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Published September 16, 2002 | Published
Journal Article Open

Secondary organic aerosol 2. Thermodynamic model for gas/particle partitioning of molecular constituents


A model that predicts secondary organic aerosol (SOA) formation based on the thermodynamic equilibrium partitioning of secondary organic oxidation products has been developed for implementation into atmospheric models. Hydrophobic secondary products are assumed to partition to an absorbing organic aerosol consisting of primary organic aerosol (POA) and other secondary hydrophobic organics according to an equilibrium partitioning coefficient calculated iteratively for each secondary compound present. The hydrophobic module is evaluated by studying the partitioning of octadecanoic acid to surrogate POA species. As expected, the amount of octadecanoic acid predicted to be present in the aerosol phase increases as the total amount of absorbing material increases or as the total amount of acid present increases. Hydrophilic secondary compounds partition to an aqueous phase via Henry's law; the fraction of each compound's mass that partitions is determined by its Henry's law constant and its acid dissociation constant(s). The available liquid water content (LWC) of the aerosol is determined iteratively between an inorganic aerosol module and the hydrophilic module, which is evaluated by studying the partitioning of glyoxalic and malic acids. While glyoxalic acid tends to remain in the gas phase, malic acid partitions strongly to the aqueous phase, with ions being the dominant form in the aqueous phase. As expected, an increase in relative humidity increases the amount of water associated with the organics (ΔLWC), and a lower aerosol pH favors molecular solutes over ionized forms. Increasing pH results in higher effective Henry's law constants for the acids, yielding higher organic aerosol concentrations. Results also indicate that increasing ΔLWC induces additional partitioning of inorganics to the aqueous phase.

Additional Information

Copyright 2002 by the American Geophysical Union. Received 23 February 2001; revised 15 January 2002; accepted 18 January 2002; published 11 September 2002. This work was funded by the United States Environmental Protection Agency under grant R826371-01, by the State of California Air Resources Board under contract 98-314, and by the Electric Power Research Institute Contract WO 8221-01. Although the research described in this article has been funded in part by the U. S. Environmental Protection Agency's STAR program through grant R826371-01, it has not been subjected to any EPA review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.

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