Criegee intermediates (CIs), carbonyl oxides formed in ozonolysis of alkenes, play key roles in the troposphere. The decomposition of CIs can be a significant source of OH to the tropospheric oxidation cycle especially during nighttime and winter months. A variety of model-measurement studies have estimated surface-level stabilized Criegee intermediate (sCI) concentrations on the order of 1 × 10⁴ cm⁻³ to 1 × 10⁵ cm⁻³, which makes a non-negligible contribution to the oxidising capacity in the terrestrial boundary layer. The reactions of sCI with the water monomer and the water dimer have been found to be the most important bimolecular reactions to the tropospheric sCI loss rate, at least for the smallest carbonyl oxides; the products from these reactions (e.g. hydroxymethyl hydroperoxide, HMHP) are also of importance to the atmospheric oxidation cycle. The sCI can oxidise SO₂ to form SO₃, which can go on to form a significant amount of H₂SO₄ which is a key atmospheric nucleation species and therefore vital to the formation of clouds. The sCI can also react with carboxylic acids, carbonyl compounds, alcohols, peroxy radicals and hydroperoxides, and the products of these reactions are likely to be highly oxygenated species, with low vapour pressures, that can lead to nucleation and SOA formation over terrestrial regions.

Significance: Ozonolysis is an important sink of alkenes in Earth’s troposphere, leading to the formation of Criegee intermediates (CIs), whose reactions significantly impact tropospheric composition. The reactivity of the four-carbon unsaturated CIs derived from isoprene, the most abundant alkene, has remained unknown until now. Direct measurements of one such CI, methyl vinyl ketone oxide (MVK-oxide), with water vapor, SO ₂ , and formic acid are reported herein, substantiated by high-level theory, revealing the long lifetime of syn- MVK-oxide with respect to unimolecular decay and reaction with water vapor. Through a combination of direct experiment, high-level theory, and global modelling, syn -MVK-oxide is shown to survive high-humidity tropospheric environments and play a role in sulfuric acid formation and formic acid removal.

Ammonia and amines are emitted into the troposphere by various natural and anthropogenic sources, where they have a significant role in aerosol formation. Here, we explore the significance of their removal by reaction with Criegee intermediates, which are produced in the troposphere by ozonolysis of alkenes. Rate coefficients for the reactions of two representative Criegee intermediates, formaldehyde oxide (CH₂OO) and acetone oxide ((CH₃)₂COO) with NH₃ and CH₃NH₂ were measured using cavity ring-down spectroscopy. Temperature-dependent rate coefficients, k(CH₂OO + NH₃) = (3.1 ± 0.5) × 10⁻²⁰T² exp(1011 ± 48/T) cm³ s⁻¹ and k(CH₂OO + CH₃NH₂) = (5 ± 2) × 10⁻¹⁹T² exp(1384 ± 96/T) cm³ s⁻¹ were obtained in the 240 to 320 K range. Both the reactions of CH₂OO were found to be independent of pressure in the 10 to 100 Torr (N₂) range, and average rate coefficients k(CH₂OO + NH₃) = (8.4 ± 1.2) × 10⁻¹⁴ cm³ s⁻¹ and k(CH₂OO + CH₃NH₂) = (5.6 ± 0.4) × 10⁻¹² cm³ s⁻¹ were deduced at 293 K. An upper limit of ≤2.7 × 10⁻¹⁵ cm³ s⁻¹ was estimated for the rate coefficient of the (CH₃)₂COO + NH₃ reaction. Complementary measurements were performed with mass spectrometry using synchrotron radiation photoionization giving k(CH₂OO + CH₃NH₂) = (4.3 ± 0.5) × 10⁻¹² cm³ s⁻¹ at 298 K and 4 Torr (He). Photoionization mass spectra indicated production of NH₂CH₂OOH and CH₃N(H)CH₂OOH functionalized organic hydroperoxide adducts from CH₂OO + NH₃ and CH₂OO + CH₃NH₂ reactions, respectively. Ab initio calculations performed at the CCSD(T)(F12*)/cc-pVQZ-F12//CCSD(T)(F12*)/cc-pVDZ-F12 level of theory predicted pre-reactive complex formation, consistent with previous studies. Master equation simulations of the experimental data using the ab initio computed structures identified submerged barrier heights of −2.1 ± 0.1 kJ mol⁻¹ and −22.4 ± 0.2 kJ mol⁻¹ for the CH₂OO + NH₃ and CH₂OO + CH₃NH₂ reactions, respectively. The reactions of NH₃ and CH₃NH₂ with CH₂OO are not expected to compete with its removal by reaction with (H₂O)₂ in the troposphere. Similarly, losses of NH₃ and CH₃NH₂ by reaction with Criegee intermediates will be insignificant compared with reactions with OH radicals.

Methanol is a benchmark for understanding tropospheric oxidation, but is underpredicted by up to 100% in atmospheric models. Recent work has suggested this discrepancy can be reconciled by the rapid reaction of hydroxyl and methylperoxy radicals with a methanol branching fraction of 30%. However, for fractions below 15%, methanol underprediction is exacerbated. Theoretical investigations of this reaction are challenging because of intersystem crossing between singlet and triplet surfaces – ∼45% of reaction products are obtained via intersystem crossing of a pre-product complex – which demands experimental determinations of product branching. Here we report direct measurements of methanol from this reaction. A branching fraction below 15% is established, consequently highlighting a large gap in the understanding of global methanol sources. These results support the recent high-level theoretical work and substantially reduce its uncertainties.

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.