Growth Rate Dependence of Secondary Organic Aerosol on Seed Particle Size, Composition, and Phase

DEVON HIGGINS, Michael S. Taylor, Justin Krasnomowitz, Murray Johnston, University of Delaware

     Abstract Number: 354
     Working Group: Aerosol Chemistry

Abstract
Atmospheric nanoparticle growth, which is a key process associated with formation of cloud condensation nuclei (CCN), is thought to be dominated by the condensation of condensable organic vapor (COV), as produced in autooxidation reactions when a biogenic precursor is oxidized. However, it has been recently established by our group that seed particle size, phase, and composition affect particle growth in the 40-100 nm size range. When seed particles composed of ammonium sulfate or freshly generated a-pinene SOA were exposed to a-pinene and ozone, they were found to grow by different amounts, which varied with aerosol liquid water (ALW) content. Seed particle growth is represented by the growth yield (GY), which is defined as the fraction of ⍺-pinene molecules that react with ozone to give a product that grows the particle. Overall, SOA seed particles gave the lowest growth yield, which corresponded to the yield of nonvolatile, highly oxidized molecules produced by autooxidation that are able to condensationally grow the particles. Effloresced ammonium sulfate particles gave somewhat higher growth yields and increased with increasing relative humidity, indicating that reactions of semivolatile molecules at the particle surface can contribute to particle growth. Deliquesced ammonium sulfate particles gave the highest growth yields, indicating that reactions of semivolatile molecules within the particle volume can contribute to particle growth. The growth yield for effloresced ammonium sulfate particles was independent of particle size, while liquid-like seed particles (SOA and deliquesced ammonium sulfate) gave somewhat higher growth yields for largest particles studied. These size dependencies suggest that particles grow by both surface- and volume- limited kinetics. The particle growth results will be presented alongside molecular composition measurements via mass spectrometry to discuss the reaction mechanisms responsible for growth under the various conditions studied.