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Nighttime NO3-initiated oxidation of biogenic volatile organic compounds (BVOCs) such as monoterpenes is important for the atmospheric formation and growth of secondary organic aerosol (SOA), which has significant impact on climate, air quality, and human health. In such SOA formation and growth, highly oxygenated organic molecules (HOM) may be crucial, but their formation pathways and role in aerosol formation have yet to be clarified. Among monoterpenes, limonene is of particular interest for its high emission globally and high SOA yield. In this work, HOM formation in the reaction of limonene with nitrate radical (NO3) was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). About 280 HOM products were identified, grouped into 19 monomer families, 11 dimer families, and 3 trimer families. Both closed-shell products and open-shell peroxy radicals (RO2⚫) were observed, and many of them have not been reported previously. Monomers and dimers accounted for 47 % and 47 % of HOM concentrations, respectively, with trimers making up the remaining 6 %. In the most abundant monomer families, C10H15−17NO6−14, carbonyl products outnumbered hydroxyl products, indicating the importance of RO2⚫ termination by unimolecular dissociation. Both RO2⚫ autoxidation and alkoxy–peroxy pathways were found to be important processes leading to HOM. Time-dependent concentration profiles of monomer products containing nitrogen showed mainly second-generation formation patterns. Dimers were likely formed via the accretion reaction of two monomer RO2⚫, and HOM-trimers via the accretion reaction between monomer RO2⚫ and dimer RO2⚫. Trimers are suggested to play an important role in new particle formation (NPF) observed in our experiment. A HOM yield of 1.5%-0.7%+1.7% was estimated considering only first-generation products. SOA mass growth could be reasonably explained by HOM condensation on particles assuming irreversible uptake of ultra-low volatility organic compounds (ULVOCs), extremely low volatility organic compounds (ELVOCs), and low volatility organic compounds (LVOCs). This work provides evidence for the important role of HOM formed via the limonene +NO3 reaction in NPF and growth of SOA particles.

Reaction with ozone is an important atmospheric removal process for alkenes. The ozonolysis reaction produces carbonyls and carbonyl oxides (Criegee intermediates, CI), which can rapidly decompose to yield a range of closed shell and radical products, including OH radicals. Consequently, it is essential to accurately represent the complex chemistry of Criegee intermediates in atmospheric models in order to fully understand the impact of alkene ozonolysis on atmospheric composition. A mechanism construction protocol is presented which is suitable for use in automatic mechanism generation. The protocol defines the critical parameters for describing the chemistry following the initial reaction, namely the primary carbonyl/CI yields from the primary ozonide fragmentation, the amount of stabilisation of the excited CI, the unimolecular decomposition pathways, rates and products of the CI, and the bimolecular rates and products of atmospherically important reactions of the stabilised CI (SCI). This analysis implicitly predicts the yield of OH from the alkene–ozone reaction. A comprehensive database of experimental OH, SCI and carbonyl yields has been collated using reported values in the literature and used to assess the reliability of the protocol. The protocol provides estimates of OH, SCI and carbonyl yields with root mean square errors of 0.13 and 0.12 and 0.14, respectively. Areas where new experimental and theoretical data would improve the protocol and its assessment are identified and discussed.

The oxidation of carbonyl sulfide (OCS) is the primary, continuous source of stratospheric sulfate aerosol particles, which can scatter shortwave radiation and catalyze heterogeneous reactions in the stratosphere. While it has been estimated that the oxidation of dimethyl sulfide (DMS), emitted from the surface ocean, accounts for 8-20% of the global OCS source, there is no existing DMS oxidation mechanism relevant to the marine atmosphere that is consistent with an OCS source of this magnitude. We describe new laboratory measurements and theoretical analyses of DMS oxidation that provide a mechanistic description for OCS production from hydroperoxymethyl thioformate (HPMTF), an ubiquitous, soluble DMS oxidation product.The mechanism for OCS formation from DMS + OH is found to proceed through several intermediate stages, including secondary OH-initiated oxidation of hydroperoxymethyl thioformate (HOOCH2SCH=O), thioperformic anhydride (O=CHSCH=O), and thioperformic acid (HOOCH=S and HOSCH=O). Several of these reactions are affected by chemical activation, leading to prompt product formation. A theoretical kinetic analysis of these reactions and of conditions representative of the marine boundary layer shows several potential OCS formation channels, which combined lead to a high yield of OCS under OH-initiated oxidation of DMS.We incorporate this chemical mechanism into a global chemical transport model, showing that OCS production from DMS is a factor of 3 smaller than current estimates and displays a maximum in the tropics consistent with field observations. A critical factor in the conversion of DMS to OCS is the heterogeneous loss of the soluble intermediates, making the OCS yield sensitive to multiphase cloud chemistry and reducing the total OCS formation.

The gas-phase reaction of isoprene with the nitrate radical (NO3) was investigated in experiments in the outdoor SAPHIR chamber under atmospherically relevant conditions specifically with respect to the chemical lifetime and fate of nitrato-organic peroxy radicals (RO2). Observations of organic products were compared to concentrations expected from different chemical mechanisms: (1) the Master Chemical Mechanism, which simplifies the NO3 isoprene chemistry by only considering one RO2 isomer; (2) the chemical mechanism derived from experiments in the Caltech chamber, which considers different RO2 isomers; and (3) the FZJ-NO3 isoprene mechanism derived from quantum chemical calculations, which in addition to the Caltech mechanism includes equilibrium reactions of RO2 isomers, unimolecular reactions of nitrate RO2 radicals and epoxidation reactions of nitrate alkoxy radicals. Measurements using mass spectrometer instruments give evidence that the new reactions pathways predicted by quantum chemical calculations play a role in the NO3 oxidation of isoprene. Hydroperoxy aldehyde (HPALD) species, which are specific to unimolecular reactions of nitrate RO2, were detected even in the presence of an OH scavenger, excluding the possibility that concurrent oxidation by hydroxyl radicals (OH) is responsible for their formation. In addition, ion signals at masses that can be attributed to epoxy compounds, which are specific to the epoxidation reaction of nitrate alkoxy radicals, were detected. Measurements of methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations confirm that the decomposition of nitrate alkoxy radicals implemented in the Caltech mechanism cannot compete with the ring-closure reactions predicted by quantum chemical calculations. The validity of the FZJ-NO3 isoprene mechanism is further supported by a good agreement between measured and simulated hydroxyl radical (OH) reactivity. Nevertheless, the FZJ-NO3 isoprene mechanism needs further investigations with respect to the absolute importance of unimolecular reactions of nitrate RO2 and epoxidation reactions of nitrate alkoxy radicals. Absolute concentrations of specific organic nitrates such as nitrate hydroperoxides would be required to experimentally determine product yields and branching ratios of reactions but could not be measured in the chamber experiments due to the lack of calibration standards for these compounds. The temporal evolution of mass traces attributed to product species such as nitrate hydroperoxides, nitrate carbonyl and nitrate alcohols as well as hydroperoxy aldehydes observed by the mass spectrometer instruments demonstrates that further oxidation by the nitrate radical and ozone at atmospheric concentrations is small on the timescale of one night (12 h) for typical oxidant concentrations. However, oxidation by hydroxyl radicals present at night and potentially also produced from the decomposition of nitrate alkoxy radicals can contribute to their nocturnal chemical loss.

Oxidation of isoprene by nitrate radicals (NO3) or by hydroxyl radicals (OH) under high NOx conditions forms a substantial amount of organonitrates (ONs). ONs impact NOx concentrations and consequently ozone formation while also contributing to secondary organic aerosol. Here we show that the ONs with the chemical formula C4H7NO5 are a significant fraction of isoprene-derived ONs, based on chamber experiments and ambient measurements from different sites around the globe. From chamber experiments we found that C4H7NO5 isomers contribute 5%–17% of all measured ONs formed during nighttime and constitute more than 40% of the measured ONs after further daytime oxidation. In ambient measurements C4H7NO5 isomers usually dominate both nighttime and daytime, implying a long residence time compared to C5 ONs which are removed more rapidly. We propose potential nighttime sources and secondary formation pathways, and test them using a box model with an updated isoprene oxidation scheme.

The reactions of biogenic volatile organic compounds (BVOC) with the nitrate radicals (NO

Based on the first measurements of gas-phase pyruvic acid (CH3C(O)C(O)OH) in the boreal forest, we derive effective emission rates of pyruvic acid and compare them with monoterpene emission rates over the diel cycle. Using a data-constrained box model, we determine the impact of pyruvic acid photolysis on the formation of acetaldehyde (CH3CHO) and the peroxy radicals CH3C(O)O2 and HO2 during an autumn campaign in the boreal forest. The results are dependent on the quantum yield (φ) and mechanism of the photodissociation of pyruvic acid and the fate of a likely major product, methylhydroxy carbene (CH3COH). With the box model, we investigate two different scenarios in which we follow the present IUPAC (IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation, 2021) recommendations with φ = 0.2 (at 1 bar of air), and the main photolysis products (60 %) are acetaldehyde + CO2 with 35 % C–C bond fission to form HOCO and CH3CO (scenario A). In the second scenario (B), the formation of vibrationally hot CH3COH (and CO2) represents the main dissociation pathway at longer wavelengths (∼ 75 %) with a ∼ 25 % contribution from C–C bond fission to form HOCO and CH3CO (at shorter wavelengths). In scenario 2 we vary φ between 0.2 and 1 and, based on the results of our theoretical calculations, allow the thermalized CH3COH to react with O2 (forming peroxy radicals) and to undergo acid-catalysed isomerization to CH3CHO. When constraining the pyruvic acid to measured mixing ratios and independent of the model scenario, we find that the photolysis of pyruvic acid is the dominant source of CH3CHO with a contribution between ∼ 70 % and 90 % to the total production rate. We find that the photolysis of pyruvic acid is also a major source of the acetylperoxy radical, with contributions varying between ∼ 20 % and 60 % dependent on the choice of φ and the products formed. HO2 production rates are also enhanced, mainly via the formation of CH3O2. The elevated production rates of CH3C(O)O2 and HO2 and concentration of CH3CHO result in significant increases in the modelled mixing ratios of CH3C(O)OOH, CH3OOH, HCHO, and H2O2.

The reactions of carbonyl oxide Criegee intermediates with acids proceed predominantly by an insertion mechanism. We characterise the products from one of the simplest reactions of carbonyl oxides with inorganic acids, CH2OO + hydrogen chloride, which occurs via a 1,2-insertion in the H–Cl bond. Reactions of both HCl and DCl isotopologues yield product signal at the mass of the insertion product chloro(hydroperoxy)methane and a dissociative ionisation peak at the mass of the protonated (or deuteronated) Criegee intermediate. The isotopic composition of the insertion product has been measured for reaction mixtures where both HCl isotopologues are present, and the H/D ratio of the product is consistently higher (by a factor of 1.6 ± 0.3) than that of the reactants. This isotope selectivity in the products has smaller uncertainty than the ratio of measured rate coefficients and suggests a normal (kH > kD) kinetic isotope effect in the reaction. Theoretical kinetics calculations predict a small normal kinetic isotope effect for the overall reaction (kH / kD = 1.35 at 20 Torr N2 and kH / kD = 1.2 at 1 atm N2) but predict a substantial inverse kinetic isotope effect (kD > kH) for the stabilisation fraction, in disagreement with the experimental observation.

Experiments at atmospherically relevant conditions were performed in the simulation chamber SAPHIR, investigating the reaction of isoprene with NO3 and its subsequent oxidation. Due to the production of NO3 from the reaction of NO2 with O3 as well as the formation of OH in subsequent reactions, the reactions of isoprene with O3 and OH were estimated to contribute up to 15% of the total isoprene consumption each in these experiments. The ratio of RO2 to HO2 concentrations was varied by changing the reactant concentrations, which modifies the product distribution from bimolecular reactions of the nitrated RO2. The reaction with HO2 or NO3 was found to be the main bimolecular loss process for the RO2 radicals under all conditions examined.Yields of the first-generation isoprene oxygenated nitrates as well as the sum of methyl vinyl ketone (MVK) and methacrolein (MACR) were determined by high resolution proton mass spectrometry using the Vocus PTR-TOF. The experimental time series of these products are compared to model calculations based on the MCM v3.3.1,1 the isoprene mechanism as published by Wennberg et al.2 and the newly developed FZJ-NO3-isoprene mechanism,3 which incorporates theory-based rate coefficients for a wide range of reactions.Among other changes, the FZJ-NO3-isoprene mechanism contains a novel fast oxidation route through the epoxidation of alkoxy radicals, originating from the formation of nitrated peroxy radicals. This inhibits the formation of MVK and MACR from the NO3-initiated oxidation of isoprene to practically zero, which agrees with the observations from chamber experiments. In addition, the FZJ-NO3-isoprene mechanism increases the level of agreement for the main first-generation oxygenated nitrates. 1 M. E. Jenkin, J. C. Young and A. R. Rickard, The MCM v3.3.1 degradation scheme for isoprene, Atmospheric Chem. Phys., 2015, 15, 11433–11459.2 P. O. Wennberg at al., Gas-Phase Reactions of Isoprene and Its Major Oxidation Products, Chem. Rev., 2018, 118, 3337–3390. 3 L. Vereecken et al., Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene, Phys. Chem. Chem. Phys., submitted.

International audience; The role of water vapor (H2O) and its hydrogen-bonded complexes in the gas-phase reactivity of organic compounds with hydroxyl (OH) radicals has been the subject of many recent studies. Contradictory effects have been reported at temperatures between 200 and 400 K. For the OH + acetaldehyde reaction, a slight catalytic effect of H2O was previously reported at temperatures between 60 and 118 K. In this work, we used Laval nozzle expansions to reinvestigate the impact of H2O on the OH-reactivity with acetaldehyde between 21.7 and 135.0 K. The results of this comprehensive study demonstrate that water, instead, slows down the reaction by factors of ∼3 (21.7 K) and ∼2 (36.2-89.5 K), and almost no effect of added H2O was observed at 135.0 K. © 2021 Author(s).

The reactions of biogenic volatile organic compounds (BVOC) with the nitrate radicals (NO3) are major night-time sources of organic nitrates and secondary organic aerosols (SOA) in regions influenced by BVOC and anthropogenic emissions. In this study, the formation of gas-phase highly oxygenated organic molecules-organic nitrates (HOM-ON) from NO3-initiated oxidation of a representative monoterpene, β-pinene, was investigated in the SAPHIR chamber (Simulation of Atmosphere PHotochemistry In a large Reaction chamber). Six monomer (C = 7-10, N = 1-2, O = 6-16) and five accretion product (C = 17-20, N = 2-4, O = 9-22) families were identified and further classified into first- or second-generation products based on their temporal behavior. The time lag observed in the peak concentrations between peroxy radicals containing odd and even number of oxygen atoms, as well as between radicals and their corresponding termination products, provided constraints on the HOM-ON formation mechanism. The HOM-ON formation can be explained by unimolecular or bimolecular reactions of peroxy radicals. A dominant portion of carbonylnitrates in HOM-ON was detected, highlighting the significance of unimolecular termination reactions by intramolecular H-shift for the formation of HOM-ON. A mean molar yield of HOM-ON was estimated to be 4.8% (-2.6%/+5.6%), suggesting significant HOM-ON contributions to the SOA formation.

Terpenoids are an important class of multi-unsaturated volatile organic compounds emitted to the atmosphere. During their oxidation in the troposphere, unsaturated peroxy radicals are formed, which may undergo ring closure reactions by an addition of the radical oxygen atom on either of the carbons in the C[double bond, length as m-dash]C double bond. This study describes a quantum chemical and theoretical kinetic study of the rate of ring closure, finding that the reactions are comparatively fast with rates often exceeding 1 s-1 at room temperature, making these reactions competitive in low-NOx environments and allowing for continued autoxidation by ring closure. A structure-activity relationship (SAR) is presented for 5- to 8-membered ring closure in unsaturated RO2 radicals with aliphatic substituents, with some analysis of the impact of oxygenated substituents. H-migration in the cycloperoxide peroxy radicals formed after the ring closure was found to be comparatively slow for unsubstituted RO2 radicals. In the related cycloperoxide alkoxy radicals, migration of H-atoms implanted on the ring was similarly found to be slower than for non-cyclic alkoxy radicals and is typically not competitive against decomposition reactions that lead to cycloperoxide ring breaking. Ring closure reactions may constitute an important reaction channel in the atmospheric oxidation of terpenoids and could promote continued autoxidation, though the impact is likely to be strongly dependent on the specific molecular backbone.

Highly oxygenated organic molecules (HOM) are found to play an important role in the formation and growth of secondary organic aerosol (SOA). SOA is an important type of aerosol with significant impact on air quality and climate. Compared with the oxidation of volatile organic compounds by O3 and OH, HOM formation in the oxidation by NO3 radical, an important oxidant at night-time and dawn, has received less attention. In this study, HOM formation in the reaction of isoprene with NO3 was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). A large number of HOM including monomers (C5), dimers (C10), and trimers (C15), both closed-shell compounds and open-shell peroxy radicals, were identified and were classified into various series according to their formula. Their formation pathways were proposed based on the peroxy radicals observed and known mechanisms in the literature, which were further constrained by the time profiles of HOM after sequential isoprene addition to differentiate first- and second-generation products. HOM monomers containing one to three N atoms (1–3N monomers) were formed, starting with NO3 addition to carbon double bond, forming peroxy radicals (RO2), followed by autoxidation. 1N monomers were formed by both the direct reaction of NO3 with isoprene and of NO3 with first-generation products. 2N-monomers (e.g. C5H8N2On (n = 8–13), C5H10N2On (n = 8–14)) were likely the termination products of C5H9N2On•, which was formed by the addition of NO3 to C5-hydroxynitrate (C5H9NO4), a first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for ~34 % of all HOM, indicating the important role of second-generation oxidation in HOM formation in isoprene+NO3 under our reaction conditions. H-shift of alkoxy radicals to form peroxy radicals and subsequent autoxidation (alkoxy-peroxy pathway) was found to be an important pathway of HOM formation. HOM dimers were mostly formed by the accretion reaction of various HOM monomer RO2 and via the termination reactions of dimer RO2 formed by further reaction of closed-shell dimers with NO3 and possibly by the reaction of C5-RO2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO2 with monomer RO2. The concentrations of different HOM showed distinct time profiles during the reaction, which was linked to their formation pathway. HOM concentrations either showed a typical time profile of first-generation products, or of second-generation products, or a combination of both, indicating multiple formation pathways and/or multiple isomers. Total HOM molar yield was estimated to be 1.2 %+1.3 %−0.7 %, which corresponded to a SOA yield of ~3.6 % assuming the molecular weight of C5H9NO6 as the lower limit. This yield suggests that HOM may contribute a significant fraction to SOA yield in the reaction of isoprene with NO3.

Highly oxygenated organic molecules (HOM) are found to play an important role in the formation and growth of secondary organic aerosol (SOA). SOA is an important type of aerosol with significant impact on air quality and climate. Compared with the oxidation of volatile organic compounds by O3 and OH, HOM formation in the oxidation by NO3 radical, an important oxidant at night-time and dawn, has received less attention. In this study, HOM formation in the reaction of isoprene with NO3 was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). A large number of HOM including monomers (C5), dimers (C10), and trimers (C15), both closed-shell compounds and open-shell peroxy radicals, were identified and were classified into various series according to their formula. Their formation pathways were proposed based on the peroxy radicals observed and known mechanisms in the literature, which were further constrained by the time profiles of HOM after sequential isoprene addition to differentiate first- and second-generation products. HOM monomers containing one to three N atoms (1–3N monomers) were formed, starting with NO3 addition to carbon double bond, forming peroxy radicals (RO2), followed by autoxidation. 1N monomers were formed by both the direct reaction of NO3 with isoprene and of NO3 with first-generation products. 2N-monomers (e.g. C5H8N2On (n = 8–13), C5H10N2On (n = 8–14)) were likely the termination products of C5H9N2On•, which was formed by the addition of NO3 to C5-hydroxynitrate (C5H9NO4), a first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for ~34 % of all HOM, indicating the important role of second-generation oxidation in HOM formation in isoprene+NO3 under our reaction conditions. H-shift of alkoxy radicals to form peroxy radicals and subsequent autoxidation (alkoxy-peroxy pathway) was found to be an important pathway of HOM formation. HOM dimers were mostly formed by the accretion reaction of various HOM monomer RO2 and via the termination reactions of dimer RO2 formed by further reaction of closed-shell dimers with NO3 and possibly by the reaction of C5-RO2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO2 with monomer RO2. The concentrations of different HOM showed distinct time profiles during the reaction, which was linked to their formation pathway. HOM concentrations either showed a typical time profile of first-generation products, or of second-generation products, or a combination of both, indicating multiple formation pathways and/or multiple isomers. Total HOM molar yield was estimated to be 1.2 %+1.3 %−0.7 %, which corresponded to a SOA yield of ~3.6 % assuming the molecular weight of C5H9NO6 as the lower limit. This yield suggests that HOM may contribute a significant fraction to SOA yield in the reaction of isoprene with NO3.

Peracetic acid (CH3C(O)OOH) is one of the most abundant organic peroxides in the atmosphere, yet the kinetics of its reaction with OH, believed to be the major sink, have been studied only once experimentally. In this work we combine a pulsed-laser photolysis kinetic study of the title reaction with theoretical calculations of the rate coefficient and mechanism. We demonstrate that the rate coefficient is orders of magnitude lower than previously determined, with an experimentally derived upper limit of ≤ 4 × 10−14 cm3 molecule−1 s−1. The relatively low rate coefficient is in good agreement with the theoretical result of 3 × 10−14 cm3 molecule−1 s−1 at 298 K, increasing to ~ 6 × 10−14 in the cold upper troposphere, but with associated uncertainty of a factor-two. The reaction proceeds mainly via abstraction of the peroxidic-hydrogen via a relatively weakly bonded and short-lived pre-reaction complex, in which H-abstraction occurs only slowly due to a high barrier and low tunneling probabilities. Our results imply that the lifetime of CH3C(O)OOH with respect to OH-initiated degradation in the atmosphere is of the order of one year (and not days as previously believed) and that its major sink in the free and upper troposphere is likely to be photolysis, with dry-deposition important in the boundary layer. Similar conclusions can be made for other, saturated peroxy-acids.

Criegee intermediates (CIs) are formed in the ozonolysis of unsaturated hydrocarbons and play a role in atmospheric chemistry as a non-photolytic OH source or a strong oxidant. Using a relative rate method in an ozonolysis experiment, Newland et al. (2015) reported high reactivity of isoprene-derived Criegee intermediates towards dimethyl sulfide (DMS) relative to that towards SO2 with the ratio of the rate coefficients kDMS+CI/kSO2+CI = 3.5 ± 1.8. Here we reinvestigated the kinetics of DMS reactions with two major Criegee intermediates formed in isoprene ozonolysis, CH2OO, and methyl vinyl ketone oxide (MVKO). The individual CI was prepared following the reported photolytic method with suitable (diiodo) precursors in the presence of O2. The concentration of CH2OO or MVKO was monitored directly in real time through their intense UV–visible absorption. Our results indicate the reactions of DMS with CH2OO and MVKO are both very slow; the upper limits of the rate coefficients are 4 orders of magnitude smaller than the rate coefficient reported by Newland et al. (2015) These results suggest that the ozonolysis experiment could be complicated such that interpretation should be careful and these CIs would not oxidize atmospheric DMS at any substantial level.

The oxidation of most organic matter emitted to the atmosphere proceeds by radical reaction steps, where peroxy radicals, ROO•, are critical intermediates formed by addition of O2 molecules to carbon-based radicals. The chemistry of these RO2 radicals in high-NOx conditions is well-known, forming alkoxy radicals and NO2. In low-NOx and pristine conditions, the RO2 radicals react with HO2 and other R'O2 radicals, but can have a sufficiently long lifetime to also undergo unimolecular reactions. Hydrogen atom migration, forming a hydroperoxide (-OOH) and a new peroxy radical site after addition of an additional O2 on the newly formed radical site, has been studied extensively in some compounds, such as isoprene where it was shown to be the a critical step in OH radical regeneration. RO2 ring closure reactions have likewise been studied, where for β-pinene it has been shown to be a critical step governing the yield of the decomposition products such as acetone and nopinone.Despite the interest in RO2 unimolecular reactions, and the potential impact on atmospheric chemistry, no widely applicable structure-activity relationships (SARs) have been proposed to allow systematic incorporation of such unimolecular reactions in gas phase atmospheric kinetic models. In this work, we present a series of systematic theoretical predictions on the site-specific rate coefficients for such reactions for a wide range of molecular substitutions. Combined with extensive literature data this allows for the formulation of a SAR for RO2 unimolecular reactions, covering aliphatic, branched, and unsaturated RO2 with oxo, hydroxy, hydroperoxy, nitrate, carboxylic acid, and ether substitutions.The predictions are compared to experimental and theoretical data, including multi-functionalized species. Though some molecular classes are well represented in the training set (e.g. aliphatic RO2), other classes have little data available and additional work is needed to enhance and validate the reliability of the SAR. Direct experimental data is scarce for all RO2 classes. The fastest H-migrations are found to be for unsaturated RO2, with the double bond outside the H-migration TS ring. Ring closure of unsaturated RO2 are likewise fast if the product radical carbon is exocyclic to the newly formed peroxide ring.

The impact of chemical loss processes of isocyanic acid was studied by a combined theoretical and modeling study. The potential energy surfaces of the reactions of HNCO with OH and NO3 radicals, Cl atoms, and ozone, were studied using high-level CCSD(T)/CBS(DTQ)//M06-2X/aug-cc-pVTZ quantum chemical methodologies, followed by TST theoretical kinetic predictions of the rate coefficients at temperatures of 200–3000 K. It was found that the reactions are all slow in atmospheric conditions, with k(300 K) ≤ 7 × 10−16 cm3 molecule−1 s−1; the predictions are in good agreement with earlier experimental work, where available. The reverse reactions of NCO radicals, of importance mostly in combustion, were also examined briefly. The global model confirms that gas phase chemical loss of HNCO is a negligible process, contributing less than 1 %. Removal of HNCO by clouds and precipitation is a larger sink, contributing for about 10 % of the total loss, while globally dry deposition is the main sink, accounting for ~ 90 %. The global simulation also shows that due to its long chemical lifetime in the free troposphere, HNCO can be efficiently transported into the UTLS by deep convection events. Average daily concentrations of HNCO are found to rarely exceed levels considered potentially toxic, though locally instantaneous toxic levels are expected.the free troposphere, HNCO can be efficiently transported into the UTLS by deep convection events. Average daily concentrations of HNCO are found to rarely exceed levels considered potentially toxic, though locally instantaneous toxic levels are expected.

A large data set of rate coefficients for H-migration in peroxy radicals is presented, and supplemented with literature data to derive a structure-activity relationship (SAR), for the title reaction class. The SAR supports aliphatic RO2 radicals, as well as unsaturated bonds and β-oxo substitution both endo- and exo-cyclic to the transition state ring, and α‑oxo (aldehyde), −OH, −OOH, and −ONO2 substitutions, including migration of O-based hydrogen atoms. Also discussed are −C(= O)OH and −OR substitutions. The SAR allows predictions of rate coefficients k(T) for a temperature range of 200 to 450 K, with migrations spans ranging from 1,4 to 1,9-H-shifts depending on the functionalities. The performance of the SAR reflects the uncertainty of the underlying data, reproducing the scarce experimental data on average to a factor of 2, and the wide range of theoretical data to a factor 10 to 100 depending also on the quality of the data. The SAR evaluation discusses the performance in multi-functionalized species. For aliphatic RO2, we also present some experimental product identification that validates the expected mechanisms. The proposed SAR is a valuable tool for mechanism development, experimental design, and guides future theoretical work, which should allow rapid improvements of the SAR in the following years; relative multi-conformer TST (rel-MC-TST) kinetic theory is introduced as an aid for systematic kinetic studies.

This perspective gives our views on general aspects and future directions of gas‐phase atmospheric chemical kinetic mechanism development, emphasizing on the work needed for the sustainable development of chemically detailed mechanisms that reflect current kinetic, mechanistic, and theoretical knowledge. Current and future mechanism development efforts and research needs are discussed, including software‐aided autogeneration and maintenance of kinetic models as a future‐proof approach for atmospheric model development. There is an overarching need for the evaluation and extension of structure‐activity relationships (SARs) that predict the properties and reactions of the many multifunctionalized compounds in the atmosphere that are at the core of detailed mechanisms, but for which no direct chemical data are available. Here, we discuss the experimental and theoretical data needed to support the development of mechanisms and SARs, the types of SARs relevant to atmospheric chemistry, the current status and limitations of SARs for various types of atmospheric reactions, the status of thermochemical estimates needed for mechanism development, and our outlook for the future. The authors have recently formed a SAR evaluation working group to address these issues.

The reaction of fulminic acid, HCNO, with NH2 radicals was studied using quantum chemical and theoretical kinetic methodologies. B3LYP/6-311++G(3df,2p) calculations combined with CCSD(T) energy calculations at the basis set limit reveal a complex potential energy surface, where only two entrance channels contribute significantly to the product formation. Transition state theory and RRKM master equation calculations find a rate coefficient ranging from 7.2 × 10-12 cm3 molecule−1 s−1 at room temperature, to > 1 × 10−10 cm3 molecule−1 s−1 at 3000 K. Despite a reduced efficiency in product formation due to fast redissociation of the adducts to the reactants at high temperatures, the title reaction can thus be an efficient sink for HCNO at combustion temperatures in nitrogen-rich environments. At 1 bar and below, H2NCO + NO is the dominant product, with H2NCN + OH and HCN + NHOH contributing weakly. This work presents rate coefficients and product distributions for the temperature range 300–3000 K, and pressure range of 10−3–103 bar; a brute-force error analysis examines the expected uncertainty interval for these predictions.

The reaction mechanism of the reaction of the Criegee intermediate CH2OO with NO2 was investigated using quantum chemical and theoretical kinetic methodologies. The reaction shows a rich chemistry, though the number of channels that effectively contribute at room temperature is limited. The theoretical characterization of the entrance transition states was hampered by strongly multireference wave functions. The predicted rate coefficient k(298 K) = 4.4 × 10−12 cm3 molecule−1 s−1 thus carries a large uncertainty, but is in agreement with literature data. We find that the CH2OO + NO2 reaction reacts by adduct formation, near-exclusively forming nitro-peroxy radicals, •OOCH2NO2. These will react as other alkylperoxy radicals in the atmosphere, ultimately generating CH2O and regenerating NO2 in most reaction conditions. The product predictions contrast with earlier experimental work showing NO3 formation, but support other observations of adduct products.

The stable carbon kinetic isotope effect (KIE) of ethane photo-oxidation by OH radicals was deduced by employing both laboratory measurements and theoretical calculations. The investigations were designed to elucidate the temperature dependence of KIE within atmospherically relevant temperature range. The experimental KIE were derived from laboratory compound specific isotope analyses of ethane with natural isotopic abundance exposed to OH at constant temperature, showing e values of 7.16 ± 0.54 ‰ (303 K), 7.45 ± 0.48 ‰ (288 K), 7.36 ± 0.28 ‰ (273 K), 7.61 ± 0.28 ‰ (263 K), 8.89 ± 0.90‰ (253 K), and 9.42 ± 2.19% (243 K). Compared to previous studies, a significant improvement of the measurement precision was reached at the high end of the investigated temperature range. The KIE was theoretically determined as well, in the temperature range of 150 K to 400 K, by calculating the reaction rate coefficients of 12C and singly 13C substituted ethane isotopologues applying chemical quantum mechanics together with transition state theory. Tunneling effect and internal rotations were also considered. The agreement between experimental and theoretical results for rate coefficients and KIE in an atmospherically relevant temperature range is discussed. However, both laboratory observations and computational predictions show no significant temperature dependence of the KIE for the ethane oxidation by OH radicals.

Photolysis of NO2 is the only major pathway for O3 formation as products from the reaction of OH and NO under atmospheric conditions in competition to the formation of HONO has been investigated experimentally and theoretically. Experiments have been carried out by directly measuring the formation of HO2 radicals using laser photolysis coupled to cw-CRDS. OH radicals have been generated from the reaction of F atoms with H2O, and absolute HO2 and OH profiles have been recorded at different NO concentrations. The potential energy surface has been calculated and the rate constant has been obtained from RRKM master equation modeling. Both experiment and theory show that the OH + NO reaction in the presence of O2 bath gas is not a competitive source of HO2 + NO2.

The reaction of CH2OO, the smallest carbonyl oxide (Criegee intermediate, CI), with several acids was investigated using the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ quantum chemical method, as well as microvariational transition state theory and RRKM master equation theoretical kinetic methodologies. For oxoacids HNO3 and HCOOH, a 1,4-insertion mechanism allows for barrierless reactions with high rate coefficients, in agreement with literature experimental data. This mechanism relies on the presence of a double bond in the α-position to the acidic OH group. We predict that reactions of CI with enols will likewise have high rate coefficients, proceeding through a similar mechanism. The hydracid HCl was found to react through a less favorable 1,2-insertion reaction, leading to lower rate coefficients, again in good agreement with the literature. We conclude that the reaction mechanism is the main indicator for the reaction rate for CH2OO + acid reactions, with acidity only of secondary influence. At room temperature and 1 atm the main product for all reactions was found to be the thermalized hydroperoxide initial adduct, with minor yields of fragmentation products. One of the product channels characterized is a novel reaction path involving intramolecular H-abstraction after a roaming reaction in the OH + product radical complex formed by the dissociation of the hydroperoxide adduct; this channel is the lowest fragmentation route for some of the reactions studied.

Recent advances in the spectroscopy of Criegee intermediates (CI) have enabled direct kinetic studies of these highly reactive chemical species. The impact of CI chemistry is currently being incorporated into atmospheric models, including their reactions with trace organic and inorganic compounds. Isoprene, C5H8, is a doubly-unsaturated hydrocarbon that accounts for the largest share of all biogenic emissions around the globe and is also a building block of larger volatile organic compounds. We report direct measurements of the reaction of the simplest CI (CH2OO) with isoprene, using time-resolved cavity-enhanced UV absorption spectroscopy. We find the reaction to be pressure-independent between 15–100 Torr, with a rate coefficient that varies from (1.5 ± 0.1) × 10−15 cm3 molecule−1 s−1 at room temperature to (23 ± 2) × 10−15 cm3 molecule−1 s−1 at 540 K. Quantum chemical and transition-state theory calculations of 16 unique channels for CH2OO + isoprene somewhat underpredict the observed T-dependence of the total reaction rate coefficient, but are overall in good agreement with the experimental measurements. This reaction is broadly similar to those with smaller alkenes, proceeding by 1,3-dipolar cycloaddition to one of the two conjugated double bonds of isoprene.

Theoretical, laboratory and chamber studies have shown fast regeneration of hydroxyl radical (OH) in the photochemistry of isoprene largely due to previously disregarded unimolecular reactions which were previously thought not to be important under atmospheric conditions. Based on early field measurements, nearly complete regeneration was hypothesized for a wide range of tropospheric conditions, including areas such as the rainforest where slow regeneration of OH radicals is expected due to low concentrations of nitric oxide (NO). In this work the OH regeneration in the isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospheric conditions (i.e. NO between 0.15 and 2 ppbv and temperature between 25 and 41°C) in the atmospheric simulation chamber SAPHIR. These conditions cover remote areas partially influenced by anthropogenic NO emissions, giving a regeneration efficiency of OH close to one, and areas like the Amazonian rainforest with very low NO, resulting in a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3 higher than explainable in the absence of unimolecular reactions. The measured radical concentrations were compared to model calculations and the best agreement was observed when at least 50% of the total loss of isoprene peroxy radicals conformers (weighted by their abundance) occurs via isomerization reactions for NO lower than 0.2 parts per billion (ppbv). For these levels of NO, up to 50% of the OH radicals are regenerated from the products of the 1,6 α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO2 radicals through photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehyde hydrogen shift (rate constant ~10 s-1 at 300K) in di-hydroperoxy carbonyl peroxy radicals (di-HPCARP-RO2), depending on their relative yield. The agreement between all measured and modelled trace gases (hydroxyl, hydroperoxy and organic peroxy radicals, carbon monoxide and the sum of methyl vinyl ketone, methacrolein and hydroxyl hydroperoxides) is nearly independent on the adopted yield of HPALD and di-HPCARP-RO2 as both degrade relatively fast (< 1 h), forming OH radical and CO among other products. Taking into consideration this and earlier isoprene studies, considerable uncertainties remain on the oxygenated products distribution, which affect radical levels and organic aerosol downwind of unpolluted isoprene dominated regions.

International audience; The reactivity of methanol (CH3OH) toward the hydroxyl (OH) radical was investigated in the temperature range 11.7-177.5 K using the CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique. In the present study, the temperature dependence of the rate coefficient for the OH + CH3OH reaction, k(T), has been revisited and additional experimental and computational data are reported. New kinetic measurements were performed to fill the existing gaps (andlt;22 K, 22-42 K and 88-123 K), reporting k(T andlt; 20 K) for the first time. The lowest temperature ever achieved by a pulsed CRESU has been obtained in this work (11.7 K). k(T) abruptly increases by almost 2 orders of magnitude from 177.5 K to around 100 K. At T andlt; 100 K, this increase is less pronounced, reaching the capture limit at temperatures below 22 K. The pressure dependence of k(T) has been investigated for selected temperatures and gas densities (1.5 × 10 16 to 4.3 × 10 17 cm -3 ), combining our results with those previously reported. No dependence was observed within the experimental uncertainties below 110 K. The high- and low-pressure rate coefficients, k HPL (T) and k LPL (T), were also studied in detail using high-level quantum chemical and theoretical kinetic methodologies, closely reproducing the experimental data between 20 and 400 K. The results suggest that the experimental data are near the high pressure limit at the lowest temperatures, but that the reaction remains a fast and effective source of CH2OH and CH3O at the low pressures and temperatures prevalent in the interstellar medium. © 2019 the Owner Societies.

The autoxidation of organic peroxy radicals (RO2 ) into hydroperoxy-alkyl radicals (QOOH), then hydroperoxy-peroxy radicals (HOOQO2 ) is now considered to be important in the Earth's atmosphere. To avoid mechanistic uncertainties these reactions are best studied by monitoring the radicals. But for the volatile and aliphatic RO2 radicals playing key roles in the atmosphere this has long been an instrumental challenge. This work reports the first study of the autoxidation of aliphatic RO2 radicals and is based on monitoring RO2 and HOOQO2 radicals. The rate coefficients, kiso (s-1 ), were determined both experimentally and theoretically using MC-TST kinetic theory based on CCSD(T)//M06-2X quantum chemical methodologies. The results were in excellent agreement and confirmed that the first H-migration is strongly rate-limiting in the oxidation of non-oxygenated volatile organic compounds (VOCs). At higher relative humidity (2-30 %) water complexes were evidenced for HOOQO2 radicals, which could be an important fate for HOO-substituted RO2 radicals in the atmosphere.

The photolysis of nitrophenols was proposed as a source of reactive radicals and NOx compounds in polluted air. The S0 singlet ground state and T1 first excited triplet state of nitrophenol were investigated to assess the energy dependence of the photofragmentation product distribution as a function of the reaction conditions, based on quantum chemical calculations at the G3SX//M06–2X/aug-cc-pVTZ level of theory combined with RRKM master equation calculations. On both potential energy surfaces, we find rapid isomerization with the aci-nitrophenol isomer, as well as pathways forming NO, NO2, OH, HONO, and H-, and O-atoms, extending earlier studies on the T1 state and in agreement with available work on other nitroaromatics. We find that accessing the lowest photofragmentation channel from the S0 ground state requires only 268 kJ/mol of activation energy, but at a pressure of 1 atm collisional energy loss dominates such that significant fragmentation only occurs at internal energies exceeding 550 kJ/mol, making this surface unimportant for atmospheric photolysis. Intersystem crossing to the T1 triplet state leads more readily to fragmentation, with dissociation occurring at energies of ∼450 kJ/mol above the singlet ground state even at 1 atm. The main product is found to be OH + nitrosophenoxy, followed by formation of hydroxyphenoxy + NO and phenyloxyl + HONO. The predictions are compared against available experimental data.