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

Ayumi Iwata,Atsushi Matsuki

doi:10.5194/acp-18-1785-2018

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(NO3)2, and (NH4)2SO4 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.

Highlights

Ice nucleation in clouds substantially affects the climate by significantly impacting the radiation balance and precipitation processes in the Earth’s atmosphere (Lohmann and Feichter, 2005; Rosenfeld et al, 2008; Flato et al, 2013)
Before evaluating the ice nucleation activity of the standard samples by individual droplet freezing method (IDFM), we measured the freezing temperature of pure water droplets using the same method, which can be regarded as the onset temperature of homogeneous freezing
Homogeneous ice nucleation was initiated at approximately −36.5 ◦C or below, and all droplets spontaneously froze in a narrow temperature range within the same experimental run

Summary

Introduction

Ice nucleation in clouds substantially affects the climate by significantly impacting the radiation balance and precipitation processes in the Earth’s atmosphere (Lohmann and Feichter, 2005; Rosenfeld et al, 2008; Flato et al, 2013). Most of the initial precipitation process in midlatitude regions involves ice nucleation in mixed-phase clouds, where supercooled water droplets and ice crystals coexist (Pruppacher and Klett, 1997; Murray et al, 2012). Matsuki: Characterization of individual ice residual particles water droplets These pure water droplets spontaneously freeze by cooling to approximately −37 ◦C and below (homogeneous nucleation) (e.g., Koop, 2000; Rosenfeld and Woodley, 2000; Murray et al, 2010, 2012). Supercooled water droplets in the atmosphere generally form ice crystals at higher temperatures due to the presence of aerosol particles that can nucleate ice (heterogeneous nucleation). The response of ice nucleation processes to changes in host aerosol properties is still poorly understood due to a lack of understanding of the basic aerosol particle interactions leading to ice crystal formation. Considerable uncertainty still exists regarding the prediction of ice nucleation that would lead to climate changes in the atmosphere

Methods

Results

Discussion

Conclusion