AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Deliquescence, Efflorescence, and Phase Miscibility of Mixed Particles of Aqueous Ammonium Sulfate and Isoprene-Derived Secondary Organic Material
MACKENZIE SMITH, Allan Bertram, Scot Martin, Harvard University
Abstract Number: 383 Working Group: Aerosol Chemistry
Abstract 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 photo-oxidation at organic mass concentrations of 20 to 30 micro-gram m$^(-3) and oxygen-to-carbon ratios of 0.67 to 0.74. We show that the organic material produced by isoprene photo-oxidation exerts a measurable influence on the hygroscopic properties of ammonium sulfate. Compared to an efflorescence relative humidity (ERH) of 30 to 35% for pure ammonium sulfate, efflorescence was eliminated for mixed aqueous particles having organic volume fractions epsilon of approximately 0.6 and greater. Compared to a deliquescence relative humidity (DRH) of 80% for pure ammonium sulfate, the DRH steadily decreased for increasing epsilon, approaching a DRH of 40% for epsilon of 0.9. Parameterizations of DRH(epsilon) and ERH(epsilon) and a new account of partial dissolution of ammonium sulfate for RH < DRH (i.e., initial deliquescence) are presented for these mixed particles.
These results imply that isoprene photo-oxidation products are miscible with the aqueous inorganic phase and are therefore able to alter the hygroscopic behavior of ammonium sulfate. This phase miscibility is in contrast to the liquid-liquid separation that occurs for some other types of secondary organic material (e.g., terpene dark ozonolysis products). The differences in phase miscibility are 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.