On mineral dust aerosol hygroscopicity

Abstract

Abstract. Despite its importance, hygroscopicity of mineral dust aerosol remains highly uncertain. In this work, we investigated water adsorption and hygroscopicity of different mineral dust samples at 25 °C, via measuring sample mass at different relative humidity (RH, up to 90 %) using a very sensitive balance. Mineral dust samples examined (twenty one in total) included seven authentic mineral dust samples from different regions in the world and fourteen major minerals contained in mineral dust aerosol. At 90 % RH, mass ratios of adsorbed water to the dry mineral ranged from 0.0011 to 0.3080, largely depending on the BET surface areas of mineral dust samples. The surface coverages of adsorbed water were determined to vary between 1.26 and 8.63 at 90 % RH, and it was found that the Frenkel-Halsey-Hill (FHH) adsorption isotherm could well describe surface coverages of adsorbed water as a function of RH, with AFHH and BFHH parameters in the range of 0.15–4.39 and 1.10–1.91, respectively. The comprehensive and robust data obtained would largely improve our knowledge of hygroscopicity of mineral dust aerosol.

Heterogeneous Reaction of CaCO3 With NO2 at Different Relative Humidities: Kinetics, Mechanisms, and Impacts on Aerosol Hygroscopicity

“…For reacted CaCO 3 shown in Figure 4b, m w / m 0 was measured to be ∼0.052 at 20% RH, ∼0.159 at 60% RH and 0.573 at 90% RH. In comparison, m w / m 0 was determined to be <0.002 at 90% RH for unreacted CaCO 3 in our previous work (Chen et al., 2020), suggesting heterogeneous reaction of CaCO 3 with NO 2 at 40% RH led to significant increase in its hygroscopicity. As also revealed by Figure 4, reacted CaCO 3 particles started to take up substantial amount of water even at 20% RH, while the deliquescence RH of Ca(NO 3 ) 2 ∙4H 2 O is ∼50% (Guo et al., 2019; Kelly & Wexler, 2005).…”

“…CaCO 3 (>99.5% Alfa Aesar) particles used in our work have a BET surface area of 2.18 ± 0.01 m 2 /g and an average diameter of 3.12 ± 0.56 μm (Chen et al., 2020). The particle‐loaded filters used for heterogeneous reactions were prepared according to the procedure used in our previous work (R. Li et al., 2020): (a) approximately 250 mg CaCO 3 particles were transferred into 500 mL HPLC‐grade ethanol, and the CaCO 3 /ethanol mixture (0.5 g/L) was stirred using a magnetic stirrer to keep it homogeneous; (b) 10 mL aqueous mixture was transferred onto a PTFE filter (47 mm in diameter, Whatman, USA) to form a particle film on the filter after ethanol was evaporated; (c) the mass of CaCO 3 particles on individual filters prepared was determined to be 5.0 ± 1.0 mg, via measuring the mass of filters before and after CaCO 3 particles were loaded.…”

“…When the mass change of the sample was <0.05% in 60 min, we believed that an equilibrium was reached and RH was then changed to the next step. The VSA instrument was routinely calibrated (Chen et al., 2020; Gu et al., 2017), and all the measurements were carried out at 25°C.…”

“…The first part (after VSA measurements) and the second part of the filter were separately immersed in 10 mL deionized water, and the mixtures were stirred for 2 h using an oscillating table to extract water soluble inorganic ions (Chen et al., 2020; R. Li et al., 2020). Afterward, 0.22 μm polyethersulfone filters (Anpel, Shanghai, China) were used to filter these extracts, and the resulting solutions were then analyzed using ion chromatography (R. Li et al., 2020) to quantify the amount of nitrate formed.…”

“…The hygroscopicity of unreacted CaCO 3 is very low (Chen et al., 2020; Gustafsson et al., 2005; Ma et al., 2012; Sullivan et al., 2009a). Heterogeneous reactions of CaCO 3 with acidic trace gases produce more soluble and hygroscopic specie (for example, Ca(NO 3 ) 2 ) (Gibson et al., 2006; Guo et al., 2019; Sullivan et al., 2009a; M. J. Tang et al., 2016).…”