Temperature-Dependent Rate Coefficient for the Reaction of CH3SH with the Simplest Criegee Intermediate

The kinetics of the reaction of the simplest Criegee intermediate CH₂OO with CH₃SH was measured with transient IR absorption spectroscopy in a temperature-controlled flow reaction cell, and the bimolecular rate coefficients were measured from 278 to 349 K and at total pressure from 10 to 300 Torr. The measured bimolecular rate coefficient at 298 K and 300 Torr is (1.01 ± 0.17)   ×  10^-12 cm³ s^-1. The results exhibit a weak negative temperature dependence: the activation energy E_a ( k = Ae^{- E_a/ RT}) is -1.83 ± 0.05 kcal mol^-1, measured at 30 and 100 Torr. Quantum chemistry calculations of the reaction rate coefficient at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level (1.6  ×  10^-12 cm³ s^-1 at 298 K; E_a = - 2.80 kcal mol^-1) are in reasonable agreement with the experimental results. The experimental and theoretical results of the reaction of CH₂OO with CH₃SH are compared to the reactions of CH₂OO with methanol and hydrogen sulfide, and the trends in reactivity are discussed. The results of the present work indicate that this reaction has a negligible influence to atmospheric CH₂OO or CH₃SH.

Electronic spectroscopy of methyl vinyl ketone oxide: A four-carbon unsaturated Criegee intermediate from isoprene ozonolysis

Ozonolysis of isoprene, one of the most abundant volatile organic compounds in the atmosphere, proceeds through methyl vinyl ketone oxide (MVK-oxide), methacrolein oxide, and formaldehyde oxide (CH₂OO) Criegee intermediates. The present study focuses on MVK-oxide, a four-carbon unsaturated carbonyl oxide intermediate, using vacuum ultraviolet photoionization at 118 nm and UV-visible induced depletion of the m/z = 86 mass channel to characterize its first π^* ← π electronic transition. The electronic spectrum is broad and unstructured with its peak at 388 nm (3.2 eV). The MVK-oxide spectrum is shifted to a significantly longer wavelength than CH₂OO and alkyl-substituted Criegee intermediates studied previously due to extended conjugation across the vinyl and carbonyl oxide groups. Electronic excitation results in rapid dissociation at λ ≤ 430 nm to methyl vinyl ketone and O ¹D products, the latter detected by 2 + 1 resonance enhanced multiphoton ionization using velocity map imaging. Complementary electronic structure calculations (CASPT2(12,10)/AVDZ) predict two π^* ← π transitions with significant oscillator strength for each of the four conformers of MVK-oxide with vertical excitation energies (and corresponding wavelengths) in the 3.1-3.6 eV (350-400 nm) and 4.5-5.5 eV (220-280 nm) regions. The computed electronic absorption profile of MVK-oxide, based on a Wigner distribution of ground state configurations and summed over the four conformers, is predicted to peak at 397 nm. UV-visible spectroscopy on the first π^* ← π transition is shown by a combination of experiment and theory to provide a sensitive method for detection of the MVK-oxide Criegee intermediate that will enable further studies of its photochemistry and unimolecular and bimolecular reaction dynamics.

Identification of the Criegee intermediate reaction network in ethylene ozonolysis: impact on energy conversion strategies and atmospheric chemistry

The reaction network of the simplest Criegee intermediate (CI) CH₂OO has been studied experimentally during the ozonolysis of ethylene.

Theoretical investigation on the reaction mechanism and kinetics of a Criegee intermediate with ethylene and acetylene

The competition among the possible pathways, the branching ratios of the adduct and the decomposition products at different temperatures and pressures have been evaluated.

Experimental and computational studies of Criegee intermediate reactions with NH3 and CH3NH2

The significance of removal of atmospheric ammonia and amines by reaction with Criegee intermediates is assessed by kinetic studies.

Effects of water vapor on the reaction of CH2OO with NH3

A strong synergic effect of water and ammonia molecules may enhance the formation of H₂NCH₂OOH.

Criegee Intermediate Reaction with Alcohol Is Enhanced by a Single Water Molecule

The role of water in gas-phase reactions has gained considerable interest. Here we report a direct kinetic measurement of the reaction of syn-CH₃CHOO (a Criegee intermediate or carbonyl oxide) with methanol at various relative humidity (RH = 0-80%) under near-ambient conditions (298 K, 250-755 Torr). The data indicate that a single water molecule expedites the reaction by up to a factor of three. The rate coefficient of the corresponding reaction, syn-CH₃CHOO + CH₃OH + H₂O → products, has been determined to be (1.95 ± 0.11) × 10^-32 cm⁶ s^-1 at 298 K, with no observable pressure dependence for 250-755 Torr. Quantum chemistry calculation shows that the dominating pathway involves a hydrogen-bonded ring structure, in which methanol is donating a hydrogen atom to water, water is donating a hydrogen atom to the terminal oxygen atom of the Criegee intermediate, and, on the product side, H₂O is reformed and acts as a catalyst.

An Extended Computational Study of Criegee Intermediate-Alcohol Reactions

High-level ab initio calculations (DF-LCCSD(T)-F12a//B3LYP/aug-cc-pVTZ) are performed on a range of stabilized Criegee intermediate (sCI)-alcohol reactions, computing reaction coordinate energies, leading to the formation of α-alkoxyalkyl hydroperoxides (AAAHs). These potential energy surfaces are used to model bimolecular reaction kinetics over a range of temperatures. The calculations performed in this work reproduce the complicated temperature-dependent reaction rates of CH₂OO and (CH₃)₂COO with methanol, which have previously been experimentally determined. This methodology is then extended to compute reaction rates of 22 different Criegee intermediates with methanol, including several intermediates derived from isoprene ozonolysis. In some cases, sCI-alcohol reaction rates approach those of sCI-(H₂O)₂. This suggests that in regions with elevated alcohol concentrations, such as urban Brazil, these reactions may generate significant quantities of AAAHs and may begin to compete with sCI reactions with other trace tropospheric pollutants such as SO₂. This work also demonstrates the ability of alcohols to catalyze the 1,4-H transfer unimolecular decomposition of α-methyl substituted sCIs.

Absolute Infrared Absorption Cross Section of the Simplest Criegee Intermediate Near 1285.7 cm-1

The ν₄ fundamental of the simplest Criegee intermediate, CH₂OO, has been monitored with high-resolution infrared (IR) transient absorption spectroscopy under total pressures of 4-94 Torr. This IR spectrum provides an unambiguous identification of CH₂OO and is potentially useful to determine the number density of CH₂OO in various laboratory studies. Here we utilized an ultraviolet (UV) and IR coupled spectrometer to measure the UV and IR absorption spectra of CH₂OO simultaneously; the absolute IR cross section can then be determined by using a known UV cross section. Due to significant pressure broadening in the studied pressure range, we integrated the IR absorption spectra between 1285.2 and 1286.4 cm^-1 (covering the Q branch), and then we converted this integrated absorbance to the absolute integral IR cross section of CH₂OO (for the Q branch); its absolute value is (3.7 ± 0.6) × 10^-19 cm·molecule^-1 or 2.2 ± 0.4 km·mol^-1. The whole rotational band (P, Q, and R branches) can be adequately simulated by using the precise spectroscopic parameters from the literature, yielding the absolute integral IR cross section (full ν₄ band) to be 19.2 ± 3.5 km·mol^-1. For a practical detection of CH₂OO, this work also reports the peak cross section as a function of total pressure (4-94 Torr O₂). At low pressure (≤4 Torr), where the pressure broadening is insignificant, the absorption cross section of the highest peak is (6.2 ± 0.9) × 10^-18 cm²·molecule^-1 (at the system line width of 0.004 cm^-1 fwhm).

Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production

The reaction of ozone with isoprene, one of the most abundant volatile organic compounds in the atmosphere, produces three distinct carbonyl oxide species (RR'COO) known as Criegee intermediates: formaldehyde oxide (CH₂OO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R' = H, CH₃, CH═CH₂) and conformations of the Criegee intermediates control their subsequent chemistry in the atmosphere. In particular, unimolecular decay of MVK-OO is predicted to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis. This study reports the initial laboratory synthesis and direct detection of MVK-OO through reaction of a photolytically generated, resonance-stabilized monoiodoalkene radical with O₂. MVK-OO is characterized utilizing infrared (IR) action spectroscopy, in which IR activation of MVK-OO with two quanta of CH stretch at ca. 6000 cm^-1 is coupled with ultraviolet detection of the resultant OH products. MVK-OO is identified by comparison of the experimentally observed IR spectral features with theoretically predicted IR absorption spectra. For syn-MVK-OO, the rate of appearance of OH products agrees with the unimolecular decay rate predicted using statistical theory with tunneling. This validates the hydrogen atom transfer mechanism and computed transition-state barrier (18.0 kcal mol^-1) leading to OH products. Theoretical calculations reveal an additional roaming pathway between the separating radical fragments, which results in other products. Master equation modeling yields a thermal unimolecular decay rate for syn-MVK-OO of 33 s^-1 (298 K, 1 atm). For anti-MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products.

Effect of unsaturated substituents in the reaction of Criegee intermediates with water vapor

Criegee intermediates (CIs), formed in the reactions of unsaturated hydrocarbons with ozone, are very reactive carbonyl oxides and have recently been suggested as important oxidants in the atmosphere. In this work, we studied the substituent effect on the water monomer and dimer reaction with CIs which include up to three carbon atoms at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level. Our calculation showed that for saturated CIs with a hydrogen atom on the same side as the terminal oxygen atom, the reaction with water vapor would likely dominate the removal processes of these CIs in the atmosphere. On the other hand, for unsaturated CIs, the reactivity toward water vapor decreases compared to the saturated species allowing them to survive in humid atmospheric environments. We also evaluated the kinetic isotope effect in the reaction between CI and water vapor by performing calculations with deuterated water. We found that tunneling is not important and the kinetic isotope effect mainly comes from the difference in the zero point energy between water and deuterated water.

Structure-dependent reactivity of Criegee intermediates studied with spectroscopic methods

Criegee intermediates are very reactive carbonyl oxides that are formed in reactions of unsaturated hydrocarbons with ozone (ozonolysis). Recently, Criegee intermediates have gained significant attention since a new preparation method has been reported in 2012, which employs the reaction of iodoalkyl radical with molecular oxygen: for instance, CH₂I + O₂ → CH₂OO + I. This new synthesis route can produce Criegee intermediates with a high number density, which allows direct detection of the Criegee intermediate via various spectroscopic tools, including vacuum UV photoionization mass spectrometry, absorption and action spectroscopy in the UV and IR regions, and microwave spectroscopy. Criegee intermediates have been thought to play important roles in atmospheric chemistry, such as in OH radical formation as well as oxidation of atmospheric gases such as SO₂, NO₂, volatile organic compounds, organic and inorganic acids, and even water. These reactions are relevant to acid rain and aerosol formation. Kinetics data including rate coefficients, product yields and their temperature and pressure dependences are important for understanding and modeling relevant atmospheric chemistry. In fundamental physical chemistry, Criegee intermediates have unique and interesting features, which have been partially revealed through spectroscopic, kinetic, and dynamic investigations. Although previous review articles have discussed Criegee intermediates, new data and knowledge on Criegee intermediates are still being accumulated. In this tutorial review, we have focused on structure-dependent reactivity of Criegee intermediates and various spectroscopic tools that have been utilized to probe the kinetics of Criegee intermediates.

Criegee intermediates and their impacts on the troposphere

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.

Unimolecular decay strongly limits the atmospheric impact of Criegee intermediates

Stabilized Criegee intermediates (SCI) are reactive oxygenated species formed in the ozonolysis of hydrocarbons. Their chemistry could influence the oxidative capacity of the atmosphere by affecting the HO_x and NO_x cycles, or by the formation of low-volatility oxygenates enhancing atmospheric aerosols known to have an important impact on climate. The concentration of SCI in the atmosphere has hitherto not been determined reliably, and very little is known about their speciation. Here we show that the concentration of biogenic SCI is strongly limited by their unimolecular decay, based on extensive theory-based structure-activity relationships (SARs) for the reaction rates for decomposition. Reaction with water vapor, H₂O and (H₂O)₂ molecules, is the second most important loss process; SARs are also proposed for these reactions. For SCI derived from the most common biogenic VOCs, we find that unimolecular decay is responsible for just over half of the loss, with reaction with water vapor the main remaining loss process. Reactions with SO₂, NO₂, or acids have negligible impact on the atmospheric SCI concentration. The ambient SCI concentrations are further characterized by analysis of field data with speciated hydrocarbon information, and by implementation of the chemistry in a global chemistry model. The results show a highly complex SCI speciation, with an atmospheric peak SCI concentrations below 1 × 10⁵ molecule cm^-3, and annual average SCI concentrations less than 7 × 10³ molecule cm^-3. We find that SCI have only a negligible impact on the global gas phase H₂SO₄ formation or removal of oxygenates, though some contribution around the equatorial belt, and in select regions, cannot be excluded.

Impact of the water dimer on the atmospheric reactivity of carbonyl oxides

The reactions of twelve carbonyl oxides or Criegee intermediates with the water monomer and with the water dimer have been investigated employing high level theoretical methods. The study includes all possible carbonyl oxides arising from the isoprene ozonolysis and the methyl and dimethyl carbonyl oxides that originated from the reaction of ozone with several hydrocarbons. These reactions have great significance in the chemistry of the atmosphere because Criegee intermediates have recently been identified as important oxidants in the troposphere and as precursors of secondary organic aerosols. Moreover, water vapor is one of the most abundant trace gases in the atmosphere and the water dimer can trigger the atmospheric decomposition of Criegee intermediates. Our calculations show that the nature and position of the substituents in carbonyl oxides play a very important role in the reactivity of these species with both the water monomer and the water dimer. This fact results in differences in rate constants of up to six orders of magnitude depending on the carbonyl oxide. In this work we have defined an effective rate constant (keff) for the atmospheric reaction of carbonyl oxides with water vapor, which depends on the temperature and on the relative humidity as well. With this keff we show that the water dimer, despite its low tropospheric concentration, enhances the atmospheric reactivity of Criegee intermediates, but its effect changes with the nature of carbonyl oxide, ranging between 59 and 295 times in the most favorable case (syn-methyl carbonyl oxide), and between 1.4 and 3 times only in the most unfavorable case.

Reactivity of Criegee Intermediates toward Carbon Dioxide

Recent theoretical work by Kumar and Francisco suggested that the high reactivity of Criegee intermediates (CIs) could be utilized for designing efficient carbon capture technologies. Because the anti-CH₃CHOO + CO₂ reaction has the lowest barrier in their study, we chose to investigate it experimentally. We probed anti-CH₃CHOO with its strong UV absorption at 365 nm and measured the rate coefficient to be ≤2 × 10^-17 cm³ molecule^-1 s^-1 at 298 K, which is consistent with our theoretical value of 2.1 × 10^-17 cm³ molecule^-1 s^-1 at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level but inconsistent with their results obtained at the M06-2X/aug-cc-pVTZ level, which tends to underestimate the barrier heights. The experimental result indicates that the reaction of a Criegee intermediate with atmospheric CO₂ (400 ppmv) would be inefficient (k_eff < 0.2 s^-1) and cannot compete with other decay processes of Criegee intermediates like reactions with water vapor (∼10³ s^-1) or thermal decomposition (∼10² s^-1).

Re-examining ammonia addition to the Criegee intermediate: converging to chemical accuracy

Theory shows ammonia is unlikely to be significant in Criegee chemistry and demonstrates the importance of perturbative quadruple excitations in Criegee chemistry.

Kinetics of the reaction of the simplest Criegee intermediate with ammonia: a combination of experiment and theory

The negative temperature dependence of the rate coefficient for CH₂OO + NH₃ reaction was observed using an OH laser-induced fluorescence method.

Catalysis and tunnelling in the unimolecular decay of Criegee intermediates

Semi-classical Transition State theory can be applied to catalysed atmospheric reactions, but reaction mode anharmonicity must be treated carefully.

Kinetics of the simplest Criegee intermediate reaction with ozone studied using a mid-infrared quantum cascade laser spectrometer

The first experimental study of Criegee intermediate reactions with ozone, clarifying its role in atmosphere and laboratory studies of ozonolysis.

Kinetic studies of C1 and C2 Criegee intermediates with SO2 using laser flash photolysis coupled with photoionization mass spectrometry and time resolved UV absorption spectroscopy

Kinetics of CH₂OO + SO₂ confirmed over a wide range of [SO₂]. Acetaldehyde observed as a major product of the reaction of CH₃CHOO + SO₂.

QM/MM studies on ozonolysis of α-humulene and Criegee reactions with acids and water at air–water/acetonitrile interfaces

QM/MM electronic structure calculations reveal important mechanistic insights on the ozonolysis of α-humulene and Criegee reactions with acids and water at air–water/acetonitrile interfaces.

Selective deuteration illuminates the importance of tunneling in the unimolecular decay of Criegee intermediates to hydroxyl radical products

Ozonolysis of alkenes, an important nonphotolytic source of hydroxyl (OH) radicals in the atmosphere, proceeds through unimolecular decay of Criegee intermediates. Here, we report a large kinetic isotope effect associated with the rate-limiting hydrogen-transfer step that releases OH radicals for a prototypical Criegee intermediate, CH3CHOO. IR excitation of selectively deuterated syn-CD3CHOO is shown to result in deuterium atom transfer and release OD radical products. Vibrational activation of syn-CD3CHOO is coupled with direct time-resolved detection of OD products to measure a 10-fold slower rate of unimolecular decay upon deuteration in the vicinity of the transition state barrier, which is confirmed by microcanonical statistical theory that incorporates quantum mechanical tunneling. The corresponding kinetic isotope effect of ∼10 is attributed primarily to the decreased probability of D-atom vs. H-atom transfer arising from tunneling. Master equation modeling is utilized to compute the thermal unimolecular decay rates for selectively and fully deuterated syn methyl-substituted Criegee intermediates under atmospheric conditions. At 298 K (1 atm), tunneling is predicted to enhance the thermal decay rate of syn-CH3CHOO compared with the deuterated species, giving rise to a significant kinetic isotope effect of ∼50.

Mechanism of gas-phase ozonolysis of sabinene in the atmosphere

Sabinene is one of the monoterpenes of biogenic origin in the atmosphere. Ozonolysis is one of the important oxidation removal reactions for sabinene in the atmosphere during the daytime, leading to the formation of secondary organic aerosols. In this study, we investigate the mechanism of gas-phase ozonolysis of sabinene using quantum chemistry and kinetic calculations. The reaction starts with the formation of four primary ozonides (POZs), which decompose to primary product channels CH₂OO + sabinaketone and CH₂O + two Criegee intermediates (CI-1 and CI-2) with branching ratios of 17%, 45%, and 38%, respectively, at 298 K and 760 Torr. Calculations showed that the stabilized CI-1 would undergo a rapid intramolecular H-shift to a vinyl hydroperoxide (VHP) at a rate of ∼2700 s^-1 followed by rapid decomposition to an OH radical and a vinoxy-type radical (VTR) and CI-2 would slowly isomerize to dioxirane at a rate of 0.97 s^-1. In the atmosphere, CI-2 would instead react with water and the water dimer, forming α-hydroxyalkyl hydroperoxides (αHAHPs), which would decompose to sabinaketone and H₂O₂via heterogeneous processes. The reaction of CI-2 with SO₂ would also be significant in a dry and cold atmosphere. The yield of sabinaketone of 47%, from primary POZ decomposition and secondary reactions of αHAHPs, agrees with the previously measured values of 35-50%. The OH radical, formed from CI-1, could reach 44%, agreeing with the previously reported value of (33 ± 6)%. Further reaction of the VTR radical would form highly-oxidized multifunctional products containing carboxylic and/or carbonyl groups which might contribute substantially to SOA formation.

An Estimation of the Levels of Stabilized Criegee Intermediates in the UK Urban and Rural Atmosphere Using the Steady-State Approximation and the Potential Effects of These Intermediates on Tropospheric Oxidation Cycles

Levels of the stabilized Criegee Intermediate (sCI), produced via the ozonolysis of unsaturated volatile organic compounds (VOCs), were estimated at two London urban sites (Marylebone Road and Eltham) and one rural site (Harwell) in the UK over the period of 1998-2012. The steady-state approximation was applied to data obtained from the NETCEN (National Environmental Technology Centre) database, and the levels of annual average sCI were estimated to be in the range of 30-3000 molecules cm-3 for UK sites. A consistent diurnal cycle of sCI concentration is estimated for the UK sites with increasing levels during daylight hours, peaking just after midday. The seasonal pattern of sCI shows higher levels in spring with peaks around May due to the higher levels of O3. The ozone weekend effect resulted in higher sCI in UK urban areas during weekend. The sCI data were modeled using the information provided by the Air Quality Improvement Research Program (AQIRP) and found that the modeled production was five- to six-fold higher than our estimated data, and therefore the estimated sCI concentrations in this study are thought to be lower estimates only. Compared with nighttime, 1.3- to 1.8-fold higher sCI exists under daytime conditions. Using the levels of sCI estimated at Marylebone Road, globally the oxidation rates of NO2 + sCI (22.4 Gg/yr) and SO2 + sCI (37.6 Gg/yr) in urban areas can increase their effect in the troposphere and potentially further alter the oxidizing capacity of the troposphere. Further investigations of modeled sCI show that CH3CHOO (64%) and CH2OO (13%) are dominant among all contributing sCI at the UK sites.

Criegee Chemistry on Aqueous Organic Surfaces

In the troposphere, the fate of gas-phase Criegee intermediates (CIs) is deemed to be determined by their reactions with water molecules. Here it is shown that CIs produced in situ on the surface of water/acetonitrile (W/AN) solutions react competitively with millimolar carboxylic acids. Present experiments probe, via online electrospray mass spectrometry, CIs' chemistry on the surface of α-humulene and β-caryophyllene in W/AN microjets exposed to O₃(g) for <10 μs. Mass-specific identification lets us establish the progeny of products and intermediates generated in the early stages of CIs' reactions with H₂O, D₂O, H₂¹⁸O, and n-alkyl-COOH (n = 1-7). It is found that n-alkyl-COOH competes for CIs with interfacial water, their competitiveness being an increasing function of n. Present findings demonstrate that CIs can react with species other than H₂O on the surface of aqueous organic aerosols due to the low water concentrations prevalent in the outermost interfacial layers.

Water vapour effects on secondary organic aerosol formation in isoprene ozonolysis

A portion of stabilized Criegee intermediates formed in isoprene ozonolysis can be involved in SOA formation even under humid conditions.

Introductory lecture: atmospheric chemistry in the Anthropocene

The term “Anthropocene” was coined by Professor Paul Crutzen in 2000 to describe an unprecedented era in which anthropogenic activities are impacting planet Earth on a global scale. Greatly increased emissions into the atmosphere, reflecting the advent of the Industrial Revolution, have caused significant changes in both the lower and upper atmosphere. Atmospheric reactions of the anthropogenic emissions and of those with biogenic compounds have significant impacts on human health, visibility, climate and weather. Two activities that have had particularly large impacts on the troposphere are fossil fuel combustion and agriculture, both associated with a burgeoning population. Emissions are also changing due to alterations in land use. This paper describes some of the tropospheric chemistry associated with the Anthropocene, with emphasis on areas having large uncertainties. These include heterogeneous chemistry such as those of oxides of nitrogen and the neonicotinoid pesticides, reactions at liquid interfaces, organic oxidations and particle formation, the role of sulfur compounds in the Anthropocene and biogenic–anthropogenic interactions. A clear and quantitative understanding of the connections between emissions, reactions, deposition and atmospheric composition is central to developing appropriate cost-effective strategies for minimizing the impacts of anthropogenic activities. The evolving nature of emissions in the Anthropocene places atmospheric chemistry at the fulcrum of determining human health and welfare in the future.

Atmospheric Chemistry of Criegee Intermediates: Unimolecular Reactions and Reactions with Water

Criegee intermediates are produced in the ozonolysis of unsaturated hydrocarbons in the troposphere, and understanding their fate is a prerequisite to modeling climate-controlling atmospheric aerosol formation. Although some experimental and theoretical rate data are available, they are incomplete and partially inconsistent, and they do not cover the tropospheric temperature range. Here, we report quantum chemical rate constants for the reactions of stabilized formaldehyde oxide (CH₂OO) and acetaldehyde oxide (syn-CH₃CHOO and anti-CH₃CHOO) with H₂O and for their unimolecular reactions. Our results are obtained by combining post-CCSD(T) electronic structure benchmarks, validated density functional theory potential energy surfaces, and multipath variational transition state theory with multidimensional tunneling, coupled-torsions anharmonicity, and high-frequency anharmonicity. We consider two different types of reaction mechanisms for the bimolecular reactions, namely, (i) addition-coupled hydrogen transfer and (ii) double hydrogen atom transfer (DHAT). First, we show that the MN15-L exchange-correlation functional has kJ/mol accuracy for the CH₂OO + H₂O and syn-CH₃CHOO + H₂O reactions. Then we show that, due to tunneling, the DHAT mechanism is especially important in the syn-CH₃CHOO + H₂O reaction. We show that the dominant pathways for reactions of Criegee intermediates depend on altitude. The results we obtain eliminate the discrepancy between experiment and theory under those conditions where experimental results are available, and we make predictions for the full range of temperatures and pressures encountered in the troposphere and stratosphere. The present results are an important cog in clarifying the atmospheric fate and oxidation processes of Criegee intermediates, and they also show how theoretical methods can provide reliable rate data for complex atmospheric processes.

Isomerization of the methoxy radical revisited: the impact of water dimers

Investigations carried out at MP2 and CCSD(T) levels of theory show that water dimers can compete with formic and sulfuric acids in catalyzing isomerization of methoxy radicals in the lower troposphere.