Laboratory-generated mixtures of mineral dust particles with biological substances: characterization of the particle mixing state and immersion freezing behavior

摘要

Biological particles such as bacteria, fungal spores or pollen areknown to be efficient ice nucleating particles. Their ability tonucleate ice is due to ice nucleation active macromolecules(INMs). It has been suggested that these INMs maintain theirnucleating ability even when they are separated from their originalcarriers. This opens the possibility of an accumulation of such INMsin soils, resulting in an internal mixture of mineral dust andINMs.If particles from such soils which contain biological INMs arethen dispersed into the atmosphere due to wind erosion oragricultural processes, they could induce ice nucleation attemperatures typical for biological substances, i.e., above −20 upto almost 0 °C, while they might be characterized as mineraldust particles due to a possibly low content of biological material.We conducted a study within the research unit INUIT (Ice Nucleation researchUnIT), where we investigated the ice nucleation behavior of mineral dust particlesinternally mixed with INM. Specifically, we mixed a pure mineraldust sample (illite-NX) with ice active biological material (birchpollen washing water) and quantified the immersion freezing behaviorof the resulting particles utilizing the Leipzig Aerosol CloudInteraction Simulator (LACIS). A very important topic concerning theinvestigations presented here as well as for atmospheric application is thecharacterization of the mixing state of aerosol particles. In the presentstudy we used different methods like single-particle aerosol massspectrometry, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray analysis (EDX), and a Volatility–Hygroscopicity TandemDifferential Mobility Analyser(VH-TDMA) to investigate the mixing state of our generated aerosol. Not allapplied methods performed similarly well in detecting small amounts ofbiological material on the mineral dust particles. Measuring thehygroscopicity/volatility of the mixed particles with the VH-TDMA was themost sensitive method. We found that internally mixed particles, containingiceactive biological material, follow the ice nucleation behaviorobserved for the pure biological particles. We verified this by modelingthe freezing behavior of the mixed particles with the Soccerball model (SBM).It can be concluded that a single INM located on a mineral dustparticle determines the freezing behavior of that particle with the resultthat freezing occurs at temperatures at which pure mineral dust particles arenot yet iceactive.