Investigation of the hygroscopic properties of Ca(NO<sub>3</sub>)<sub>2</sub> and internally mixed Ca(NO<sub>3</sub>)<sub>2</sub>/CaCO<sub>3</sub> particles by micro-Raman spectrometry

Abstract

Abstract. To understand and predict the role of mineral aerosol particles processed by reactive nitrogen species in the atmosphere, the hygroscopic properties of both Ca(NO3)2 and Ca(NO3)2-containing mineral particles must be well understood. Using a micro-Raman system, the dehydration and hydration processes of micro-sized individual Ca(NO3)2 and internally mixed Ca(NO3)2/CaCO3 particles were investigated systematically. In addition to accurate quantification of the dependence of water content on relative humidity (RH), significant new spectroscopic evidence related to chemical structure was also obtained to confirm the occurrence of amorphous solid state and to better understand the phase transition process. The Ca(NO3)2 particles exhibit reversible behavior in the dehydration and hydration processes; they are in the state of solution droplets above 10% RH and amorphous hydrates below 7% RH, and phase transition occurs at 7–10% RH. The hygroscopic behavior of Ca(NO3)2/CaCO3 particles is identical to that of pure Ca(NO3)2 particles, suggesting a negligible effect of the inclusion of slightly soluble CaCO3.

Amorphous and crystalline aerosol particles interacting with water vapor: conceptual framework and experimental evidence for restructuring, phase transitions and kinetic limitations

“…Krueger et al (2003) found similar deliquescence behavior of Ca(NO 3 ) 2 formed by heterogeneous reaction of CaCO 3 particles with gaseous nitric acid. By Raman-microscopy Liu et al (2008) observed that pure and mixed Ca(NO 3 ) 2 and Ca(NO 3 ) 2 /CaCO 3 microparticles exhibit reversible water uptake and form amorphous hydrates below 7% RH. Rosenoern et al (2008) suggested that the hygroscopic properties of nanocrystalline aerosol particles may also deviate from those of crystalline bulk material, and several studies pointed out that the drying conditions have a strong influence on the morphology of solid aerosol particles (e.g., Mikhailov et al, 2004;Biskos et al, 2006;Rose et al, 2008;Rosenoern et al, 2008).…”

“…Mechanistically, the ability for continuous and reversible water uptake has been attributed to the presence of residual water in non-effloresced, (quasi-)liquid particles Prenni et al, 2003;Mochida and Kawamura, 2004;Moore and Raymond, 2008;Zardini et al, 2008) or to the morphology or amorphous state of (quasi-)solid particles (Cohen et al, 1987;Onasch et al, 2000;Marcolli et al, 2004;Liu et al, 2008).…”

“…For example, aerosol particles composed of certain watersoluble inorganic salts can reversibly take up water vapor without the prompt deliquescence and efflorescence phase transitions and hysteresis effects typically observed for the corresponding crystalline bulk material: CaCl 2 and MnSO 4 (Cohen at al., 1987); NH 4 NO 3 (Tang, 1980;Dougle et al, 1998;Cziczo and Abbatt, 2000;Lee and Hsu, 2000;Mikhailov et al, 2004;Schlenker et al, 2004;Schlenker and Martin, 2005;Svenningson et al, 2006), NH 4 HSO 4 (Tang and Munkelwitz, 1994;Cziczo et al, 1997;Cziczo and Abbatt, 2000;Lee and Hsu, 2000;Martin et al, 2003;Colberg et al, 2003Colberg et al, , 2004Schlenker et al, 2004;Schlenker and Martin, 2005), NaNO 3 (McInnes et al, 1996;Hsu, 2000, Gysel et al, 2002;Hoffmann et al, 2004), Ca(NO 3 ) 2 (Liu et al, 2008), and sea salt .…”

Water Content and Phase Transitions in Particles of Inorganic and Organic Species and their Mixtures Using Micro-Raman Spectroscopy

“…More recently, micro-Raman spectroscopic systems have been employed to investigate the molecular interactions in supersaturated droplets of different inorganic species (Wang et al 2005;Li et al 2006;Dong et al 2007) and the polymorphic transformation of NH 4 NO 3 particles deposited on a hydrophobic substrate (Wu and Chan 2008). Such micro-Raman systems have also been used to study hygroscopicity and phase transitions of Ca(NO 3 ) 2 and Ca(NO 3 ) 2 /CaCO 3 (Liu et al 2008a), and ammonium sulfate-adipic acid mixtures (Yeung et al 2009). Moreover, spatial spectroscopic imagining has proved useful for the phase and chemical characterization of heterogeneous particles (Liu et al 2008a;Yeung et al 2009).…”

“…Such micro-Raman systems have also been used to study hygroscopicity and phase transitions of Ca(NO 3 ) 2 and Ca(NO 3 ) 2 /CaCO 3 (Liu et al 2008a), and ammonium sulfate-adipic acid mixtures (Yeung et al 2009). Moreover, spatial spectroscopic imagining has proved useful for the phase and chemical characterization of heterogeneous particles (Liu et al 2008a;Yeung et al 2009). A similar technique that utilizes micro-FTIR to study the hygroscopic behavior of pure inorganic and sea-salt particles has also been reported (Liu et al 2008b).…”