Abstract
Abstract. Surface-collected dust from three different locations around the world was examined with respect to its ice nucleation activity (INA) with the ETH Portable Ice Nucleation Chamber (PINC). Ice nucleation experiments were conducted with particles of 200 and 400 nm in diameter in the temperature range of 233–243 K in both the deposition nucleation and condensation freezing regimes. Several treatments were performed in order to investigate the effect of mineralogical composition, as well as the presence of biological and proteinaceous, organic and soluble compounds on the INA of mineral and soil dust. The INA of untreated dust particles correlated well with the total feldspar and K-feldspar content, corroborating previously published results. The removal of heat-sensitive proteinaceous and organic components from the particle surface with heat decreased the INA of dusts. However, the decrease in the INA was not proportional to the amount of these organic components, indicating that different proteinaceous and organic species have different ice nucleation activities, and the exact speciation is required in order to determine why dusts respond differently to the heating process. The INA of certain dusts increased after the removal of soluble material from the particle surface, demonstrating the low INA of the soluble compounds and/or the exposition of the underlying active sites. Similar to the proteinaceous organic compounds, soluble compounds seem to have different effects on the INA of surface-collected dusts, and a general conclusion about how the presence of soluble material on the particle surface affects its INA is not possible. The investigation of the heated and washed dusts revealed that mineralogy alone is not able to fully explain the observed INA of surface-collected dusts at the examined temperature and relative humidity conditions. The results showed that it is not possible to predict the INA of surface-collected soil dust based on the presence and amount of certain minerals or any particular class of compounds, such as soluble or proteinaceous/organic compounds. Instead, at temperatures of 238–243 K the INA of the untreated, surface-collected soil dust in the condensation freezing mode can be roughly approximated by one of the existing surrogates for atmospheric mineral dust, such as illite NX. Uncertainties associated with mechanical damage and possible changes to the mineralogy during treatments, as well as with the BET surface area and its immediate impact on the number of active sites (ns,BET), are addressed.
Highlights
Atmospheric aerosol particles are well known to modify the microphysical properties of clouds, such as their albedo, lifetime and precipitation patterns, in what is known as the indirect effects of aerosols on climate (e.g. Lohmann and Feichter, 2005)
Primary nucleation of ice in the atmosphere can occur in the absence of insoluble material by way of freezing of supercooled liquid water droplets; this process is known as homogeneous freezing, and it requires temperatures below ∼ −37 ◦C (236 K; e.g. Murray et al, 2010; Vali et al, 2015)
The results of the heating and washing treatments revealed that mineralogy alone is not able to fully explain the observed ice nucleation activity (INA) of surface-collected dusts at the examined temperature and relative humidity conditions
Summary
Atmospheric aerosol particles are well known to modify the microphysical properties of clouds, such as their albedo, lifetime and precipitation patterns, in what is known as the indirect effects of aerosols on climate (e.g. Lohmann and Feichter, 2005). In the atmosphere aerosol particles can form both cloud droplets and ice crystals; in such cases these aerosol particles are referred to as cloud condensation nuclei (CCN) and ice nucleating particles (INPs) forming liquid and solid hydrometeors, respectively. Primary nucleation of ice in the atmosphere can occur in the absence of insoluble material by way of freezing of supercooled liquid water droplets; this process is known as homogeneous freezing, and it requires temperatures below ∼ −37 ◦C The presence of INPs results in the onset of freezing at higher temperatures in what is known as heterogeneous freezing This process has four known mechanisms: deposition nucleation, condensation freezing, immersion freezing and contact freezing (Vali, 1985; Vali et al, 2015). Since the concentration of ice crystals in the atmosphere is typically orders of magnitude larger than the INP concentration (Cantrell and Heymsfield, 2005), several secondary production mechanisms are believed to contribute to the observed ice crystal number concentrations (e.g. Hallett and Mossop, 1974; Field et al, 2017)
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