Measuring and Modeling Ultrafine Particle Growth by Isoprene Ozonolysis Secondary Organic Aerosol

MICHAEL S. TAYLOR, Murray Johnston, University of Delaware

     Abstract Number: 402
     Working Group: Aerosol Chemistry

Abstract
Through condensation and partitioning of oxidation products, biogenic volatile organic compounds (BVOC) can contribute significantly to the growth of particles in the Aitken mode size range (10-100 nm), which often represent the largest number fractions in ambient air. Understanding the mechanisms across conditions found in the environment allow for cloud condensation nuclei (CCN) to be better predicted. Monoterpenes such as α-pinene have been shown to produce a significant number of low volatility organic compounds via gas phase oxidation reactions. These products vary significantly in terms of structure, volatility, and yield, altering the SOA composition and rate of particle growth. Recent work in our group has shown seed particle size, phase, and interfacial water on the growth rate of ammonium sulfate seed particles, both through theoretical simulations and experimental measurements (Taylor Jr. et al. 2022, Higgins et. al. 2022). These studies introduce the term “growth yield” (GY), which is a measure of the molar fraction of BVOC ozonolysis products that are taken into the particle, causing it to grow. For α-pinene ozonolysis products specifically, growth on ammonium sulfate seed particles is enhanced by surface water (effloresced particles exposed to increasing RH) and especially by aerosol liquid water (deliquesced particles). Isoprene, an acyclic alkene, is known to produce a substantially lower yield of low volatility products when reacting with ozone, and therefore, the growth yield is expected to be reduced when compared to that of α-pinene. However, higher volatility products of isoprene ozonolysis can produce a significant amount of SOA from aqueous phase reactions. Here, we present results on the growth of ammonium sulfate seed particles by isoprene ozonolysis under low (10%) and high (60%) relative humidity as well as effloresced vs. deliquesced seed phases. Additionally, the effects of seed acidity are examined to elucidate how this effects growth from particle phase reactions. SMPS measurements and corresponding growth yield values will be presented and discussed.