Analyzing the chemical composition, morphology, and size of ice-nucleating particles by coupling a scanning electron microscope to an offline diffusion chamber

Abstract

To understand and predict the formation of clouds and precipitation and their influence on our climate, it is crucial to know the characteristics and abundance of ice-nucleating particles (INPs) in the atmosphere. As the ice-nucleating efficiency is a result of individual particle properties, detailed knowledge of these properties is essential. Here, an offline method for the comprehensive single-particle analysis of ambient INPs that benefits from the combination of two instruments already used for ice nucleation measurements is presented, focusing on the methodological description of the coupling, whereby strengths and weaknesses of the method are discussed. First, the aerosol is sampled on silicon wafers. INPs are then activated at different temperature and humidity conditions in the deposition nucleation and condensation freezing mode using a static diffusion chamber. The positions of grown ice crystals are defined by a coordinate system, which allows for recovery and detailed analysis of the individual INPs by a scanning electron microscope. Based on their physico-chemical properties (elemental composition and morphology) the INPs can be classified into categories. In combination with the size information, a size-resolved distribution of the INP classes can be determined. Such results are useful for evaluating INP-type-specific parametrizations, e.g., for use in atmospheric modeling and in closure studies. A case study from the high-altitude research station Jungfraujoch, Switzerland shows that the targeted INP analysis as obtained by this method is able to identify the main INP classes in reliable proportions. Most of the deposition-nucleation-mode and condensation-freezing-mode INPs activated at − 30 °C, indicating a geogenic mineral origin (mainly aluminosilicates / Al-rich particles but also carbonates and silica). Other major contributors were carbonaceous particles, consisting of both smaller soot particles and larger biological particles and mixed particles (mostly Al / C mixed particles). The INPs had projected area diameters ranging from 300 nm–35 µm, with a distinct maximum at 1–2 µm. Mineral particles were present throughout the entire size range, while mixed particles were identified in higher abundances at sizes of 3 µm and above. Minor contributions were seen from sulfates and metal oxides, the latter with an increased proportion in the size range below 500 nm. During a Saharan dust event, a significant increase in mineral particles in the INP composition was detected.