Abstract
Abstract. Atmospheric aerosol particles are often partially or completely composed of inorganic salts, such as ammonium sulfate and sodium chloride, and therefore exhibit hygroscopic properties. Many inorganic salts have well-defined deliquescence and efflorescence points at which they take up and lose water, respectively. Field measurements have shown that atmospheric aerosols are not typically pure inorganic salt, instead, they often also contain organic species. There is ample evidence from laboratory studies that suggests that mixed particles exist in a phase-separated state, with an aqueous inorganic core and organic shell. Although phase separation has not been measured in situ, there is no reason it would not also take place in the atmosphere. Here, we investigate the deliquescence and efflorescence points, phase separation and ability to exchange gas-phase components of mixed organic and inorganic aerosol using a flow tube coupled with FTIR (Fourier transform infrared) spectroscopy. Ammonium sulfate aerosol mixed with organic polyols with different O : C ratios, including 1,4-butanediol, glycerol, 1,2,6-hexanetriol, 1,2-hexanediol, and 1,5-pentanediol have been investigated. Those constituents correspond to materials found in the atmosphere in great abundance and, therefore, particles prepared in this study should mimic atmospheric mixed-phase aerosol particles. Some results of this study tend to be in agreement with previous microscopy experiments, but others, such as phase separation properties of 1,2,6-hexanetriol, do not agree with previous work. Because the particles studied in this experiment are of a smaller size than those used in microscopy studies, the discrepancies found could be a size-related effect.
Highlights
Tropospheric aerosol particles containing organics can exist as solids or liquids with a range of viscosities, depending on temperature and relative humidity conditions
Initial experiments were conducted on the binary (NH4)2SO4/water system to determine deliquescence RH (DRH) and efflorescence RH (ERH) for comparison to the literature
Liquid–liquid phase separation is expected to occur in these systems if the O : C ratio of the organic is below ∼ 0.7 (Bertram et al, 2011; You et al, 2013; Song et al, 2012)
Summary
Tropospheric aerosol particles containing organics can exist as solids or liquids with a range of viscosities, depending on temperature and relative humidity conditions. In the case of inorganic aerosol, droplets form when crystalline aerosol is exposed to humidified air For pure salts, such as sodium chloride or ammonium sulfate, the water uptake starts rapidly within a very narrow range of relative humidity (RH). Many microscopy techniques require depositing particles on a hydrophobic slide, which represents a possible surface for heterogeneous phase transition (Liu et al, 2008; Bertram et al, 2011) Levitation techniques, such as electrodynamic balance, acoustic suspension and light pressure suspension coupled with spectroscopy are well suited for studying condensation and freezing events on single particles but not the properties of a multi-particle flow (Zhao et al, 2008; Parsons et al, 2006). The (NH4)2SO4/glycerol system in this study is the only system expected not to undergo phase separation based on its O : C ratio (Bertram et al, 2011; Song et al, 2012)
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