A comprehensive evaluation of water uptake on atmospherically relevant mineral surfaces: DRIFT spectroscopy, thermogravimetric analysis and aerosol growth measurements

Abstract

Abstract. The hygroscopicity of mineral aerosol samples has been examined by three independent methods: diffuse reflectance infrared Fourier transform spectroscopy, thermogravimetric analysis and differential mobility analysis. All three methods allow an evaluation of the water coverage of two samples, CaCO 3 and Arizona Test dust, as a function of relative humidity. For the first time, a correlation between absolute gravimetric measurements and the other two (indirect) methods has been established. Water uptake isotherms were reliably determined for both solids which at 298 K and 80% relative humidity exhibited similar coverages of ∼4 monolayers. However, the behaviour at low relative humidity was markedly different in the two cases, with Arizona Test Dust showing a substantially higher affinity for water in the contact layer. This is understandable in terms of the chemical composition of these two materials. The mobility analysis results are in good accord with field observations and with our own spectroscopic and gravimetric measurements. These findings are of value for an understanding of atmospheric chemical processes.

Heterogeneous reaction of N<sub>2</sub>O<sub>5</sub> with illite and Arizona test dust particles

“…Indeed, while CaCO 3 is the least reactive at low humidity, the uptake coefficient of N 2 O 5 at RH close to 70 % is similar for both CaCO 3 and Saharan dust, and only slightly lower than for illite (which displays the opposite trend with RH as described above). The significant increase of γ (N 2 O 5 ) onto CaCO 3 at high RH bears resemblance to its water adsorption isotherm (Gustafsson et al, 2005) and is likely to be related to the formation of more reactive Ca(OH)(CO 3 H) on the surface at higher RH (Al-Hosney et al, 2005).…”

“…The lower uptake coefficients may be related to the mineral composition of ATD which consists mainly of feldspar and quartz (Broadley et al, 2012), which may have less (and/or less reactive) surface OH groups. The weak dependence on RH is probably related to the fact that the hygroscopic growth of ATD particles is very small (Gustafsson et al, 2005;Vlasenko et al, 2005) and therefore even at high RH, the amount of adsorbed water on the surface does not contribute significantly to N 2 O 5 solvation/ionization but may still result in deactivation of surface OH groups as shown by Goodman et al (2001). Surface sensitive methods would be required to confirm this postulate.…”

Heterogeneous reaction of HO<sub>2</sub> with airborne TiO<sub>2</sub> particles and its implication for climate change mitigation strategies

“…For ATD, γ (HO 2 ) was only determined at four values of RH, but a general increase with RH was observed (although there was a dip around RH = 50 % before a further increase), consistent with HO 2 uptake being driven by the number of defects in the crystal lattice of mineral dust surfaces, which provides bridging OH groups upon exposure to water and subsequently by the number of monolayers of water adsorbed onto the surface of such aerosols. The number of monolayers of water on the ATD surface at different RH has been determined (Gustafsson et al, 2005) and also showed a general increase with RH but with a shoulder around RH = 50 %, where the observed γ (HO 2 ) also contained a small dip.…”

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

“…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.…”