Ice Nucleation by Particles Collected in the Eastern North Atlantic and by Internally and Externally Mixed Mineral Dust Particles in Experiment and Model

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Description

Particles collected at the ground site of Graciosa Island during the Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) campaign in summer 2017 and laboratory generated externally and internally mixed mineral dust particles are investigated for their ice nucleation efficiency. During ACE-ENA, particles were collected for day and nighttime periods. Ice nucleation experiments were conducted for temperatures between 210 and 240 K. All particle samples initiate ice formation heterogeneously, either at saturation or below the homogeneous freezing limit. Below 230 K the particles demonstrate high ice nucleation propensity, typically observed for dust particles. Particles sampled during nighttime tend to initiate ice nucleation at higher RH compared to daytime samples. We conclude that the marine boundary layer contains ice nucleating particles (INPs). These experiments will be accompanied by scanning electron microscopy with energy dispersive x-ray spectroscopy for INPs’ and particles’ elemental analyses and scanning transmission X-ray microscopy with near edge X-ray absorption spectroscopy for mixing state analysis. Immersion freezing is recognized as the most important ice crystal formation process in mixed-phase cloud environments. Here, the focus is placed on how different mineral dust species such as illite, kaolinite and feldspar, initiate freezing of water droplets when present in internal and external mixtures during constant cooling rate or constant temperature freezing experiments. Analyses of single and multicomponent mineral dust droplet samples include different stochastic and deterministic models. Parameter sets derived from freezing data of single mineral dust containing droplets are evaluated for prediction of freezing of multicomponent mineral dust samples. We apply the Earth system model ModelE2 with newly implemented aerosol mineral fraction method to improve the simulation of the size distribution of the aerosolized mineral fractions. We then calculate INP numbers from minerals for an active site representation. The global K-feldspar INP are reduced by a factor 2-3, compared to a simple size distribution of dust. The decrease is up to a factor of five in some geographical regions. The results vary little between external and internal mixing and different activation temperatures, except for the coldest temperatures. In the sectional size distribution, the size range 2-4 µm contributes most to INP numbers.