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 rain 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, a detailed knowledge on these properties is essential. Here, we present an offline method for the comprehensive analysis of ambient INPs that benefits from the combination of two instruments already used for ice nucleation measurements. 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. Activated INPs are located in a coordinate system, which allows for recovery of the individual particles causing the nucleation in a scanning electron microscope. Here, the size, chemistry and morphology of the particles are identified. Finally, the INPs are classified into categories based on their measured properties. As a result, a size resolved spectrum of the INP classes can be determined.

The performance of this coupling method is investigated in a case study on samples from the high-altitude field side Jungfraujoch (JFJ), Switzerland. 200 individual INPs from 14 samples obtained during a 5-week period were classified. Most deposition nucleation / condensation freezing mode INPs from Jungfraujoch, activated at −30 °C, were of irregular shape and had projected area diameters in the range from 300 nm to 35 µm, with a distinct maximum at 1–2 µm. A major contribution of mineral particles, mainly aluminosilicates / Al-rich particles, but also carbonates and silica, was identified for the entire INP size spectrum at −30°C. Further contributions were from carbon-rich particles, consisting of both smaller soot particles and larger biological particles. Mixed particles, here mostly particles rich in Al and C, were identified in higher abundances primarily between 3 µm and 9 µm. Minor contributions were seen from sulfates and metal oxides, with the latter ones found with increased proportion in the size range below 500 nm.

Such results are useful for evaluating INP type-specific parametrizations, e.g., for use in atmospheric modeling, and in closure studies.