10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Measurements of Acid and Organic Partitioning between Phase-separated SOA/Aqueous Phases
BENJAMIN DEMING, Paul Ziemann, University of Colorado
Abstract Number: 1503 Working Group: Aerosol Chemistry
Abstract Measurements of aerosol pH vary, but they are typically very acidic, with values as low as 1 not being uncommon. This has important implications for the composition of organic aerosol, which can be influenced by particle-phase reactions involving acid catalysis. For example, alcohols may be converted to alkenes, which are highly reactive towards ozonolysis, and oligomers can be formed through accretion reactions. However, many aerosols are phase-separated, with distinct organic and aqueous portions. There is therefore some question as to the amount of acid an organic species may be exposed to. The purpose of this work is to determine the distribution of acid and SOA components within a model phase-separated system. To this end, we have generated SOA via the reaction of α-pinene with ozone in an environmental chamber and collected filter samples for offline analyses. This reaction has been well characterized by our group and others, and has the added benefit of allowing for the easy formation of milligram quantities of SOA. Partitioning of the SOA was measured directly by mixing it with a chosen aqueous phase and optionally additional organic material, separating the two phases, evaporating the solvents, and weighing with a microbalance. The phase preferences for compounds containing carboxyl, carbonyl, hydroxyl, peroxide, and ester groups was also determined using derivatization-spectrophotometric methods we developed previously for microscale analysis of complex organic aerosol. The partitioning of acid between the two phases was measured by acidifying the two-phase model system, separating the two phases, and precipitating the acid out of the organic phase by the addition of ammonia. The solvent was then evaporated and the residual ammonium sulfate salt weighed using a sensitive microbalance. The organic phases used included SOA formed from α-pinene ozonolysis, and a suite of commercially available solvents that included 2-butanone, ethyl acetate, and propylene carbonate, which span the O/C ratios commonly measured for atmospheric aerosols. The aqueous phase consisted of either pure water or an ammonium sulfate solution, also typical of ambient aerosols. These measurements were then used to derive distribution coefficients for both acid and SOA in phase-separated aerosol. The results from this work will have important implications for the viability of acid-catalyzed mechanisms in models of SOA production.