Ice Nucleation Activity of Soil Dust Aerosols

Only a minor, but strongly temperature and humidity dependent fraction of atmospheric aerosol particles act as ice nucleating particles (INPs) and thereby induce primary ice formation in tropospheric clouds. This primary formation of the ice-phase is often followed by secondary ice multiplication processes, and thereby markedly influences cloud radiative properties as well as the formation and distribution of precipitation. Mineral dust aerosols originating from dry and vegetation free desert sources are considered to significantly contribute to the abundance of atmospheric INPs. More recent investigations have shown that arable soil dust has an enhanced ice nucleation activity compared to this predominantly pure mineral dust probably because of its content of soil organic matter and biological material. It is the major subject of this thesis to comprehensively investigate the sources for this enhanced ice nucleation of soil dust dominated aerosol, and to quantify the ice nucleation activity for a selection of soil dust samples of relevance for the atmospheric aerosol. The laboratory investigations presented in this study made use of the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud chamber to study the immersion freezing and deposition nucleation behavior in the temperature range between 190K and 256K. Four different soil dust samples were used for the cloud simulation experiments and were additionally characterized with other offline and online INP measurement methods. After dispersing the dry dust samples into the AIDA chamber, aerosol filter samples were, for example, taken and shipped to the Max Planck Institute for Chemistry for INP analysis with their MINA (mono ice nucleation assay) instrument. The new laboratory based continuous flow diffusion chamber INKA (Ice Nucleation instrument of the KArlsruhe Institute of Technology) was built as part of this thesis in collaboration with the Colorado State University (CSU), and was used for immersion freezing measurements with the same aerosols as used in the AIDA experiments. The advantage of INKA are fast automated scans of the aerosol ice nucleation behavior at well controlled temperatures (213K to 263K) and relative humidities. Another instrument for INP measurements, that was developed as part of this thesis in cooperation with CSU is the Ice Nucleation SpEctrometer of the Karlsruhe Institute of Technology (INSEKT) that adopted the design of the CSU Ice Spectrometer instrument (CSU-IS). INSEKT was used for measuring the temperature dependent INP concentration in aerosols sampled from both the AIDA chamber and a field site in Cyprus during the A-LIFE campaign in April 2017. The advantage of this method is its high sensitivity for INP measurements, thereby extending and complementing the AIDA and INKA data to higher temperatures and providing concentrations of atmospheric INPs in a wide temperature range from 248K to about 268K. The INSEKT development was motivated by, and strongly benefited from two research stays at the CSU from July 2015 to October 2015 and from August 2016 to September 2016. The first research stay included the participation in a field campaign at the SPL (Storm Peak Laboratory) in September 2015, which was part of the Fifth International Workshop in Ice Nucleation (FIN). During this field campaign, atmospheric INP concentrations where measured with the CSU-IS at temperatures between 246K to about 268K. Further, the CSU-IS was used to measure INP temperature spectra of aerosols collected from the AIDA chamber as well as of atmospheric aerosols sampled at the Cyprus Atmospheric Observatory (CAO) during the INUIT-BACCHUS-ACTRIS campaign in April 2016. The laboratory experiments were analyzed in terms of the so-called ice nucleation active surface site (INAS) density as a function of temperature, whereas for the field experiments temperature-dependent spectra of the INP concentrations were obtained. The laboratory and field measurements in the mixedphase cloud regime, for temperatures between 243K and 268K, show two distinct regimes in the temperature spectrum of INP concentrations or INAS densities. Below 253K, the ice nucleation activity follows an exponential temperature dependence with only a small variability between different samples. Heat and hydrogen peroxide treatment has almost no effect, indicating that the ice nucleation activity in this regime is dominated by mineral dust particles. At temperatures above 253K, the ice nucleation activity varies largely between the individual samples. Heat and hydrogen peroxide treatment reduce the ice nucleation activity, clearly demonstrating the important contribution of heat-sensitive or chemically reactive organic compounds to the ice nucleation activity of multi-component soil dust aerosols at temperatures above 253K. A day with high mineral dust aerosol concentrations during the INUIT-BACCHUS-ACTRIS campaign provided a unique opportunity to sample and analyze the INP temperature spectrum of atmospheric mineral dust aerosol. The INP concentration followed an exponential temperature dependency over the entire temperature range (246K to 265K) and was independent of the applied heat and hydrogen peroxide treatment. This result underlines the conclusion that mineral dust particles mainly contribute to the atmospheric INP abundance at lower mixed-phase cloud temperatures. Laboratory measurements in the cirrus cloud regime below about 238K indicate that the ice nucleation activity of soil dust aerosol in this regime is independent of the soil organic matter content. A series of deposition nucleation experiments with one soil dust sample showed u-shaped isolines of the INAS density parameter in the temperature-saturation ratio diagram, similar to recent literature results for pure mineral dust aerosols. The here investigated soil dust is more ice active at low ice supersaturations than pure mineral dust aerosols investigated in previous AIDA studies.