Uptake and release of gaseous species accompanying the reactions of isoprene photo-oxidation products with sulfate particles.

Gaseous species produced via the HO2 reaction pathways of isoprene photo-oxidation were reacted with liquid, partially neutralized sulfate aerosol particles at 293 ± 1 K and <5% relative humidity. Isoprene-derived epoxydiols (IEPOX) were taken up for all neutralizations so long as the liquid phase was maintained. By comparison, isoprene-derived hydroperoxides (ISOPOOH) were taken up only for low neutralization (i.e., high acidity). The release of product molecules to the gas phase increased for low neutralization, corresponding to the release of at least 60 product molecules for the uptake of 100 reactant molecules at the lowest neutralization. A major reaction pathway was hydroperoxide cleavage in the particle phase to produce volatile products. Product species larger than the C5 chain of isoprene were also released to the gas phase, implying that some accretion products in the particle phase were sufficiently volatile to partition to the gas phase. The study results show that the dependence of reactive uptake on neutralization varies by species. Furthermore, in addition to functionalization and accretion, decomposition and re-volatilization should be considered in mass balance formulations of reactive uptake by atmospheric particles.