Ice Nucleation Activities of Carbon-Bearing Materials in Deposition Mode: From Graphite to Airplane Soot Surrogates

Abstract

Soot particles emitted by aircraft engines may act as ice nuclei within the atmosphere, subsequently triggering the formation of condensation trails. Such contrails might further evolve as cirrus clouds and thus greatly influence the Earth’s radiative budget and impact the amount of precipitation. In order to monitor in situ deliquescence, efflorescence, and nucleation processes followed by ice growth in the laboratory, we developed the ice and droplet nucleation experimental setup, which combines optical imaging and micro-Raman measurements to follow nucleation events in a pressure-, temperature-, and humidity-controlled optical chamber. We first compare against the literature data and later confirm the deliquescence relative humidities of micrometer-sized sodium chloride salt crystals in the −5 to −35 °C temperature range. Then, we investigate the ice nucleation activity of graphite and aircraft soot analogues, in the −15 to −45 °C temperature range, when exposed to humid nitrogen (N2/H2O gas flow). Soot samples exhibiting various surface chemistries, morphologies, and sizes are thoroughly examined via mass spectrometry and spectroscopic and optical techniques. All carbon-bearing samples are found to be active at nucleating ice at low ice saturation ratios (Sice determined when the first crystal is detected). When normalizing Sice to the total surface area of a sample, one can derive the ice-active surface site density (ns). This parameter provides a means to compare the ice nucleating behavior of various particle types with distinct surface areas. As all samples studied in our work feature large surface areas, we provide ice nucleation data (Sice and ns) for a range of surface areas that remained largely unexplored to date. We find that the interplay between surface composition and morphology (micro, meso, and macro pores, surface roughness) influences the ice onset relative humidity.