Reactive uptake of Isoprene-derived epoxydiols to submicron aerosol particles: implications for IEPOX lifetime and SOA formation

The reactive uptake of isoprene-derived epoxydiols (IEPOX) is thought to be a significant source of atmospheric secondary organic aerosol (SOA). However, the IEPOX reaction probability (γ_IEPOX) and its dependence upon particle composition remain poorly constrained. We report measurements of γ_IEPOX for trans-b-IEPOX, the predominant IEPOX isomer, on submicron particles as a function of composition, acidity, and relative humidity (RH). Particle acidity had the strongest effect. γ_IEPOX is more than 500 times larger on ammonium bisulfate (γ ~ 0.05) than on ammonium sulfate (γ ≤ 1 x 10^-4). We could accurately predict γ_IEPOX using an acid-catalyzed, epoxide ring-opening mechanism and a high Henry’s law coefficient (1.6 x 10⁸ M/atm). Suppression of γ_IEPOX was observed in particles containing both ammonium bisulfate and polyethylene glycol (PEG-300), likely due to diffusion and solubility limitations within a PEG-300 coating, suggesting that IEPOX uptake could be self-limiting. Using the measured uptake kinetics, the predicted atmospheric lifetime of IEPOX is a few hours in the presence of highly acidic particles (pH < 0), but is greater than a day on less acidic particles (pH > 3). We connect these net reactive uptake measurements to chamber studies of the SOA yield from IEPOX multiphase chemistry and discuss the implications of these findings for modeling the anthropogenic influence upon SOA formation from isoprene.