Effect of Atmospherically-Relevant Organic Coatings, Humidity, and Aerosol Acidity on Multiphase Chemistry of Isoprene Epoxydiols

Multiphase chemistry of isomeric isoprene epoxydiols (IEPOX) has been shown to be the dominant source of isoprene-derived secondary organic aerosol (SOA). Recent studies have reported particles composed of ammonium bisulfate (ABS) mixed with model organics exhibit slower rates of IEPOX uptake compared to pure ABS particles. We will present the results of a study, in which we investigated, for the first time, the effect of atmospherically-relevant organic coatings of α-pinene SOA on the reactive uptake of trans-β-IEPOX onto ABS particles under different conditions and coating thicknesses. Single particle mass spectrometry was used to characterize in real-time particle size, shape, density, and quantitative composition before and after reaction with IEPOX. We demonstrate that IEPOX uptake by pure ABS and deliquesced ammonium sulfate particles is a volume-controlled process, which results in particles with uniform concentration of IEPOX-derived SOA across a wide range of sizes. Aerosol acidity was shown to enhance IEPOX-derived SOA formation, consistent with recent studies. We find that the weight fraction of IEPOX-derived SOA for ABS particles is 41% as compared to 31% for ammonium sulfate particles. The presence of water has a weaker impact on IEPOX-derived SOA yield for ABS particles, but significantly enhanced formation of 2-methyltetrols, consistent with offline filter analysis. In contrast, we show that presence of α-pinene SOA coatings greatly impact IEPOX heterogeneous chemistry. IEPOX uptake by ABS particles coated with α-pinene SOA is lower compared to that of pure ABS particles, and strongly dependent on particle composition, and therefore on particle size. For example, in the experiment with thinly (~18 nm) coated ABS particles, the weight fraction of IEPOX-derived SOA increases rapidly from 0 to 30% with respect to ABS seed, as the seed aerosol diameter increases from 60 to 100 nm, and then remains nearly constant at ~30% for diameters > 100 nm.

In addition, we will present the results of volatility measurements of IEPOX-derived SOA and compare it to the volatility of SOA formed by photo oxidation of isoprene.