Deliquescence, efflorescence, and phase miscibility of mixed particles of aqueous ammonium sulfate and isoprene-derived secondary organic material

Description

The hygroscopic phase transitions of particles composed of laboratory-generated secondary organic material and ammonium sulfate were investigated using a dual-arm tandem differential mobility analyzer. Organic material was generated via isoprene photooxidation at organic mass concentrations of 20 to 30 μg m-3 and oxygen-to-carbon ratios of 0.67 to 0.74. We show that the organic material produced by isoprene photooxidation exerts a measurable influence on the hygroscopic properties of ammonium sulfate. For particles of high organic volume fraction, both the efflorescence and deliquescence relative humidity (ERH and DRH, respectively) of mixed particles decreased by greater than 20% relative humidity compared to pure ammonium sulfate values. The partial dissolution of ammonium sulfate was also induced at relative humidities less than the final DRH. The implication is that isoprene photooxidation products are miscible with the aqueous inorganic phase and are therefore able to alter the hygroscopic behavior of ammonium sulfate. This implication is consistent with a parameterization predicting phase separation as a function of organic material oxygen-to-carbon ratio. In the context of previous work, these results show that the influence of secondary organic material on the hygroscopic properties of ammonium sulfate varies with organic composition and confirm that the degree of oxygenation of the organic material, including complex organic materials, is an important variable influencing the hygroscopic properties of mixed organic-inorganic particles.