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

Abstract

The atmospheric oxidation of isoprene by OH radicals under low NO x 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 NO x 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, R 2 = 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.

Role of Hydroperoxyl Radicals in Heterogeneous Oxidation of Oxygenated Organic Aerosols

“…The relative abundance of ROOH for these OA surrogates ranges from 0.37 ± 0.02 to 10.37 ± 1.10% under [HO 2 • ]/[ • OH] < 1 but is enhanced to 1.83 ± 0.06 to 24.95 ± 0.53% under [HO 2 • ]/[ • OH] ∼ 40. It should also be emphasized that ROOH are known to be thermally labile and could undergo decomposition during thermal desorption in TD-CIMS. − To provide some quantitative constraint of the thermal decomposition, we generated a well-known gas-phase ROOH, isoprene hydroxy hydroperoxide (ISOPOOH, C 5 H 10 O 3 ) ,− in the FTR from isoprene oxidation, which is the dominant product detected at m/Q 245 Th by I – -CIMS . The isoprene oxidation products including ISOPOOH were sampled by the TD-CIMS under room temperature and 180 °C (the same operation temperature for the oxidized OA model systems).…”

Observation-Constrained Kinetic Modelling of Isoprene SOA Formation in the Atmosphere

“…(2) isomerization reactions for the major RO2 based on recent studies (D'ambro et al, 2017;Wennberg et al, 2018;Vereecken et al, 2021;Mettke et al, 2023); and (3) dimer formation from several RO2 + RO2 reactions that were supported by prior chamber experiments (Ng et al, 2008;Mettke et al, 2023). This mechanism was implemented into F0AM-WAM to simulate published chamber experimental data under different conditions.…”

“…1) than that used in the Caltech mechanism (Wennberg et al, 2018) and other models (D'ambro et al, 2017;Thornton et al, 2020). Recent work by Mettke et al (2023) suggested that the ISOPOOHOO isomerization is ~ 0.002 s -1 , rather than the order of 0.1 s -1 reported by D'ambro et al ( 2017) and Wennberg et al (2018). Thus, in the UCR-ISOP mechanism, we chose to use a rate coefficient 10 times slower than the Caltech mechanism (i.e., 0.01 s -1 ), which is between the two very different suggested rate coefficients.…”

“…This change greatly affects the yields of IDHPE vs. IDHDP. In addition, we consider the rapid ISOPOOHOO self-reaction to form the corresponding carbonyl (C5H10O6), alcohol (C5H12O6), and dimer (C10H22O10), suggested by Mettke et al (2023), with a rate coefficient of 1×10 -11 cm 3 molecule -1 s -1 . This dimer formation pathway could partly explain the slightly lower C5-LV simulation using the UCR-ISOP mechanism under high concentrations (Fig.…”

“…A well-studied example is that isoprene hydroxy hydroperoxide (ISOPOOH), the direct precursor of IEPOX, was found to undergo OH-oxidation and form highly oxidized low-volatility products that form SOA (see Fig. 1) (Krechmer et al, 2015;Liu et al, 2016;D'ambro et al, 2017;Mettke et al, 2023). In addition, N-containing multifunctional low-volatility species (C5-NLV) from chamber and field measurements 70 suggests that further oxidation of isoprene nitrates could be another SOA source (Lee et al, 2016;Schwantes et al, 2019).…”