Photolysis of glyoxal in air

Infrared characterization of hydrated products of glyoxal in aqueous solution

The simplest and most abundant dicarbonyl in the atmosphere, glyoxal ((CHO)2), and its geminal diols via stepwise hydration reactions, monohydrate (CHOCH(OH)2) and dihydrate ((HC(OH)2)2), are proposed to be responsible for the generation of atmospheric acid and the increase in aerosol viscosity. In this work, the hydrates of glyoxal were prepared by dissolving glyoxal trimer dihydrate (C6H10O8) in H2O and D2O and probed by infrared absorption spectrometry at varied temperatures. In glyoxal aqueous solution at a concentration of < 1 wt%, the monomeric dihydrate is predominant. Coupled with the predicted vibrational wavenumbers and the corresponding intensities using the B3LYP/aug-cc-pVTZ method, the intense IR bands at 1075 cm-1 and 1073 cm-1 are attributed to the C-O stretching modes of dihydrate and deuterium substituted dihydrate at the hydroxyl groups, denoted as d4-dihydrate ((HC(OD)2)2). Upon heating of the d4-dihydrate solution to cause dehydration, a new band generated at 1745 cm-1 was attributed to the C=O stretching mode of d2-monohydrate (CHOCH(OD)2). Comparing the predicted wavenumbers of glyoxal monohydrate and the observed vibrational wavenumbers of the glycolaldehyde (HCOCH2OH), the wavenumber of the C=O stretching mode of monohydrate is reasonably presumed to be 1745 ± 5 cm-1. These infrared characterizations of the glyoxal hydrates provide suitable detection windows for further investigating the roles of glyoxal and its hydrates in atmospheric and aerosol chemistry, as well as studying the relevant reaction kinetics.

Impact of Covid-19 lockdown regulations on PM2.5 and trace gases (NO2, SO2, CH4, HCHO, C2H2O2 and O3) over Lahore, Pakistan

The COVID-19 pandemic altered the human mobility and economic activities immensely, as authorities enforced unprecedented lock down regulations. In order to reduce the spread of COVID-19, a complete lockdown was observed between 24 March - 31 May 2020 in Pakistan. This paper aims at investigating the PM_2.5, AOD and column amounts of six trace gases (NO₂, SO₂, CH₄, HCHO, C₂H₂O₂, and O₃) by comparing periods of reduced emissions during lockdown periods with reference periods without emission reductions over Lahore, Pakistan. HYSPLIT cluster trajectory analyses were performed, which confirmed similar meteorological flow conditions during lockdown and reference periods. This provides confidence that any change in air quality conditions would be due to changes in human activities and associated emissions. The results show about 38% reduction in ambient surface PM_2.5 levels during the lockdown period. This change also positively correlated with MODIS_DB and AERONET_AOD data with a decrease of AOD by 42% and 35%, respectively. Reductions for tropospheric columns of NO₂ and SO₂ were about 20% and 50%, respectively during a semi lockdown period, while no reduction in the CH_4, C₂H₂O₂, HCHO and O₃ levels occurred. During the lockdown period NO_2, O₃ and CH₄ were about 50%, 45% and 25% lower, respectively, but no reduction in SO₂, C₂H₂O₂ and HCHO levels were noticed compared to the reference lockdown period for Lahore. HYSPLIT cluster trajectory analysis revealed the greatest impact on Lahore air quality through local emissions and regional transport from the east (agricultural burning and industry).

Investigating the Association Reactions of HOCH2CO and HOCHCHO with O2: A Quantum Computational and Master Equation Study

Glycolaldehyde, HOCH₂CHO, is an important multifunctional atmospheric trace gas formed in the oxidation of ethylene and isoprene and emitted directly from burning biomass. The initial step in the atmospheric photooxidation of HOCH₂CHO yields HOCH₂CO and HOCHCHO radicals; both of these radicals react rapidly with O₂ in the troposphere. This study presents a comprehensive theoretical investigation of the HOCH₂CO + O₂ and HOCHCHO + O₂ reactions using high-level quantum chemical calculations and energy-grained master equation simulations. The HOCH₂CO + O₂ reaction results in the formation of a HOCH₂C(O)O₂ radical, while the HOCHCHO + O₂ reaction yields (HCO)₂ + HO₂. Density functional theory calculations have identified two open unimolecular pathways associated with the HOCH₂C(O)O₂ radical that yield HCOCOOH + OH or HCHO + CO₂ + OH products; the former novel bimolecular product pathway has not been previously reported in the literature. Master equation simulations based on the potential energy surface calculated here for the HOCH₂CO + O₂ recombination reaction support experimental product yield data from the literature and indicate that, even at total pressures of 1 atm, the HOCH₂CO + O₂ reaction yields ∼11% OH at 298 K.

The underappreciated role of monocarbonyl-dicarbonyl interconversion in secondary organic aerosol formation during photochemical oxidation of m-xylene

Photochemical oxidation (including photolysis and OH-initiated reactions) of aromatic hydrocarbon produces carbonyls, which are involved in the formation of secondary organic aerosols (SOA). However, the mechanism of this process remains incompletely understood. Herein, the monocarbonyl-dicarbonyl interconversion and its role in SOA production were investigated via a series of photochemical oxidation experiments for m-xylene and representative carbonyls. The results showed that SOA mass concentration peaked at 113.5 ± 3.5 μg m^-3 after m-xylene oxidation for 60 min and then decreased. Change in the main oxidation products from dicarbonyl (e.g., glyoxal, methylglyoxal) to monocarbonyl (e.g., formaldehyde) was responsible for this decrease. The photolysis of methylglyoxal or glyoxal produced formaldehyde, favoring SOA formation, while photopolymerization of formaldehyde to glyoxal decreased SOA production. The presence of ·OH altered the balance of photolysis interconversion, resulting in greater production of formaldehyde and SOA from glyoxal than methylglyoxal. Both photolysis and OH-initiated transformations of glyoxal to formaldehyde were suppressed by methylglyoxal, while glyoxal accelerated the reaction of ·OH with methylglyoxal to generate products which reversibly converted to glyoxal and methylglyoxal. These interconversion reactions reduced SOA production. The present study provides a new research perspective for the contribution mechanism of carbonyls in SOA formation and the findings are also helpful to efficiently evaluate the atmospheric fate of aromatic hydrocarbons.

Kinetics of the Reactions of CH2OO with Acetone, α-Diketones, and β-Diketones

Rate constants for the reactions between the simplest Criegee intermediate, CH₂OO, with acetone, the α-diketones biacetyl and acetylpropionyl, and the β-diketones acetylacetone and 3,3-dimethyl-2,4-pentanedione have been measured at 295 K. CH₂OO was produced photochemically in a flow reactor by 355 nm laser flash photolysis of diiodomethane in the presence of excess oxygen. Time-dependent concentrations were measured using broadband transient absorption spectroscopy, and the reaction kinetics was characterized under pseudo-first-order conditions. The bimolecular rate constant for the CH₂OO + acetone reaction is measured to be (4.1 ± 0.4) × 10^-13 cm³ s^-1, consistent with previous measurements. The reactions of CH₂OO with the β-diketones acetylacetone and 3,3-dimethyl-2,5-pentanedione are found to have broadly similar rate constants of (6.6 ± 0.7) × 10^-13 and (3.5 ± 0.8) × 10^-13 cm³ s^-1, respectively; these values may be cautiously considered as upper limits. In contrast, α-diketones react significantly faster, with rate constants of (1.45 ± 0.18) × 10^-11 and (1.29 ± 0.15) × 10^-11 cm³ s^-1 measured for biacetyl and acetylpropionyl. The potential energy surfaces for these 1,3-dipolar cycloaddition reactions are characterized at the M06-2X/aug-cc-pVTZ and CBS-QB3 levels of theory and provide additional support to the observed experimental trends. The reactivity of carbonyl compounds with CH₂OO is also interpreted by application of frontier molecular orbital theory and predicted using Hammett substituent constants. Finally, the results are compared with other kinetic studies of Criegee intermediate reactions with carbonyl compounds and discussed within the context of their atmospheric relevance.

Sensitive Detection of Gas-Phase Glyoxal by Electron Attachment Reaction Ionization Mass Spectrometry

Glyoxal (GLY) acts as a key contributor to tropospheric ozone production and secondary organic aerosol (SOA) formation on local to regional scales. The detection of GLY provides useful indicators of fast photochemistry occurring in the lower troposphere. The fast and sensitive detection of GLY is thus important, while traditional chemical ionization such as the proton-transfer reaction (PTR) is extremely limited by the poor detection limit and extensive fragmentation. To address these limitations, electron attachment reaction (EAR) ionization was applied to detect GLY. The generation of parent anions (GLY^-) without fragmentation was observed, and cryogenic photoelectron imaging spectroscopy further characterized the structure of GLY^-. The detection limit was estimated to be as low as (52 ± 1) pptv (parts per trillion by volume) with 1 min measurements. Other components in ambient air, such as water, carbon dioxide, and trace gases (acetone, propanal, etc.) have no effect on the detection of GLY. The EAR ionization is more promising than PTR ionization in detecting GLY. The detection of GLY in ambient air by the EAR ionization has been demonstrated.

Investigating the impact of Glyoxal retrieval from MAX-DOAS observations during haze and non-haze conditions in Beijing

This study presents the Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements for Glyoxal (CHOCHO) in Beijing, China (39.95°N, 116.32°E). CHOCHO is the smallest compound of di-carbonyl group. As a primary sink of CHOCHO, its photolysis with NOx (oxides of nitrogen) results in the production of tropospheric ozone. Therefore, the focus of CHOCHO DOAS measurements is increasing in trend. We did the measurements from 09 May 2017 to 09 September 2017. The study was conducted to compare different retrieval settings in order to reveal best DOAS fit settings for CHOCHO; furthermore, effect of haze and non-haze days on CHOCHO concentration was examined. The root mean square of residual and Differential Slant Column density (dSCD) error was reduced when measurements were done with lower wavelength limit around 432-438 nm and upper intervals around 455-460 nm. Thus, lower wavelength intervals around 432-438 nm and upper intervals around 457-460 nm were best for the retrieval of dSCDs for CHOCHO. Meteorological conditions like haze or non-haze days did not have significant effect on DOAS fit parameters. The CHOCHO vertical column densities range from 1.33E+14 to 9.77E+14 molecules/cm² during the study period with average of 6.16E+14 molecules/cm². The results indicated that during haze days CHOCHO concentration was higher because of lower rate of photolysis and atmospheric oxidation potential. Our results did not show any significant weekend effect on CHOCHO atmospheric concentration.

Products and mechanism of the OH-initiated photo-oxidation of perfluoro ethyl vinyl ether, C2F5OCF[double bond, length as m-dash]CF2

The OH-initiated photo-oxidation of perfluoro ethyl vinyl ether (C2F5OCF[double bond, length as m-dash]CF2, PEVE) in air (298 K, 50 and 750 Torr total pressure) was studied in a photochemical reactor using in situ detection of PEVE and its products by Fourier transform IR absorption spectroscopy. The relative rate technique was used to derive the rate coefficient, k1, for the reaction of PEVE with OH as k1 = (2.8 ± 0.3) × 10-12 cm3 molecule-1 s-1. The photo-oxidation of PEVE in the presence of NOx at 1 bar results in formation of C2F5OCFO, FC(O)C(O)F and CF2O in molar yields of 0.50 ± 0.07, 0.46 ± 0.07 and 1.50 ± 0.22, respectively. FC(O)C(O)F and CF2O are formed partially in secondary, most likely heterogeneous processes. At a reduced pressure of 50 Torr, the product distribution is shifted towards formation of FC(O)C(O)F, indicating the important role of collisional quenching of initially formed association complexes, and enabling details of the reaction mechanism to be elucidated. An atmospheric photo-oxidation mechanism for PEVE is presented and the environmental implications of PEVE release and degradation are discussed.

Mechanism and Product Distribution of the O3-Initiated Degradation of (E)-2-Heptenal, (E)-2-Octenal, and (E)-2-Nonenal

The O₃-molecule initiated degradation of three 2-alkenals (E)-2-heptenal, (E)-2-octenal, and (E)-2-nonenal has been investigated in a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air using in situ FTIR spectroscopy to monitor the reactants and products. The experiments were performed in the absence of an OH scavenger. The molar yields of the primary products formed were glyoxal (49 ± 4) % and pentanal (34 ± 3) % from the reaction of (E)-2-heptenal with O₃, glyoxal (41 ± 3) % and hexanal (39 ± 3) % from the reaction of (E)-2-octenal with O₃, and glyoxal (45 ± 3) % and heptanal (46 ± 3) % from the reaction of (E)-2-nonenal with O₃. The residual bands in the infrared product spectra for each of the studied reactions are attributed to 2-oxoaldehyde compounds. Based on the observed products, a general mechanism for the ozonolysis reaction of long chain unsaturated aldehydes is proposed, and the results are compared with the available literature data.

Quantitative infrared intensity studies of vapor-phase glyoxal, methylglyoxal, and 2,3-butanedione (diacetyl) with vibrational assignments

Glyoxal, methylglyoxal, and 2,3-butanedione (diacetyl) are all known biomass burning effluents and suspected aerosol precursors. Pressure-broadened quantitative infrared spectra of glyoxal, methylglyoxal, and diacetyl vapors covering the 520-6500 cm(-1) range are reported at 0.112 cm(-1) resolution, each with a composite spectrum derived from a minimum of 10 different sample pressures for the compound, representing some of the first quantitative intensity data for these analytes. Many vibrational assignments for methylglyoxal are reported for the first time, as are some near-IR and far-IR bands of glyoxal and diacetyl. To complete the vibrational assignments, the far-infrared spectra (25-600 cm(-1)) of all three vapors are also reported, those of methylglyoxal for the first time. Density functional theory and ab initio MP2 theory are used to help assign vibrational modes. Potential bands for atmospheric monitoring are discussed.

Developments in Laboratory Studies of Gas-Phase Reactions for Atmospheric Chemistry with Applications to Isoprene Oxidation and Carbonyl Chemistry

Laboratory studies of gas-phase chemical processes are a key tool in understanding the chemistry of our atmosphere and hence tackling issues such as climate change and air quality. Laboratory techniques have improved considerably with greater emphasis on product detection, allowing the measurement of site-specific rate coefficients. Radical chemistry lies at the heart of atmospheric chemistry. In this review we consider issues around radical generation and recycling from the oxidation of isoprene and from the chemical reactions and photolysis of carbonyl species. Isoprene is the most globally significant hydrocarbon, but uncertainties exist about its oxidation in unpolluted environments. Recent experiments and calculations that cast light on radical generation are reviewed. Carbonyl compounds are the dominant first-generation products from hydrocarbon oxidation. Chemical oxidation can recycle radicals, or photolysis can be a net radical source. Studies have demonstrated that high-resolution and temperature-dependent studies are important for some significant species.

Photo-induced hydrogen exchange reaction between methanol and glyoxal: formation of hydroxyketene

We study the structure and photochemistry of the glyoxal-methanol system (G-MeOH) by means of FTIR matrix isolation spectroscopy and ab initio calculations. The FTIR spectra show that the non-hydrogen-bonded complex, G-MeOH-1, is present in an inert environment of solid argon. MP2/aug-cc-pVDZ calculations indicate that G-MeOH-1 is the most stable complex among the five optimized structures. The interaction energy partitioned according to the symmetry-adapted perturbation theory (SAPT) scheme demonstrates that the dispersion energy gives a larger contribution to the stabilization of a non-hydrogen-bonded G-MeOH-1 complex than compared to the hydrogen-bonded ones. The irradiation of G-MeOH-1 with the filtered output of a mercury lamp (lambda>370 nm) leads to its photo-conversion into the hydroxyketene-methanol complex HK-MeOH-1. The identity of HK-MeOH-1 is confirmed by both FTIR spectroscopy and MP2/aug-cc-pVDZ calculations. An experiment with deuterated methanol (CH(3)OD) evidences that hydroxyketene is formed in a photo-induced hydrogen exchange reaction between glyoxal and methanol. The pathway for the photo-conversion of G-MeOH-1 to HK-MeOH-1 is studied by a coupled-cluster method [CR-CC(2,3)]. The calculations confirm our experimental findings that the reaction proceeds via hydrogen atom exchange between the OH group of methanol and CH group of glyoxal.