Mixing of Atmospheric Organic Aerosol Components: Evidence from High-Resolution Aerosol Mass Spectrometry

LEA HILDEBRANDT (1), Kaytlin Henry (1), Jesse H. Kroll (2), Spyros N. Pandis (1,3), Neil M. Donahue (1)

(1) Carnegie Mellon University, Pittsburgh, USA (2) Massachusetts Institute of Technology, Cambridge, USA (3) University of Patras, Patra, Greece

     Abstract Number: 421
     Last modified: May 12, 2010

Abstract
According to the pseudo-ideal mixing assumption employed in practically all chemical transport models, organic aerosol components from different sources interact with each other in a single solution, independent of the organic aerosol composition. This is a critical assumption that greatly affects modeled organic aerosol concentrations, but it has not been confirmed experimentally. A main experimental challenge is that organic aerosols from different sources look very similar when analyzed with an aerosol mass spectrometer, especially if the aerosols have undergone reactions after their initial formation. We developed a new experimental method to overcome this challenge, using isotopically labeled compounds ($^(13)C or D) and a High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-AMS) from Aerodyne, Inc. We generated one type of organic aerosol in the presence of another, for example isotopically labeled secondary organic aerosol (SOA) from toluene in the presence of unlabeled SOA from alpha-pinene. Using the HR-AMS data, we identified all isotopically labeled organic fragments based on their exact mass and subsequently separated anthropogenic (labeled) and biogenic (unlabeled) SOA using mathematical methods such as positive matrix factorization and chemical mass balance. We evaluated the mixing of the different organic-aerosol types by comparing their aerosol mass yields when formed in these mixtures to their yields when formed in isolation. Our results are consistent with pseudo-ideal mixing of anthropogenic and biogenic SOA components. Next, we will use this powerful new experimental method to evaluate the pseudo-ideal mixing assumption of primary and secondary organic aerosol components.