Studies of heterogeneous freezing by three different desert dust samples

Abstract

Abstract. We present results of experiments at the aerosol interactions and dynamics in the atmosphere (AIDA) chamber facility looking at the freezing of water by three different types of mineral particles at temperatures between −12 • C and −33 • C. The three different dusts are Asia Dust-1 (AD1), Sahara Dust-2 (SD2) and Arizona test Dust (ATD). The dust samples used had particle concentrations of sizes that were log-normally distributed with mode diameters between 0.3 and 0.5 µm and standard deviations, σ g , of 1.6-1.9. The results from the freezing experiments are consistent with the singular hypothesis of ice nucleation. The dusts showed different nucleation abilities, with ATD showing a rather sharp increase in ice-active surface site density at temperatures less than −24 • C. AD1 was the next most efficient freezing nuclei and showed a more gradual increase in activity than the ATD sample. SD2 was the least active freezing nuclei.We used data taken with particle counting probes to derive the ice-active surface site density forming on the dust as a function of temperature for each of the three samples and polynomial curves are fitted to this data. The curve fits are then used independently within a bin microphysical model to simulate the ice formation rates from the experiments in order to test the validity of parameterising the data with smooth curves. Good agreement is found between the measurements and the model for AD1 and SD2; however, the curve for ATD does not yield results that agree well with the observations. The reason for this is that more experiments between −20 and −24 • C are needed to quantify the rather sharp increase in ice-active surface site density on ATD in this temperature regime. The curves presented can be used as parameteriCorrespondence to: P. J. Connolly (p.connolly@man.ac.uk) sations in atmospheric cloud models where cooling rates of approximately 1 • C min −1 or more are present to predict the concentration of ice crystals forming by the condensationfreezing mode of ice nucleation. Finally a polynomial is fitted to all three samples together in order to have a parameterisation describing the average ice-active surface site density vs. temperature for an equal mixture of the three dust samples.

Automated Continuous-Flow Thermal-Diffusion-Chamber Type Ice Nucleus Counter

“…The onset temperature and humidity of IN activation for ATD and Snomax® are similar to those obtained from expansion experiments in the AIDA chamber (Connolly et al 2009) and by other groups' IN measuring instruments during ICIS2007. However the fractions of activated particles to total ATD or Snomax® particles (d > 0.01 μm) at a given temperature and humidity were smaller than those obtained by other groups although the fractions varied to a large extent, more than one order of magnitude, amongst the instruments.…”

A new temperature- and humidity-dependent surface site density approach for deposition ice nucleation

“…Mineral dust acts as an ice nucleus over a wide range of temperatures and supersaturations over ice, with the most active dusts nucleating ice at approximately 260 K (Welti et al, 2009;Eastwood et al, 2009;Hoose and Möhler, 2012;Murray et al, 2012;YakobiHancock et al, 2013). Using numerical modeling to estimate the climate impact of mineral dust through ice formation requires relations which connect aerosol properties, thermodynamic variables and resulting ice crystal concentrations.…”

“…Therefore, the deterministic approach describes ice formation as a function of temperature and -for deposition nucleation -relative humidity over ice. The proposition of active sites which seemingly nucleate ice as soon as certain thermodynamic thresholds are reached motivates the ice nucleation active surface site (INAS) density concept (Fletcher, 1969;Connolly et al, 2009;Niemand et al, 2012;Hoose and Möhler, 2012).…”

“…The INAS density concept was applied to results from cloud chamber experiments by Connolly et al (2009) to derive INAS densities n s for various mineral dusts. The ice nucleation active surface site density for immersion freezing is described by…”

Modelling the effects of gravity waves on stratocumulus clouds observed during VOCALS-UK

“…In order to observationally constrain CCN in the LEM, quad-log-normal fits to the SMPS and PCASP data (see Allen et al, 2011, which are reproduced in Table 1) were used as input to the ACPIM, bin-microphysics, parcel model (see Connolly et al, 2009Connolly et al, , 2012 In the context of this study the coastal CCN concentrations will be referred to as 'high'; transitional will be referred to as 'medium'; and remote will be referred to as 'low'. It is recognised that these CCN values do not represent a large variation in CCN number 300 concentration, nevertheless the descriptors, 'high', 'medium' and 'low' are relevant in the context of this study.…”