Characterization of individual ice residual particles by the single droplet freezing method: a case study in the Asian dust outflow region

Abstract

Abstract. In order to better characterize ice nucleating (IN) aerosol particles in the atmosphere, we investigated the chemical composition, mixing state, and morphology of atmospheric aerosols that nucleate ice under conditions relevant for mixed-phase clouds. Five standard mineral dust samples (quartz, K-feldspar, Na-feldspar, Arizona test dust, and Asian dust source particles) were compared with actual aerosol particles collected from the west coast of Japan (the city of Kanazawa) during Asian dust events in February and April 2016. Following droplet activation by particles deposited on a hydrophobic Si (silicon) wafer substrate under supersaturated air, individual IN particles were located using an optical microscope by gradually cooling the temperature to −30 • C. For the aerosol samples, both the IN active particles and non-active particles were analyzed individually by atomic force microscopy (AFM), micro-Raman spectroscopy, and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX). Heterogeneous ice nucleation in all standard mineral dust samples tested in this study was observed at consistently higher temperatures (e.g., −22.2 to −24.2 • C with K-feldspar) than the homogeneous freezing temperature (−36.5 • C). Meanwhile, most of the IN active atmospheric particles formed ice below −28 • C, i.e., at lower temperatures than the standard mineral dust samples of pure components. The most abundant IN active particles above −30 • C were predominantly irregular solid particles that showed clay mineral characteristics (or mixtures of several mineral components). Other than clay, Ca-rich particles internally mixed with other components, such as sulfate, were also regarded as IN active particle types. Moreover, sea salt particles were predominantly found in the non-active fraction, and internal mixing with sea salt clearly acted as a significant inhibiting agent for the ice nucleation activity of mineral dust particles. Also, relatively pure or fresh calcite, Ca(NO 3 ) 2 , and (NH 4 ) 2 SO 4 particles were more often found in the non-active fraction. In this study, we demonstrated the capability of the combined single droplet freezing method and thorough individual particle analysis to characterize the ice nucleation activity of atmospheric aerosols. We also found that dramatic changes in the particle mixing states during long-range transport had a complex effect on the ice nucleation activity of the host aerosol particles. A case study in the Asian dust outflow region highlighted the need to consider particle mixing states, which can dramatically influence ice nucleation activity.

Composition of ice particle residuals in mixed phase clouds at Jungfraujoch (Switzerland): Enrichment and depletion of particle groups relative to total aerosol

“…Similar to 25 quartz, calcium containing particles showed different ice nucleation ability in previously laboratory studies (Zimmermann et al, 2008;Atkinson et al, 2013). In field experiments, however, Ca-rich particles and Ca-sulphates were observed in the IPR fraction (Ebert et al, 2011;Worringen et al, 2015;Iwata and Matsuki, 2018).…”

“…Laboratory studies (Atkinson et al, 2013;Iwata and Matsuki, 2018) showed that K-feldspar and clay minerals (Zimmermann et al, 2008;Hiranuma et al, 2015;Boose et al, 2016) have a high ice nucleation ability compared to other minerals. A high ice 20 nucleation ability of clay minerals is also reported from field experiments (Targino et al, 2006;Worringen et al, 2015).…”

“…different size, composition and particle generation methods. In field experiments, however, salts are present in the IPR fraction of both, cirrus and mixed phase clouds (Targino et al, 2006;Ebert et al, 2011;Cziczo et al, 2013;Worringen et al, 2015;Iwata and Matsuki, 2018). It is advocated by Iwata and Matsuki (2018) that pure NaCl is not ice 5 active due to molar depression of the freezing point.…”

“…In field experiments, however, salts are present in the IPR fraction of both, cirrus and mixed phase clouds (Targino et al, 2006;Ebert et al, 2011;Cziczo et al, 2013;Worringen et al, 2015;Iwata and Matsuki, 2018). It is advocated by Iwata and Matsuki (2018) that pure NaCl is not ice 5 active due to molar depression of the freezing point. However, sea salt may act as an INP due to the presence of organic particles in sea spray (Wilson et al, 2015;DeMott et al, 2016;Iwata and Matsuki, 2018).…”

“…It is advocated by Iwata and Matsuki (2018) that pure NaCl is not ice 5 active due to molar depression of the freezing point. However, sea salt may act as an INP due to the presence of organic particles in sea spray (Wilson et al, 2015;DeMott et al, 2016;Iwata and Matsuki, 2018). We cannot prove with our measurements if the ice activity is due to organic species within the salt fraction, and hence also not exclude this hypothesis.…”