Abstract

Abstract. Biological particles such as bacteria, fungal spores or pollen are known to be efficient ice nucleating particles. Their ability to nucleate ice is due to ice nucleation active macromolecules (INMs). It has been suggested that these INMs maintain their nucleating ability even when they are separated from their original carriers. This opens the possibility of an accumulation of such INMs in soils, resulting in an internal mixture of mineral dust and INMs. If particles from such soils which contain biological INMs are then dispersed into the atmosphere due to wind erosion or agricultural processes, they could induce ice nucleation at temperatures typical for biological substances, i.e., above −20 up to almost 0 °C, while they might be characterized as mineral dust particles due to a possibly low content of biological material. We conducted a study within the research unit INUIT (Ice Nucleation research UnIT), where we investigated the ice nucleation behavior of mineral dust particles internally mixed with INM. Specifically, we mixed a pure mineral dust sample (illite-NX) with ice active biological material (birch pollen washing water) and quantified the immersion freezing behavior of the resulting particles utilizing the Leipzig Aerosol Cloud Interaction Simulator (LACIS). A very important topic concerning the investigations presented here as well as for atmospheric application is the characterization of the mixing state of aerosol particles. In the present study we used different methods like single-particle aerosol mass spectrometry, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray analysis (EDX), and a Volatility–Hygroscopicity Tandem Differential Mobility Analyser (VH-TDMA) to investigate the mixing state of our generated aerosol. Not all applied methods performed similarly well in detecting small amounts of biological material on the mineral dust particles. Measuring the hygroscopicity/volatility of the mixed particles with the VH-TDMA was the most sensitive method. We found that internally mixed particles, containing ice active biological material, follow the ice nucleation behavior observed for the pure biological particles. We verified this by modeling the freezing behavior of the mixed particles with the Soccerball model (SBM). It can be concluded that a single INM located on a mineral dust particle determines the freezing behavior of that particle with the result that freezing occurs at temperatures at which pure mineral dust particles are not yet ice active.

Highlights

  • In the last years a lot of effort has been made to characterize the freezing ability of different aerosol particles, which were thought to be ice nucleating active

  • If particles from such soils which contain biological ice nucleation active macromolecules (INMs) are dispersed into the atmosphere due to wind erosion or agricultural processes, they could induce ice nucleation at temperatures typical for biological substances, i.e., above −20 up to almost 0 ◦C, while they might be characterized as mineral dust particles due to a possibly low content of biological material

  • We conducted a study within the research unit INUIT (Ice Nucleation research UnIT), where we investigated the ice nucleation behavior of mineral dust particles internally mixed with INM

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Summary

Introduction

In the last years a lot of effort has been made to characterize the freezing ability of different aerosol particles, which were thought to be ice nucleating active. To quantify the freezing ability of an internal mixture of mineral dust and biological material it is advantageous to know the freezing ability of the individual materials For this reason in the present study we mixed a well-characterized mineral dust (illite-NX, Hiranuma et al, 2015; Augustin-Bauditz et al, 2014) with well-characterized biological material (birch pollen washing water, Pummer et al, 2012; Augustin et al, 2013) and investigated the immersion freezing ability of the resulting mixed particles, utilizing the Leipzig Aerosol Cloud Interaction Simulator (LACIS). One aspect of this study is to compare these characterization methods and to assess their ability to identify the mixing state of a laboratory-generated aerosol Another important issue is the adequate interpretation of the freezing behavior of the mixed particles.

Basics of the Soccerball model
Materials
Particle generation and characterization
Freezing experiments
VH-TDMA
SEM and EDX
Single particle mass spectrometry
Immersion freezing experiments
Theoretical description and discussion
Immersion freezing properties of BPWW particles
Immersion freezing properties of illite-NX particles
Immersion freezing properties of illite–BPWW particles
Findings
Conclusions
Full Text
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