Chemical Kinetic Study of the Reaction of CHOO with CHO.

Criegee intermediates play an important role in the oxidizing capacity of the Earth's troposphere. Although extensive studies have been conducted on Criegee intermediates in the past decade, their kinetics with radical species remain underexplored. We investigated the kinetics of the simplest Criegee intermediate, CHOO, with the methyl peroxy radical, CHO, as a model system to explore the reactivities of Criegee intermediates with peroxy radicals.

A white cell based broadband transient UV-vis absorption spectroscopy with pulsed laser photolysis reactors for chemical kinetics under variable temperatures and pressures.

UV-vis spectroscopy is widely used for kinetic studies in physical chemistry, as species' absolute cross-sections are usually less sensitive to experimental conditions (i.e., temperature and pressure).

Bimolecular Reaction of Methyl-Ethyl-Substituted Criegee Intermediate with SO.

Methyl-ethyl-substituted Criegee intermediate (MECI) is a four-carbon carbonyl oxide that is formed in the ozonolysis of some asymmetric alkenes. MECI is structurally similar to the isoprene-derived methyl vinyl ketone oxide (MVK-oxide) but lacks resonance stabilization, making it a promising candidate to help us unravel the effects of size, structure, and resonance stabilization that influence the reactivity of atmospherically important, highly functionalized Criegee intermediates. We present experimental and theoretical results from the first bimolecular study of MECI in its reaction with SO, a reaction that shows significant sensitivity to the Criegee intermediate structure.

A-Band Absorption Spectrum of the ClSO Radical: Electronic Structure of the Sulfinyl Group.

Sulfur oxide species (RSO) play a critical role in many fields, ranging from biology to atmospheric chemistry. Chlorine-containing sulfur oxides may play a key role in sulfate aerosol formation in Venus' cloud layer by catalyzing the oxidation of SO to SO via sulfinyl radicals (RSO). We present results from the gas-phase UV-vis transient absorption spectroscopy study of the simplest sulfinyl radical, ClSO, generated from the pulsed-laser photolysis of thionyl chloride at 248 nm (at 40 Torr of N and 292 K).

Acetonyl Peroxy and Hydroperoxy Self- and Cross-Reactions: Temperature-Dependent Kinetic Parameters, Branching Fractions, and Chaperone Effects.

The temperature-dependent kinetic parameters, branching fractions, and chaperone effects of the self- and cross-reactions between acetonyl peroxy (CHC(O)CHO) and hydro peroxy (HO) have been studied using pulsed laser photolysis coupled with infrared (IR) wavelength-modulation spectroscopy and ultraviolet absorption (UVA) spectroscopy. Two IR lasers simultaneously monitored HO and hydroxyl (OH), while UVA measurements monitored CHC(O)CHO. For the CHC(O)CHO self-reaction ( = 270-330 K), the rate parameters were determined to be = (1.

OH Roaming and Beyond in the Unimolecular Decay of the Methyl-Ethyl-Substituted Criegee Intermediate: Observations and Predictions.

Alkene ozonolysis generates short-lived Criegee intermediates that are a significant source of hydroxyl (OH) radicals. This study demonstrates that roaming of the separating OH radicals can yield alternate hydroxycarbonyl products, thereby reducing the OH yield. Specifically, hydroxybutanone has been detected as a stable product arising from roaming in the unimolecular decay of the methyl-ethyl-substituted Criegee intermediate (MECI) under thermal flow cell conditions.

Airborne observations over the North Atlantic Ocean reveal the importance of gas-phase urea in the atmosphere.

Reduced nitrogen (N) is central to global biogeochemistry, yet there are large uncertainties surrounding its sources and rate of cycling. Here, we present observations of gas-phase urea (CO(NH)) in the atmosphere from airborne high-resolution mass spectrometer measurements over the North Atlantic Ocean. We show that urea is ubiquitous in the lower troposphere in the summer, autumn, and winter but was not detected in the spring.

Conformer-Dependent Chemistry: Experimental Product Branching of the Vinyl Alcohol + OH + O Reaction.

The concentration of formic acid in Earth's troposphere is underestimated by detailed chemical models compared to field observations. Phototautomerization of acetaldehyde to its less stable tautomer vinyl alcohol, followed by the OH-initiated oxidation of vinyl alcohol, has been proposed as a missing source of formic acid that improves the agreement between models and field measurements. Theoretical investigations of the OH + vinyl alcohol reaction in excess O conclude that OH addition to the α carbon of vinyl alcohol produces formaldehyde + formic acid + OH, whereas OH addition to the β site leads to glycoaldehyde + HO.

Spectroscopic and Kinetic Studies of the ClSO Radical from ClSO Photolysis.

Thionyl chloride (ClSO) serves as a common Cl atom source in widespread applications of chlorine chemistry though little is known about the reactivity and spectroscopy of the ClSO radical after a Cl-S bond cleavage. We performed a Pulsed Laser Photolysis experiment to detect ClSO from ClSO photolysis at 248 nm in a gas-flow reactor by time-resolved UV-vis transient absorption spectroscopy. A few chemical tests, using I and NO, suggested the structured absorption band between 260 and 320 nm belonged to ClSO radical and that the termolecular ClSO + Cl + M → ClSO association reaction occurred.

Dramatic Conformer-Dependent Reactivity of the Acetaldehyde Oxide Criegee Intermediate with Dimethylamine a 1,2-Insertion Mechanism.

The reactivity of carbonyl oxides has previously been shown to exhibit strong conformer and substituent dependencies. Through a combination of synchrotron-multiplexed photoionization mass spectrometry experiments (298 K and 4 Torr) and high-level theory [CCSD(T)-F12/cc-pVTZ-F12//B2PLYP-D3/cc-pVTZ with an added CCSDT(Q) correction], we explore the conformer dependence of the reaction of acetaldehyde oxide (CHCHOO) with dimethylamine (DMA). The experimental data support the theoretically predicted 1,2-insertion mechanism and the formation of an amine-functionalized hydroperoxide reaction product.

Isotopic signatures of methane emissions from tropical fires, agriculture and wetlands: the MOYA and ZWAMPS flights.

We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane C isotopic signatures were in the range -55 to -49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely -60 ± 1‰ from Upper Congo and Zambezi wetlands.

Functionalized Hydroperoxide Formation from the Reaction of Methacrolein-Oxide, an Isoprene-Derived Criegee Intermediate, with Formic Acid: Experiment and Theory.

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).

Pressure and Temperature Dependencies of Rate Coefficients for the Reaction OH + NO + M → Products.

The OH + NO reaction is a critically important process for radical chain termination in the atmosphere with a major impact on the ozone budgets of the troposphere and stratosphere. Rate constants for the reaction of OH + NO + M → products have been measured under conditions relevant to the upper troposphere/lower stratosphere with a laser photolysis-laser-induced fluorescence (LP-LIF) technique augmented by in situ optical spectroscopy for quantification of [NO]. The experiments are carried out over the temperature range of 230-293 K and the pressure range 50-750 Torr of N and air and as a function of [O].

Investigating the background and local contribution of the oxidants in London and Bangkok.

The contribution of NO emissions and background O to the sources and partitioning of the oxidants [OX (= O + NO)] at the Marylebone Road site in London during the 2000s and 2010s has been investigated to see the impact of the control measures or technology changes inline with the London Mayor's Air Quality Strategy. The abatement of the pollution emissions has an impact on the trends of local and background oxidants, [OX] and [OX], decreasing by 1.4% per year and 0.

Formic acid catalyzed isomerization and adduct formation of an isoprene-derived Criegee intermediate: experiment and theory.

Isoprene is the most abundant non-methane hydrocarbon emitted into the Earth's atmosphere. Ozonolysis is an important atmospheric sink for isoprene, which generates reactive carbonyl oxide species (R1R2C[double bond, length as m-dash]O+O-) known as Criegee intermediates. This study focuses on characterizing the catalyzed isomerization and adduct formation pathways for the reaction between formic acid and methyl vinyl ketone oxide (MVK-oxide), a four-carbon unsaturated Criegee intermediate generated from isoprene ozonolysis.

Acetonyl Peroxy and Hydro Peroxy Self- and Cross-Reactions: Kinetics, Mechanism, and Chaperone Enhancement from the Perspective of the Hydroxyl Radical Product.

Pulsed laser photolysis coupled with infrared (IR) wavelength modulation spectroscopy and ultraviolet (UV) absorption spectroscopy was used to study the kinetics and branching fractions for the acetonyl peroxy (CHC(O)CHO) self-reaction and its reaction with hydro peroxy (HO) at a temperature of 298 K and pressure of 100 Torr. Near-IR and mid-IR lasers simultaneously monitored HO and hydroxyl, OH, respectively, while UV absorption measurements monitored the CHC(O)CHO concentrations. The overall rate constant for the reaction between CHC(O)CHO and HO was found to be (5.

Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate.

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.

Experimental Evidence of Dioxole Unimolecular Decay Pathway for Isoprene-Derived Criegee Intermediates.

Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth's atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide () and MACR-oxide () with the vinyl substituent adjacent to the terminal O atom.

Secondary organic aerosol reduced by mixture of atmospheric vapours.

Secondary organic aerosol contributes to the atmospheric particle burden with implications for air quality and climate. Biogenic volatile organic compounds such as terpenoids emitted from plants are important secondary organic aerosol precursors with isoprene dominating the emissions of biogenic volatile organic compounds globally. However, the particle mass from isoprene oxidation is generally modest compared to that of other terpenoids.

Experimental and computational studies of Criegee intermediate reactions with NH and CHNH.

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.

Reaction of Perfluorooctanoic Acid with Criegee Intermediates and Implications for the Atmospheric Fate of Perfluorocarboxylic Acids.

The reaction of perfluorooctanoic acid with the smallest carbonyl oxide Criegee intermediate, CHOO, has been measured and is very rapid, with a rate coefficient of (4.9 ± 0.8) × 10 cm s, similar to that for reactions of Criegee intermediates with other organic acids.

The reaction of hydroxyl and methylperoxy radicals is not a major source of atmospheric methanol.

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.

Reaction kinetics of OH + HNO under conditions relevant to the upper troposphere/lower stratosphere.

The OH initiated oxidation of HNO3 in the UT/LS plays an important role in controlling the O3 budget, removing HOx radicals whilst driving NOx/y partitioning chemistry by yielding NO3 radicals: OH + HNO3 → H2O + NO3. In this paper, k1(T, P) was measured using OH (A ← X) Laser Induced Fluorescence (LIF) and the data was modelled over the 223-298 K temperature and 25-750 Torr pressure ranges, using the modified Lindemann-Hinshelwood expression , where k0 = 5.2 × 10-14 exp(200/T) cm3 s-1, k2 = 8.

Direct kinetics study of CHOO + methyl vinyl ketone and CHOO + methacrolein reactions and an upper limit determination for CHOO + CO reaction.

Methyl vinyl ketone (MVK) and methacrolein (MACR) are important intermediate products in atmospheric degradation of volatile organic compounds, especially of isoprene. This work investigates the reactions of the smallest Criegee intermediate, CH2OO, with its co-products from isoprene ozonolysis, MVK and MACR, using multiplexed photoionization mass spectrometry (MPIMS), with either tunable synchrotron radiation from the Advanced Light Source or Lyman-α (10.2 eV) radiation for photoionization.