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Airborne dust from glacial outwash sediments may alter properties of clouds and climate at high latitudes by acting as ice nucleating particles (INPs). Nevertheless, the ice nucleating ability of airborne dust from glacial outwash sediments remains uncertain. To address this uncertainty, we measured the ice nucleating ability of airborne dust near an actively retreating glacier in Yukon, Canada during a period when airborne dust concentrations were well above background levels and most likely originated from glacial outwash sediments in the region. The airborne dust caused freezing at temperatures from -6 to -23 degrees C. Based on a heat assay and an ammonium sulfate assay, the INPs from the airborne dust that caused freezing at temperatures warmer than -15 degrees C likely contained biological materials. We show that airborne dust from the retreating glacier likely led to high concentrations of ice nucleating particles at the site for at least most of May 2018. These concentrations, at a freezing temperature of -15 degrees C, were approximately one order of magnitude higher than predictions using a global chemical transport model that included low latitude natural dust sources, but not natural high latitude dust sources., Environmental Science: Atmospheres, 2 (4), ISSN:2634-3606

Secondary organic material (SOM) constitutes a large mass fraction of atmospheric aerosol particles. Understanding its impact on climate and air quality relies on accurate models of interactions with water vapour. Recent research shows that SOM can be highly viscous and can even behave mechanically like a solid, leading to suggestions that particles exist out of equilibrium with water vapour in the atmosphere. In order to quantify any kinetic limitation we need to know water diffusion coefficients for SOM, but this quantity has, until now, only been estimated and has not yet been measured. We have directly measured water diffusion coefficients in the water soluble fraction of α-pinene SOM between 240 and 280 K. Here we show that, although this material can behave mechanically like a solid, at 280 K water diffusion is not kinetically limited on timescales of 1 s for atmospheric-sized particles. However, diffusion slows as temperature decreases. We use our measured data to constrain a Vignes-type parameterisation, which we extend to lower temperatures to show that SOM can take hours to equilibrate with water vapour under very cold conditions. Our modelling for 100 nm particles predicts that under mid- to upper-tropospheric conditions radial inhomogeneities in water content produce a low viscosity surface region and more solid interior, with implications for heterogeneous chemistry and ice nucleation.

This paper presents the studies of one and two component particles using a CO(2) laser for vaporization and VUV ionization in an ion trap mass spectrometer. The degree of fragmentation for a one component system was demonstrated to be a function of CO(2) laser energy. In a two component system, the degree of fragmentation was shown to be a function of the particle composition. This observation indicates that the analysis of mixed particles may be far more complicated than anticipated for a two step process with soft vaporization. In addition to showing that fragmentation is a function of CO(2) laser energy and particle composition, we also show that a key parameter that determines the extent of fragmentation is the energy absorbed by the particle during desorption. The ionization delay profile in a one component system is also shown to be strongly dependent on the vaporization energy. In a two component system, the delay profile is shown to strongly depend on the composition of the particle. The combined data suggest that the key parameter that governs the delay profile is the energy absorbed by the particle during desorption. This finding has implications for potential field measurements. Finally, for a two component system where the absorption crosssections are different, the change in the degree of fragmentation with particle composition resulted in a non-linear dependence of ion signal on composition. This makes any attempt at quantification difficult.