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Isoprene has the highest emission into Earth’s atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield fromisoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jetcooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO₂ and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO₃ and identifying organic hydroperoxide formation from reaction with SO₂ and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

Criegee intermediates are reactive intermediates formed in Earth’s atmosphere through ozonolysis of alkenes. Here the authors outline the fundamental chemistry that influences their highly conformer- and substituent-dependent unimolecular and bimolecular reactivity, and discuss open questions of fundamental and atmospheric interest.

Methacrolein oxide (MACR-oxide) is a four-carbon, resonance-stabilized Criegee intermediate produced from isoprene ozonolysis, yet its reactivity is not well understood. This study identifies the functionalized hydroperoxide species, 1-hydroperoxy-2-methylallyl formate (HPMAF), generated from the reaction of MACR-oxide with formic acid using multiplexed photoionization mass spectrometry (MPIMS, 298 K = 25 °C, 10 torr = 13.3 hPa). Electronic structure calculations indicate the reaction proceeds via an energetically favorable 1,4-addition mechanism. The formation of HPMAF is observed by the rapid appearance of a fragment ion at m/z 99, consistent with the proposed mechanism and characteristic loss of HO2 upon photoionization of functional hydroperoxides. The identification of HPMAF is confirmed by comparison of the appearance energy of the fragment ion with theoretical predictions of its photoionization threshold. The results are compared to analogous studies on the reaction of formic acid with methyl vinyl ketone oxide (MVK-oxide), the other four-carbon Criegee intermediate in isoprene ozonolysis.

Isoprene has the highest emission into Earth’s atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have show that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester,J. Chem. Phys.149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only thesyn-conformers are observed;anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction ofsyn-MVK-oxide with SO2and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We further confirm products through complementary multiplexed photoionization mass spectrometry, observing SO3 and identifying organic hydroperoxide formation from reaction with SO2 and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

Ozonolysis is an important sink of alkenes in Earth’s troposphere, leading to the formation of highly reactive carbonyl oxide (R1R2C=O+O−) species known as Criegee intermediates. Ozonolysis of isoprene, the most abundant non-methane volatile organic compound emitted into the atmosphere, can generate three distinct Criegee intermediates: formaldehyde oxide (CH2OO), methyl vinyl ketone oxide (MVK-oxide), and methacrolein oxide (MACR-oxide). Due to the abundance of isoprene in the atmosphere, the unimolecular and bimolecular reactions of Criegee intermediates may significantly impact the composition of the troposphere. The laboratory synthesis and direct detection of MVK-oxide and MACR-oxide is achieved through reaction of photolytically generated, resonance-stabilized iodoalkene radicals with oxygen. MVK-oxide and MACR-oxide are characterized on their first π*← π transition using a ground-state depletion method under jet-cooled conditions. These Criegee intermediates exhibit broad ultraviolet-visible (UV-vis) spectra with strong absorption (ca. 10−17 cm2 molec−1). Electronic excitation of Criegee intermediates results in nonadiabatic coupling to repulsive potentials and prompt release of O 1D products. Velocity map imaging is used to determine the angular and velocity distributions of the O-atom products following UV-vis excitation of the isoprene-derived Criegee intermediates. UV-vis transient absorption spectroscopy permits study of the bimolecular reactions of MVK-oxide with SO2, formic acid, and water vapor under thermal conditions. Complimentary experiments using multiplexed photoionization mass spectrometry (MPIMS) identify products resulting from reaction of MVK-oxide with SO2 and formic acid. The reaction of MVK-oxide with deuterated formic acid reveals multiple reaction channels including adduct formation and formic acid catalyzed isomerization yielding a vinyl hydroperoxide. Through a combination of experiment, theory, and global modeling, syn conformers of MVK-oxide are shown to survive high-humidity tropospheric environments and play a role in sulfuric acid formation and formic acid removal. In contrast, anti-MVK-oxide and syn-MACR-oxide conformers are predicted to be removed rapidly from the atmosphere via electrocyclic ring closure to form a cyclic dioxole, which subsequently decays to oxygenated hydrocarbon radical products. These radicals react rapidly with oxygen and their stable carbonyl products are detected using MPIMS.