Secondary Organic Aerosol (SOA) through Uptake of Isoprene Hydroxy Hydroperoxides (ISOPOOH) and its Oxidation Products

The atmospheric oxidation of isoprene by OH radicals under low NOx conditions primarily leads to hydroxy hydroperoxides (ISOPOOH), and further, to isoprene epoxy diols (IEPOX), which have been identified as important SOA precursors. Recent studies indicate that an additional class of highly oxidized ISOPOOH oxidation products might contribute equally to SOA. Nonetheless, kinetic investigations of the phase transfer of ISOPOOH and of its oxidation products are largely missing, resulting in large uncertainties in understanding the respective atmospheric chemistry and its implications. In the present work, the partitioning behavior of synthetic 1,2-ISOPOOH and its OH oxidation products was investigated in chamber experiments with a (NO3–)-CI-APi-ToFMS under low NOx conditions for sulfate seed particles under variation of relative humidity and particle acidity conditions. For acidic sulfate particles, a reactive uptake coefficient of γ(1,2-ISOPOOH)(pH=0) = (9 ± 4) × 10–3 was determined. For monomeric oxidation products, a parametrization of measured uptake coefficients based on estimated vapor pressures was obtained with γ(C5 product) = −1.302 × 10–2 × ln(vap.press.·atm–1) – 0.1662, n = 15, R2 = 0.727. Dimeric RO2 accretion products were observed in the gas phase for the first time. In a model study, an unexpectedly large proportion of these products was used to constrain the rate constant of intramolecular hydrogen shift reactions of C5O6H11 radicals to an upper limit of k = 0.002 s–1. To evaluate the atmospheric relevance, the results of this study were implemented in a remote-case model study performed with F0AM, which resulted in an increased SOA mass formation of 31% by way of the investigated ISOPOOH oxidation and phase transfer pathways.