Investigations of the hygroscopic properties of ammonium sulfate and mixed ammonium sulfate and glutaric acid micro droplets by means of optical levitation and Raman spectroscopy

Abstract

In the presented work an optical levitation technique performed by means of a focused laser beam, Mie and Raman spectroscopy have been utilized for measuring hygroscopic growth curves and composition of laboratory generated single ammonium sulfate (AS) and internally mixed ammonium sulfate-glutaric acid (GA) droplets in the micrometer range. The generated particles have been found to immediately supersaturate (above 45wt% for AS) at 297 K after capturing in the laser beam. Further increase of the relative humidity (RH) up to 85% does not dilute the droplets under the saturation point. A spontaneous hygroscopic growth takes place at 73.5-78% RH for pure AS. The particle grows with an average factor of 1.62 at the deliquescence relative humidity (DRH). Efflorescence of AS occurs at 43% RH with a corresponding concentration of more than 85wt%. Independent of the mixing ratios in ranges 25/75, 50/50, 75/25% AS/GA mixed particles don't exist as a metastable supersaturated solution droplets in the 35-85% RH range. Instead of growing with increasing relative humidity internally mixed particles build up a solid crystalline layer on the surface. This crystalline phase is not further influenced by ambient relative humidities.

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

“…The presence of water bands can distinguish between aqueous and dry states. Changes in the Raman shift magnitude during phase transitions of AS and AN have been reported by a few investigators (Musick et al 2000;Zhang and Chan 2002;Jordanov and Zellner 2006). The Raman peaks of solid particles are more intense and sharper relative to aqueous particles, so a sudden change in the fwhh (normalized to that at the lowest RH) of a distinct peak is also useful for phase identification (Lee et al 2008;Ling and Chan 2008).…”

“…An aerosol flow tube coupled with a Fourier transform infrared (FTIR) spectrometer has been used to study phase transitions of suspended particles (Cziczo et al 1997;Onasch et al 1999). In addition, a combined EDB/Raman spectrometer (Fung and Tang 1988) has been demonstrated to provide solid spectral evidence of the phase 270 M. C. YEUNG AND C. K. CHAN state of a particle simultaneously with hygroscopicity measurements (Zhang and Chan 2002;Jordanov and Zellner 2006;Lee et al 2008;Ling and Chan 2008). 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).…”

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

“…Jordanov and Zellner (2006) attempted to quantify the RH dependence of WSR in levitated (NH 4 ) 2 SO 4 particles with Raman spectroscopy, but were unsuccessful because of strong band distortion of NH + 4 and H 2 O caused by the morphology-dependent resonances of spherical droplets. In contrast, the disturbance of morphology-dependent resonances was successfully excluded from our experiment by depositing particles on the substrate, rather than levitating them.…”

A new approach to studying aqueous reactions using diffuse reflectance infrared Fourier transform spectrometry: application to the uptake and oxidation of SO2 on OH-processed model sea salt aerosol

“…23,24 A new technique is optical levitation of single droplets combined with Raman spectroscopy, but this has only been applied to relatively few systems to date. [25][26][27] Single particle mass spectrometry is another promising technique for the study of chemistry on aerosols; but in addition to the instrumental costs, the different approaches used for volatilization and ionization result in variable sensitivities for different components. In addition, extensive fragmentation, particularly of organics, makes speciation difficult or impossible.…”