Abstract
Abstract. In-cloud chemistry has important ramifications for atmospheric particulate matter formation and gas-phase chemistry. Recent work has shown that, like hydrogen peroxide (H2O2), the two main isomers of isoprene hydroxyl hydroperoxide (ISOPOOH) oxidize sulfur dioxide dissolved in cloud droplets (SO2,aq) to sulfate. The work revealed that the pathway of SO2,aq oxidation with ISOPOOH differs from that of H2O2. We investigate the chemical mechanisms of oxidation of SO2,aq with ISOPOOH in the cloud-relevant pH range of 3–6 and compare them with the previously reported mechanisms of oxidation of SO2,aq with H2O2, methyl hydroperoxide and peroxyacetic acid. The organic products of the reaction are identified, and two pathways are proposed. For 1,2-ISOPOOH, a higher yield pathway via proposed radical intermediates yields methyl vinyl ketone (MVK) and formaldehyde, which can react to hydroxymethanesulfonate (HMS) when SO2,aq is present. A lower yield non-fragmentation oxygen addition pathway is proposed that results in the formation of isoprene-derived diols (ISOPOH). Based on global simulations, this mechanism is not a significant pathway for formation of MVK and formaldehyde relative to their gas-phase formation but, as previously reported, it can be regionally important for sulfate production. The study adds to previous work that highlights similarities and differences between gas-phase and cloud-droplet processing of reactive organic carbon.
Highlights
Isoprene (C5H8) is the main non-methane biogenic volatile organic compound emitted to the atmosphere with global emission estimates of ∼ 470 Tg C
CH2(OH)2 is under the large D2O peak, δ ≈ 4.74 ppm, which prevents quantification (Sect. 3.2). 2-Methyl-2-vinyloxirane was used as a standard, but it was not observed as a product (Fig. S2 in the Supplement)
Some minor product signals could not be identified; for example, a signal in the SO2,aq + 1,2-ISOPOOH pH = 5.5 spectrum at δ = 1.43 ppm (Fig. 1) is likely a methyl group not adjacent to a carbonyl group based on its chemical shift; we were not able to identify the corresponding compound
Summary
Isoprene (C5H8) is the main non-methane biogenic volatile organic compound emitted to the atmosphere with global emission estimates of ∼ 470 Tg C (Guenther et al, 2006, 2012; St. Clair et al, 2016). C5H8 primarily reacts with hydroxyl radicals (OH) forming peroxy radicals (RO2) after oxygen addition (Wennberg et al, 2018). HO2-dominated (low-NO) conditions, isoprene RO2 reacts with hydroperoxyl radicals (HO2) to form multifunctional organic hydroperoxides, isoprene hydroxyl hydroperoxides (ISOPOOH, C5H10O3), of which 1hydroxyl-2-hydroperoxyl- and 4-hydroxyl-3-hydroperoxylISOPOOH (1,2-ISOPOOH and 4,3-ISOPOOH, respectively) are the most abundant isomers (Rivera-Rios et al, 2014; Krechmer et al, 2015; St. Clair et al, 2016). ISOPOOH mixing ratios of up to 1 ppb and up to ∼ 2 ppb have been reported in the Amazon rainforest and the Blodgett Forest Research Station in California, respectively (Worton et al, 2013; Liu et al, 2016)
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