Abstract
Abstract. The prediction of cloud ice formation in climate models remains a challenge, partly due to the complexity of ice-related processes. Mineral dust is a prominent aerosol in the troposphere and is an important contributor to ice nucleation in mixed-phase clouds, as dust can initiate ice heterogeneously at relatively low supercooling conditions. We characterized the ice nucleation properties of size-segregated mineral dust sampled during dust events in the eastern Mediterranean. The sampling site allowed us to compare the properties of airborne dust from several sources with diverse mineralogy that passed over different atmospheric paths. We focused on particles with six size classes determined by the Micro-Orifice Uniform Deposit Impactor (MOUDI) cutoff sizes: 5.6, 3.2, 1.8, 1.0, 0.6 and 0.3 µm. Ice nucleation experiments were conducted in the Weizmann Supercooled Droplets Observation on a Microarray (WISDOM) setup, whereby the particles are immersed in nanoliter droplets using a microfluidics technique. We observed that the activity of airborne particles depended on their size class; supermicron and submicron particles had different activities, possibly due to different composition. The concentrations of ice-nucleating particles and the density of active sites (ns) increased with the particle size and particle concentration. The supermicron particles in different dust events showed similar activity, which may indicate that freezing was dominated by common mineralogical components. Combining recent data of airborne mineral dust, we show that current predictions, which are based on surface-sampled natural dust or standard mineral dust, overestimate the activity of airborne dust, especially for the submicron class. Therefore, we suggest including information on particle size in order to increase the accuracy of ice formation modeling and thus weather and climate predictions.
Highlights
Cloud droplets can supercool to 238 K before homogeneous freezing occurs (Koop and Murray, 2016; Rosenfeld and Woodley, 2000)
We focused on particles with six size classes determined by the Micro-Orifice Uniform Deposit Impactor (MOUDI) cutoff sizes: 5.6, 3.2, 1.8, 1.0, 0.6 and 0.3 μm
Ice nucleation experiments were conducted in the Weizmann Supercooled Droplets Observation on a Microarray (WISDOM) setup, whereby the particles are immersed in nanoliter droplets using a microfluidics technique
Summary
Cloud droplets can supercool to 238 K before homogeneous freezing occurs (Koop and Murray, 2016; Rosenfeld and Woodley, 2000). While there are few measurements of AMD close to source regions (Price et al, 2018; Boose et al, 2016a; Ardon-Dryer and Levin, 2014; Schrod et al, 2017), parameterizations of ice formation in climate models are often based on the freezing properties of natural dust or soil samples collected from deserts or standard dust particles (Niemand et al, 2012; Connolly et al, 2009; Ullrich et al, 2017; Atkinson et al, 2013; Broadley et al, 2012) that may not sufficiently represent AMD (Boose et al, 2016b; Spichtinger and Cziczo, 2010). We characterized the concentrations and the density of ice-nucleation-active sites (INASs) of AMD in different size classes for several dust cases, as well as combining recent literature and available AMD data to understand how well AMD is represented in models based on recent parameterizations
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