The reaction of selected aldehydes with the hydroxyl radical (OH), the most important atmospheric oxidant, was studied in a flow tube at 1 atm pressure and a temperature range between 285 and 340 K. OH radicals were produced by flash photolysis of ozone in the flow tube and the time resolved OH concentration was observed by laser-induced fluorescence in a low-pressure measurement cell which sampled air from the flow tube. OH was consumed in the reaction with the aldehyde and could also be regenerated from the 1,4-H-migration of one isomer of the organic peroxy radicals (RO2) formed in the OH reaction with the aldehyde. By simultaneous optimisation of the OH rate coefficient and of the rate coefficient of the 1,4-H-migration, temperature dependent rate coefficients of both reactions could be derived from the observed OH concentration. In addition, high-level theoretical kinetic calculations were carried out to characterize the rate coefficients of the H-migration in aldehydic RO2 radicals, covering OH-substituted and alkylic RO2. A structure-activity relationship (SAR) is presented covering migration spans from 1,4 to 1,20-H-migration for various substitution patterns, showing good agreement with the experimental data.

Secondary organic aerosol (SOA), formed by oxidation of volatile organic compounds, significantly influence air quality and climate. Biogenic highly oxygenated organic molecules (HOM), particularly those formed from monoterpenes, play a key role in SOA formation and growth. As the most important daytime oxidant, hydroxyl radical (OH•) initiated HOM from monoterpenes is believed to be mainly formed via OH addition channel. However, for α-pinene and limonene, we found that the contribution of hydrogen abstraction channel by OH contribute a significantly to HOM formation. We will present our observations and theoretical calculations, showing the role of hydrogen-abstraction and alkoxy radicals for fast autoxidation leading to HOM formation. We also provide formation mechanisms of and yields of HOM, suggesting the non-negligible contribution of the hydrogen abstraction channel to ambient SOA, particularly in OH-rich areas.

Highly oxygenated organic compounds (HOMs) from α-pinene oxidation are of great interest because of their importance in secondary organic aerosol (SOA) formation. Despite intensive investigations, the mechanisms of HOM formation from first-generation peroxy radicals to HOM-peroxy radicals (HOM-RO2⚫) and to HOM-closed shell products are not well understood. One reason is that HOM-alkoxy radicals (HOM-RO⚫) are likely to contribute to the propagation of oxidative radical chains (alkoxy-peroxy pathway) because isomerization of functionalized alkoxy radicals can compete with their fragmentation (and reaction with O2), as shown by theoretical kinetics. However, HOM-RO⚫ reaction steps are difficult to verify in mechanisms. In this work, we have investigated HOM formation by varying the significance of the alkoxy-peroxy pathway as a function of NOx, OH⚫, and CO. HOM-RO⚫ are likely formed with high branching ratios in reactions of HOM-RO2⚫ with peroxy radicals and NO in analogy to simpler alkoxy radicals. We provide experimental evidence that for HOM-RO⚫ the branching into isomerization is about 50 % (±14 %). Thus, HOM-RO⚫ can play a central role in HOM formation, since alkoxy-peroxy pathways can compete with direct autoxidation. We observed significant concentrations of HOM-RO2⚫, despite fast termination by NO, and shifts to higher O/C for HOM-RO2⚫ and termination products with increasing NO. At NO concentrations > 1.5 ppb, the alkoxy-peroxy pathway may even prevail in propagating the oxidative radical chain leading to HOM formation. The increasing sink of HOM-RO2⚫ with increasing concentration of peroxy radicals and NO is compensated by an increasing source via the alkoxy-peroxy pathway. This manuscript is dedicated to Astrid Kiendler-Scharr (deceased, 6 February 2023).

The emission of dimethylsulfide (DMS) is an important source of sulfur in the atmosphere. Its oxidation leads to enhanced particle formation, where OCS is a critical reaction intermediate as it can reach the stratosphere and oxidize to low-volatility H2SO4 acting as a condensation nucleus. The mechanism for OCS formation from DMS is currently understood to proceed through the hydroperoxymethylthioformate intermediate (HOOCH2SCH 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000 00000000 00000000 O, HPMTF), and experimental data indicate that the OH-initiated HPMTF oxidation generates high yields of OCS. The total atmospheric OCS formation is assumed to remain limited due to competition by phase transfer of the soluble HPMTF to water droplets, but the fate of HPMTF, once it transitions to the aqueous phase, remains unclear. In this work, we theoretically study the formation of cyclic thioperhemiacetal isomers of HPMTF both in the gas phase and in acidic aqueous phase, finding that formation of thioperhemiacetal can be rapid when catalyzed by acids. The subsequent oxidation of thioperhemiacetal is shown not to form OCS, but rather lead to formic and thioformic acid, HCOOH + HCOSH. Based on these theoretical predictions we propose that thioperhemiacetal formation is the main loss process blocking OCS formation from HPMTF in the aqueous phase. To complement the models incorporating the OH-initiated HPMTF oxidation, we also theoretically predict the rate coefficients of HPMTF with Cl atoms and NO3 radicals. The reaction with Cl is found to be fast and leads primarily to OCS, while the reaction with NO3 is slow and does not contribute appreciably to HPMTF loss.

Kinetics of the gas phase reaction between the stabilized Criegee intermediate formaldehyde oxide (CH2OO) and nitrogen dioxide (NO2) have been measured using laser flash photolysis of CH2I2/O2/N2/NO2 mixtures coupled with time-resolved broadband ultraviolet absorption spectroscopy. Experiments were performed in N2 under pseudo-first-order conditions at temperatures between 242 and 353 K and pressures in the range 25 to 300 Torr. The kinetics of CH2OO + NO2 are independent of pressure, with a mean rate coefficient of k1 = (1.24 ± 0.22) × 10-12 cm3 s-1 at 298 K, where the uncertainty represents a combination of the 1σ statistical error and the systematic errors resulting from uncertainties in gas flow rates and in the concentration of NO2. Measurements indicate upper limits of <5% for production of NO3 and <5% for production of NO, and further studies of product yields are warranted. In contrast to expectations from theory, the kinetics of CH2OO + NO2 display a negative temperature dependence that can be described by k1 = (1.07 ± 0.02) × 10-12 × (T/298)-(2.9±0.2) cm3 s-1. Analysis using the Master Equation Solver for Multi-Energy well Reactions is able to reproduce a negative temperature dependence for the reaction if significant changes to barrier heights are made, but the overall agreement between the experiment and theory remains poor. This work highlights the challenges associated with calculations for systems with significant multi-reference character.

We present the measurement of the temperature dependent rate coefficient of the OH reaction with methacrolein between 280 and 340 K using an OH reactivity instrument that allows to accurately determine the loss rate of OH reactants. Experiments were performed in synthetic air (presence of oxygen). In the case of methacrolein, OH radicals are regenerated by the 1,4 H-shift reaction of the aldehyde group of the MACR-1-OH-O2 radical which is the main peroxy radicals formed its OH reaction. Therefore, the observed OH decay deviates from a single-exponential decay expected from a pseudo-first order loss reaction. This allows to determine the rate coefficient of the peroxy radical isomerization reaction using model calculations, in which reaction rates are optimized to best describe the observed OH radical decay. This method is one of few direct measurements of atmospherically relevant peroxy radical isomerization reaction coefficients that have been reported in literature so far. Experimentally derived values are compared to quantum-chemical calculations of the 1,4 H-shift reaction rate and to rate coefficients reported in literature for the isomerization reaction and the OH reactions of methacrolein.

The photooxidation of five anthropogenic volatile organic compounds (propane, propene, isopentane, n-hexane, trans-2-hexene) at different levels of nitric oxide (NO) was investigated in the atmospheric simulation chamber SAPHIR, Forschungszentrum Jülich. Measured time series of trace gases and radical concentrations are compared to zero-dimensional box model calculations, based on the Master Chemical Mechanism (agreement within 30%) and complemented by state-of-the-art structure–activity relationships (SAR). Including RO2 isomerization reactions from SAR, validated with theoretical calculations, improves particularly the model–measurement agreement by ∼20% for n-hexane. The photooxidation of the chosen compounds generates different types of peroxy radicals (RO2) which produce HO2 after one or multiple RO2+NO reaction steps, depending on the formed alkoxy radical (RO). Measurements show that the HO2/RO2 ratio is up to ∼40% lower and the number of odd oxygen (Ox = O3+NO2) formed per OH+VOC reaction (P(Ox)VOC) is up to ∼30% higher if RO regenerates RO2 instead of forming HO2 directly. Though, the formation of organic nitrates nearly completely compensates for the ozone production from the second NO reaction step for nitrate yields higher than 20%. Measured and modelled HO2/RO2 ratios agree well as does P(Ox)VOC, derived from measured/modelled radical concentrations and calculated from measured Ox.

The formation of peroxynitrates (RO2NO2) from the reaction of peroxy radicals (RO2) and nitrogen dioxide (NO2) and their subsequent redissociation are typically not included in chemical mechanisms. This is often done to save computational time as the assumption is that the equilibrium is strongly towards the RO2 + NO2 reaction for most conditions. Exceptions are the reactions of the methyl peroxy radical due to its abundance in the atmosphere and of acyl-RO2 radicals due to the long lifetime of peroxyacyl nitrates RO2NO2 (PANs). In this study, the nighttime oxidation of cis-2-butene and trans-2-hexene in the presence of NO2 is investigated in the atmospheric simulation chamber SAPHIR, Forschungszentrum J & uuml;lich, Germany, at atmospherically-relevant conditions at different temperatures (approximate to 276 K, approximate to 293 K, approximate to 305 K). Measured concentrations of peroxy and hydroperoxy radicals as well as other trace gases (ozone, NO2, volatile organic compounds) are compared to state-of-the-art zero-dimensional box model calculations. Good model-measurement agreement can only be achieved when reversible RO2 + NO2 reactions are included for all RO2 species using literature values available from the latest SAR by [Jenkin et al., Atmos. Chem. Phys., 2019, 19, 7691]. The good agreement observed gives confidence that the SAR, derived originally for aliphatic RO2, can be applied to a large range of substituted RO2 radicals, simplifying generalised implementation in chemical models. RO2NO2 concentrations from non-acyl RO2 radicals of up to 2 x 10 cm(-3) are predicted at 276 K, impacting effectively the kinetics of RO2 radicals. Under these conditions, peroxy radicals are slowly regenerated downwind of the pollution source and may be lost in the atmosphere through deposition of RO2NO2. Based on this study, 60% of RO2 radicals would be stored as RO2NO2 at a temperature of 10 degrees C and in the presence of a few ppbv of NO2. The fraction increases further at colder temperatures and/or higher NO2 mixing ratios. This does not only affect the expected concentrations of RO2 radicals but, as the peroxynitrates can react with OH radicals or photolyse, they could comprise a net sink for RO2 radicals as well as increase the production of NOx (= NO + NO2) in different locations depending on their lifetime. Omitting this chemistry from the kinetic model can lead to misinterpreted product formation and may prevent reconciling observations and model predictions.

Correction for 'Impact of temperature-dependent non-PAN peroxynitrate formation, RO2NO2, on nighttime atmospheric chemistry' by Michelle Farber et al., Phys. Chem. Chem. Phys., 2024, https://doi.org/10.1039/d3cp04163h.

The photolysis of mono-deuterated formaldehyde, CHDO, is a critical process in the deuterium enrichment of stratospheric hydrogen formed from methane. In this work, a consistent description of the quantum yields of the molecular and radical channels of the CHDO photolysis is deduced from literature data. The fluorescence measurements of Miller and Lee (1978) provided a first data set to deduce the product quantum yields. An alternative analysis is based on the measured quantum-yield spectrum for the radical channel of the CD2O photolysis by McQuigg and Calvert (1969), which is corrected for wavelength dependency and combined with the CH2O quantum-yield spectrum to provide an approximation for CHDO. Both approaches provide consistent results. Finally, the findings of Troe (1984, 2007) enable the specification of the pressure dependence of the quantum yield for CH2O and CD2O and, hence, for CHDO. We find that the radical channel does not show a pressure dependence, whereas the molecular channel is dominated by tunneling and quenching processes. Simplified representations are given that are readily implemented in kinetic atmospheric models. As an example of their application, the altitude dependence of the ratio of J(CHDO→HD+CO) and J(CH2O→H2+CO) is provided. Also, the importance of the photolysis of formaldehyde on the yield of HD in the atmosphere is shown through the altitudinal dependence of the isotopic fractionation.

Correction for ‘Impact of temperature-dependent non-PAN peroxynitrate formation, RO 2 NO 2 , on nighttime atmospheric chemistry’ by Michelle Färber et al. , Phys. Chem. Chem. Phys. , 2024, https://doi.org/10.1039/d3cp04163h.

Chemical mechanisms form the core of atmospheric models to describe degradation pathways of pollutants and ultimately inform air quality and climate policymakers and other stakeholders. The accuracy of chemical mechanisms relies on the quality of their input data, which originate from experimental (laboratory, field, chamber) and theoretical (quantum chemistry, theoretical kinetics, machine learning) studies. The development of robust mechanisms requires rigorous and transparent procedures for data collection, mechanism construction and evaluation and the creation of reduced or operationally defined mechanisms. Developments in analytical techniques have led to a large number of identified chemical species in the atmospheric multiphase system that have proved invaluable for our understanding of atmospheric chemistry. At the same time, advances in software and machine learning tools have enabled automated mechanism generation. We discuss strategies for mechanism development, applying empirical or mechanistic approaches. We show the general workflows, how either approach can lead to robust mechanisms and that the two approaches complement each other, resulting in reliable predictions. Current challenges are discussed related to global change, including shifts in emission scenarios that result in new chemical regimes (e.g., low-NO scenarios, wildfires, mega- and gigacities) and that require the development of new or expanded gas- and aqueous-phase mechanisms. In addition, new mechanisms should be developed to also target oxidation capacity and aerosol chemistry impacting climate, human and ecosystem health.

The autoxidation of alkylperoxy radicals (RO2, where R is organic) is an important degradation pathway for organic compounds in a wide range of chemical systems including Earth's atmosphere. It is thought to proceed by internal H-shift reactions and, for unsaturated radicals, cyclization. However, experimental data on specific reactions steps for unsaturated RO2 is scarce. This work investigates the unimolecular reactions of 1-butenyl-O-2, 1-pentenyl-O-2, 1-hexenyl-O-2, and 2-methyl-2-pentenyl-O-2 radicals near room temperature (302 +/- 3 K) experimentally, by monitoring the radicals directly, and theoretically. The experimental rate coefficients are in good agreement with those determined with high-level quantum calculations, confirming that cyclization can be competitive with H-shift in some cases. However, the products observed experimentally with two different mass spectrometers suggest that all the peroxy radicals studied lead to fast decomposition (k > 1 s(-1)) after the isomerization step. While the mechanisms for these decompositions could not be fully elucidated theoretically, they question whether these channels contribute to propagation or to termination of the autoxidation chains.

Correction for 'Impact of temperature-dependent non-PAN peroxynitrate formation, RO2NO2, on nighttime atmospheric chemistry' by Michelle Farber et al., Phys. Chem. Chem. Phys., 2024, https://doi.org/10.1039/d3cp04163h.

Highly oxygenated organic molecules (HOMs) play a pivotal role in the formation of secondary organic aerosol (SOA). Therefore, the distribution and yields of HOMs are fundamental to understand their fate and chemical evolution in the atmosphere, and it is conducive to ultimately assess the impact of SOA on air quality and climate change. In this study, gas-phase HOMs formed from the reaction of limonene with OH radicals in photooxidation were investigated with SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction chamber), using a time-of-flight chemical ionization mass spectrometer with nitrate reagent ion (NO3--CIMS). A large number of HOMs, including monomers (C9–10) and dimers (C17–20), were detected and classified into various families. Both closed-shell products and open-shell peroxy radicals (RO2) were identified under low NO (0.06–0.1 ppb) and high NO conditions (17 ppb). C10 monomers are the most abundant HOM products and account for over 80 % total HOMs. Closed-shell C10 monomers were formed from a two peroxy radical family, C10H15Ox⚫ (x=6–15) and C10H17Ox⚫ (x=6–15), and their respective termination reactions with NO, RO2, and HO2. While C10H17Ox⚫ is likely formed by OH addition to C10H16, the dominant initial step of limonene plus OH, C10H15Ox⚫, is likely formed via H abstraction by OH. C10H15Ox⚫ and related products contributed 41 % and 42 % of C10 HOMs at low and high NO, demonstrating that the H-abstraction pathways play a significant role in HOM formation in the reaction of limonene plus OH. Combining theoretical kinetic calculations, structure–activity relationships (SARs), data from the literature, and the observed RO2 intensities, we proposed tentative mechanisms of HOM formation from both pathways. We further estimated the molar yields of HOMs to be 1.97-1.06+2.52 % and 0.29-0.16+0.38 % at low and high NO, respectively. Our study highlights the importance of H abstraction by OH and provides the yield and tentative pathways in the OH oxidation of limonene to simulate the HOM formation and assess the role of HOMs in SOA 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 (NO 3 ) or by hydroxyl radicals (OH) under high NO x conditions forms a substantial amount of organonitrates (ONs). ONs impact NO x concentrations and consequently ozone formation while also contributing to secondary organic aerosol. Here we show that the ONs with the chemical formula C 4 H 7 NO 5 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 C 4 H 7 NO 5 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 C 4 H 7 NO 5 isomers usually dominate both nighttime and daytime, implying a long residence time compared to C 5 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 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.