as particles coated with different substances (sulfuric acid, succinic acid, levoglucosan). The most important scientific results and findings gained in course of the investigations performed are:
- Of all examined mineral dust particles (size range: a few hundred nanometers up to a few micrometers), particles from feldspar (or more specifically from microcline, a K-feldspar) are clearly more ice active than those from Arizona Test Dust, kaolinite and illite, and generally the ice nucleation activity was found to scale with particle surface area (compare e.g., Niedermeier et al. (2010), Wex et al. (2014) and Augustin-Bauditz et al. (2014)).
- The ice nucleation potential of ALL mineral dust particles (including microcline) was reduced to similar values ("clay baseline") after coating with sulfuric acid (as summarized in Augustin-Bauditz et al. (2014)), i.e. a reduction of the ice nucleation potential of mineral dusts due to atmospheric aging may be expected.
- In the water-vapor sub-saturated regime, observed ice nucleation could be described based on a parameterization developed for immersion freezing and a water-activity dependent freezing point depression (Wex et al. (2014)).
- Biological ice nucleators are macromolecules, e.g., protein complexes anchored in the cell membrane of bacteria, polysaccharides on pollen and proteinaceous macromolecules on fungi, where the latter two can easily be washed off from the respective pollen grains and fungal spores. Based on LACIS measurements, we determined ice nucleation rates for single ones of these macromolecules (Hartmann et al. (2013) for P. syringae bacteria and Augustin et al. (2013) for two different types of birch pollen).
- Different biological ice active macromolecules induce ice nucleation at different temperatures, and the size of these macromolecules was found to be related to the size of the critical ice nucleus at the respective temperatures (Pummer et al. (2015)).
- LACIS measurements clearly indicate that biological ice active macromolecules can exist separated from their original carrier without loosing their ice nucleating abilities. This suggests the possibility of accumulating ice nucleating entities in e.g. soils and their release into the atmosphere due to e.g. soil erosion processes. Such processes could significantly increase the importance of biological IN in the atmosphere, but are not considered in atmospheric models today (Hartmann et al. (2013) and Augustin et al. (2013)).
- Successful comparisons for immersion freezing measurements from a multitude of different instruments were made for a biological ice nucleator (Snomax, Wex et al. (2015)) and a mineral dust (illite, Hiranuma et al. (2015)).
- Different approaches for the parameterization of our data were used, ranging from time indepentend parameterizations (e.g., Wex et al. (2014) and Augustin-Bauditz et al. (2014)) to the Soccer Ball Model (Niedermeier et al. (2011b), Niedermeier et al. (2014)) which is based on Classical Nucleation Theory. Although ice nucleation is clearly a stochastic process, it depents much stronger on temperature than on time.
*(summary of all ice nucleation related LACIS publications)
Augustin et al. (2013), Immersion freezing of birch pollen washing water, Aerosol Chem. Phys., 13, 10989–11003.
Augustin-Bauditz et al. (2014), The immersion mode ice nucleation behavior of mineral dusts: A comparison of different pure and surface modifed dust, Geophys. Res. Lett., 41, doi:10.1002/2014GL061317.
Augustin-Bauditz et al. (2015), The immersion freezing behavior of mineral dust particles mixed with biological substances, Atmos. Chem. Phys. Discuss., 15.
Clauss et al. (2013), Application of linear polarized light for the discrimination of frozen and liquid droplets in ice nucleation experiments, Atmos. Meas. Tech., 6, 1041-1052.
Hartmann et al. (2011), Homogeneous and heterogeneous ice nucleation at LACIS: Operating principle and theoretical studies, Atmos. Chem. Phys., 11, 1753–1767.
Hartmann et al. (2013), Immersion freezing of ice nucleating active protein complexes, Atmos. Chem. Phys., 13, 5751-5766.
Hartmann et al. (2016), Immersion freezing of kaolinite - scaling with particle surface area, J. Atmos. Sci., 73, 263-278, doi:10.1175/JAS-D-15-0057.1.
Hiranuma et al. (2015), A comprehensive laboratory study on the immersion freezing behavior of illite NX particles: a comparison of seventeen ice nucleation measurement techniques, Atmos. Chem. Phys., 15, 2489–2518, doi:10.5194/acp-15-2489-2015.
Niedermeier et al. (2010), Heterogeneous freezing of droplets with immersed mineral dust particles – measurements and parameterization, Atmos. Chem. Phys., 10, 3601–3614.
Niedermeier et al. (2011a), Experimental study of the role of physicochemical surface processing on the IN ability of mineral dust particles, Atmos. Chem. Phys., 11, 11131–11144.
Niedermeier et al. (2011b), Heterogeneous ice nucleation: exploring the transition from stochastic to singular freezing behavior, Atmos. Chem. Phys., 11, 8767-8775, doi:10.5194/acp-11-8767-2011.
Niedermeier et al. (2014), A computationally efficient description of heterogeneous freezing: A simplified version of the Soccer ball model, Geophys. Res. Lett., 41, doi:10.1002/2013GL058684.
Niedermeier et al. (2015), Can we define an asymptotic value for the ice active surface site density for heterogeneous ice nucleation?, J. Geophys. Res., doi:10.1002/2014JD022814.
Pummer et al. (2015), Ice nucleation by water-soluble macromolecules, Atmos. Chem. Phys., 15, 4077–4091, doi:10.5194/acp-15-4077-2015.
Reitz et al. (2011), Surface modification of mineral dust particles by sulphuric acid processing: implications for CCN and IN abilities, Atmos. Chem. Phys., 11, 7839–7858.
Sullivan et al. (2010), Irreversible loss of ice nucleation active sites in mineral dust particles caused by sulphuric acid condensation, Atmos. Chem. Phys., 10, 11471–11487.
Tobo et al. (2012), Impacts of chemical reactivity on ice nucleation of kaolinite particles: A case study of levoglucosan and sulfuric acid, Geophys. Res. Lett., 39 (L19803), doi:10.1029/2012GL053007.
Wex et al. (2014), Kaolinite particles as ice nuclei: learning from the use of different kaolinite samples and different coatings, Atmos. Chem. Phys., 14, doi:10.5194/acp-14-5529-2014.
Wex et al. (2015), Intercomparing different devices for the investigation of ice nucleating particles using Snomax as test substance, Atmos. Chem. Phys., 15, 1463–1485, doi:10.5194/acp-15-1463-2015.