Abstract
Abstract. During the measurement campaign FROST 2 (FReezing Of duST 2), the Leipzig Aerosol Cloud Interaction Simulator (LACIS) was used to investigate the influence of various surface modifications on the ice nucleating ability of Arizona Test Dust (ATD) particles in the immersion freezing mode. The dust particles were exposed to sulfuric acid vapor, to water vapor with and without the addition of ammonia gas, and heat using a thermodenuder operating at 250 °C. Size selected, quasi monodisperse particles with a mobility diameter of 300 nm were fed into LACIS and droplets grew on these particles such that each droplet contained a single particle. Temperature dependent frozen fractions of these droplets were determined in a temperature range between −40 °C ≤T≤−28 °C. The pure ATD particles nucleated ice over a broad temperature range with their freezing behavior being separated into two freezing branches characterized through different slopes in the frozen fraction vs. temperature curves. Coating the ATD particles with sulfuric acid resulted in the particles' IN potential significantly decreasing in the first freezing branch (T>−35 °C) and a slight increase in the second branch (T≤−35 °C). The addition of water vapor after the sulfuric acid coating caused the disappearance of the first freezing branch and a strong reduction of the IN ability in the second freezing branch. The presence of ammonia gas during water vapor exposure had a negligible effect on the particles' IN ability compared to the effect of water vapor. Heating in the thermodenuder led to a decreased IN ability of the sulfuric acid coated particles for both branches but the additional heat did not or only slightly change the IN ability of the pure ATD and the water vapor exposed sulfuric acid coated particles. In other words, the combination of both sulfuric acid and water vapor being present is a main cause for the ice active surface features of the ATD particles being destroyed. A possible explanation could be the chemical transformation of ice active metal silicates to metal sulfates. The strongly enhanced reaction between sulfuric acid and dust in the presence of water vapor and the resulting significant reductions in IN potential are of importance for atmospheric ice cloud formation. Our findings suggest that the IN concentration can decrease by up to one order of magnitude for the conditions investigated.
Highlights
Various atmospheric observations of droplet freezing through heterogeneous ice nucleation show that mineral dust particles act effectively as ice nuclei (IN) (Sassen et al, 2003; DeMott et al, 2003a,b; Cziczo et al, 2004; Richardson et al, 2007; Seifert et al, 2010)
For pure Arizona Test Dust (ATD) particles it was found that two different freezing branches exist which are characterized by different slopes in the ice fraction versus temperature curves
During the measurement campaign FROST 2 the ability of Arizona Test Dust particles to act as IN in the immersion freezing mode was investigated at Leipzig Aerosol Cloud Interaction Simulator (LACIS)
Summary
Various atmospheric observations of droplet freezing through heterogeneous ice nucleation show that mineral dust particles act effectively as ice nuclei (IN) (Sassen et al, 2003; DeMott et al, 2003a,b; Cziczo et al, 2004; Richardson et al, 2007; Seifert et al, 2010). The reason is that mineral dust largely consist of aluminosilicate clays which display a high ice nucleating ability (Archuleta et al, 2005; Kanji et al, 2008; Zimmermann et al, 2008). Through their ability to nucleate ice, mineral dust particles influence microphysical and dynamical cloud properties, the formation of precipitation, cloud life time, and Earth’s climate (Martin, 2000; Sassen et al, 2003; Lohmann and Diehl, 2006; Lohmann and Hoose, 2009). As a result of these aging processes, the particle surface and particles’ IN ability may change
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