10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
The Impact of Multiphase Chemistry on Nanoparticle Growth and Composition
MICHAEL J. APSOKARDU, Murray Johnston, University of Delaware
Abstract Number: 570 Working Group: Aerosol Chemistry
Abstract Carbonaceous matter is responsible for a substantial fraction of the growth of nanoparticles in the atmosphere to a climatically relevant size. Highly oxidized organic molecules (HOMs) formed in the gas phase by oxidation of a precursor molecule will grow pre-existing particles when they partition to the particle phase. Non-volatile molecules grow particles at the condensation rate since re-evaporation is negligible. Semi-volatile molecules grow particles at a rate slower than the condensation rate since re-evaporation is significant. Chemical reactions in the particle phase that convert semi-volatile reactants into non-volatile products can increase the growth rate because additional semi-volatile molecules must flow from the gas phase to the particle phase in order to replace the molecules lost through reaction. This combination of gas-particle partitioning with subsequent reaction in the particle phase is termed multiphase chemistry. The impact of multiphase chemistry on particle growth depends on the mixing ratios and volatility distributions of HOMs in combination with their particle phase reaction rates. We have developed a kinetic model for nanoparticle growth that is able to explore these factors, and we have used it to interpret several types of experiments in flow tube reactors. For example, we find that the large fractions of oligomeric species detected by ESI-HRMS in SOA originating from OH oxidation of decamethylcyclopentasiloxane (D5) and ozone oxidation of monoterpenes require particle phase reaction rate constants in the 10-3 to 10-1 M-1s-1 range. Rate constants of this magnitude are consistent with those measured for reactions of hydroperoxides and/or peroxyacids with carbonyls to form oligomers. Most recently, we have compared experimentally measured growth rates of monodisperse seed particles to those predicted from model calculations. Calculations based on condensation of non-volatile HOMs produced in the flow tube reactor are generally able to reproduce the experimental results, though they have a tendency to under predict growth rates in some cases, suggesting that multiphase chemistry can contribute to particle growth. By systematically comparing experimental measurements with model calculations, we are able to predict the situations where multiphase chemistry is most likely to enhance nanoparticle growth in the atmosphere.