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Bimodal Size Distribution in Secondary Organic Aerosol Formed via Sequential Ozone and Nitrate Oxidation of Catechol
LAUREN A. GAROFALO, Matson A. Pothier, Carley D. Fredrickson, Brett Palm, Joel A. Thornton, John Orlando, Xuan Zhang, Delphine K. Farmer, Colorado State University
Abstract Number: 515
Working Group: Aerosol Chemistry
Abstract
The condensation of secondary organic aerosol (SOA) onto existing aerosol has profound effects not only on the chemical composition, but also on the size distribution, of the aerosol. Changes in chemical and physical properties can, in turn, alter how the aerosol interacts with the atmosphere including radiative and air quality effects. Catechol, an oxygenated aromatic that is directly emitted from wildfires as well as produced in wildfire smoke undergoes oxidation reactions with O3, OH and NO3 radical to form SOA; oxidation via NO3 or OH in the presence of NOx yields nitrocatechol – a nitrogen-containing chromophore that can drastically change the optical properties of the aerosol. In the chamber study reported here, we investigate the SOA production of catechol with various oxidants in the presence of ammonium sulfate seed. In the experiment in which catechol was first exposed to ozone and subsequently to NO3, we observe a bimodal size distribution where an organic aerosol (OA) mode appears independent of the size mode for the existing OA from ozone oxidation and that of the seed aerosol. This behavior was only observed for this sequential oxidation scheme and not for oxidation of catechol by OH/NOx. We describe possible quasi-equilibrium and non-equilibrium effects that may cause a size distribution to diverge from the size distribution expected by equilibrium growth.