Water uptake of clay and desert dust aerosol particles at sub- and supersaturated water vapor conditions

Abstract

Airborne mineral dust particles serve as cloud condensation nuclei (CCN), thereby influencing the formation and properties of warm clouds. It is therefore of atmospheric interest how dust aerosols with different mineralogy behave when exposed to high relative humidity (RH) or supersaturation (SS) with respect to liquid water. In this study the subsaturated hygroscopic growth and the supersaturated cloud condensation nucleus activity of pure clays and real desert dust aerosols were determined using a hygroscopicity tandem differential mobility analyzer (HTDMA) and a cloud condensation nuclei counter (CCNC), respectively. Five different illite, montmorillonite and kaolinite clay samples as well as three desert dust samples (Saharan dust (SD), Chinese dust (CD) and Arizona test dust (ATD)) were investigated. Aerosols were generated both with a wet and a dry disperser. The water uptake was parameterized via the hygroscopicity parameter kappa. The hygroscopicity of dry generated dust aerosols was found to be negligible when compared to processed atmospheric aerosols, with CCNC derived kappa values between 0.00 and 0.02 (the latter corresponds to a particle consisting of 96.7% by volume insoluble material and approximately 3.3% ammonium sulfate). Pure clay aerosols were generally found to be less hygroscopic than natural desert dust particles. The illite and montmorillonite samples had kappa approximately 0.003. The kaolinite samples were less hygroscopic and had kappa=0.001. SD (kappa=0.023) was found to be the most hygroscopic dry-generated desert dust followed by CD (kappa=0.007) and ATD (kappa=0.003). Wet-generated dust showed an increased water uptake when compared to dry-generated samples. This is considered to be an artifact introduced by redistribution of soluble material between the particles. Thus, the generation method is critically important when presenting such data. These results indicate any atmospheric processing of a fresh mineral dust particle which leads to the addition of more than approximately 3% soluble material will significantly enhance its hygroscopicity and CCN activity.

Cloud condensation nuclei activity and droplet activation kinetics of wet processed regional dust samples and minerals

“…For all wet samples, κ ranges between 0.15-0.61, which is considerably higher than the equivalent κ (= 0.05) for dry generated dust (Kumar et al, 2011). Increased hygroscopicity after wetting is consistent with previous studies on dust-CCN activity that used similar wet generation techniques (e.g., Koehler et al, 2009;Herich et al, 2009).…”

“…Previous studies on dust CCN activity (Koehler et al, 2009;Herich et al, 2009) attributed increased hygroscopicity of the wet generated dust aerosol to the presence of soluble salts on dust particles. Kumar et al (2009a) raised doubts to this based on the slope of the s c − D dry relationship.…”

“…8662 P. Kumar et al: Cloud condensation nuclei activity and droplet activation kinetics Two classes of theory have been proposed to describe the CCN activity of mineral aerosol: Köhler theory (KT) (Köhler, 1936), where hygroscopicity is driven by the amount of solute in the dust, and, by more recent FHH (Frenkel, Halsey, and Hill) adsorption activation theory (AT) (Sorjamaa and Laaksonen, 2007;Kumar et al, 2009a, b) that explicitly considers the effect of water vapor adsorption on the dust surface. Almost all published experimental studies on CCN activity of fresh dust (e.g., Koehler et al, 2009;Herich et al, 2009) parameterize laboratory observations using the KT framework on the assumption that dust CCN activity is controlled solely by the amount of soluble salts in the mineral aerosol, with the insoluble fraction not affecting water activity. However, studies that measured soluble ions composition on dust samples collected from various dust source regions (e.g., Song et al, 2005;Radhi et al, 2010) indicated negligible amounts of soluble salts.…”

“…Recent laboratory measurements of CCN activity of dust and calcium minerals (Hatch et al, 2008;Koehler et al, 2009;Herich et al, 2009;Sullivan et al, 2009), subsaturated hygroscopicity measurements (Gustafsson et al, 2005;Vlasenko et al, 2005;Herich et al, 2009), size distributions and chemical reactivity (Hudson et al, 2008;Gibson et al, 2006), and ice cloud particle nucleation (e.g., Koehler et al, 2009) have utilized the well-established technique of generating aerosol via atomization from an aqueous dust suspension. Recently, Sullivan et al (2010) showed that wet atomization of calcium minerals with considerably low solubility in water (typical of mineral aerosol composition) can induce artifacts in the dust properties to the point where they may not represent dust aerosols in the atmosphere.…”

Enhanced ice nucleation efficiency of microcline immersed in dilute NH<sub>3</sub> and NH<sub>4</sub><sup>+</sup>-containing solutions

“…Such chemical coatings may change the particle surface and increase the hygroscopicity of the mineral dust core, e.g. making it easier for the particles to take up water and form cloud droplets (Levin et al, 1996;Herich et al, 2009). Since dust particles are an ensemble of various minerals with potentially different IN efficiencies, the aging process and physico-chemical surface modifications add another complication in resolving their IN efficiency (Prospero, 1999;Sassen et al, 2003;DeMott et al, 2003a).…”