Photolysis and OH-Initiated Oxidation of Glycolaldehyde under Atmospheric Conditions

Carbonyls and Aerosol Mass Generation from Vaping Nicotine Salt Solutions Using Fourth- and Third-Generation E-Cigarette Devices: Effects of Coil Resistance, Coil Age, and Coil Metal Material

Aerosol formation and production yields from 11 carbonyls (carbonyl concentration per aerosol mass unit) were investigated (1) from a fourth-generation (4th gen) e-cigarette device at different coil resistances and coil age (0-5000 puffs) using unflavored e-liquid with 2% benzoic acid nicotine salt, (2) between a sub-ohm third-generation (3rd gen) tank mod at 0.12 Ω and a 4th gen pod at 1.2 Ω using e-liquid with nicotine salt, together with nicotine yield, and (3) from 3rd gen coils of different metals (stainless steel, kanthal, nichrome) using e-liquid with freebase nicotine. Coil resistance had an inverse relationship with coil temperature, and coil temperature was directly proportional to aerosol mass formation. Trends in carbonyl yields depended on carbonyl formation mechanisms. Carbonyls produced primarily from thermal degradation chemistry (e.g., formaldehyde, acetaldehyde, acrolein, propionaldehyde) increased per aerosol mass with higher coil resistances, despite lower coil temperature. Carbonyls produced primarily from chemistry initiated by reactive oxygen species (ROS) (e.g., hydroxyacetone, dihydroxyacetone, methylglyoxal, glycolaldehyde, lactaldehyde) showed the opposite trend. Coil age did not alter coil temperature nor aerosol mass formation but had a significant effect on carbonyl formation. Thermal carbonyls were formed optimally at 500 puffs in our study and then declined to a baseline, whereas ROS-derived carbonyls showed a slow rise to a maximum trend with coil aging. The 3rd gen versus 4th gen device comparison mirrored the trends in coil resistance. Nicotine yields per aerosol mass were consistent between 3rd and 4th gen devices. Coil material did not significantly alter aerosol formation nor carbonyl yield when adjusted for wattage. This work shows that sub-ohm coils may not necessarily produce higher carbonyl yields even when they produce more aerosol mass. Furthermore, carbonyl formation is dynamic and not generalizable during the coil's lifetime. Finally, studies that compare data across different e-cigarette devices, coil age, and coil anatomy should account for the aerosol chemistry trends that depend on these parameters.

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.

Organic synthesis in the study of terpene-derived oxidation products in the atmosphere

Covering: 1997 up to 2022Volatile biogenic terpenes involved in the formation of secondary organic aerosol (SOA) particles participate in rich atmospheric chemistry that impacts numerous aspects of the earth's complex climate system. Despite the importance of these species, understanding their fate in the atmosphere and determining their atmospherically-relevant properties has been limited by the availability of authentic standards and probe molecules. Advances in synthetic organic chemistry directly aimed at answering these questions have, however, led to exciting discoveries at the interface of chemistry and atmospheric science. Herein we provide a review of the literature regarding the synthesis of commercially unavailable authentic standards used to analyze the composition, properties, and mechanisms of SOA particles in the atmosphere.

Kinetics, Products, and Brown Carbon Formation by Aqueous-Phase Reactions of Glycolaldehyde with Atmospheric Amines and Ammonium Sulfate

Glycolaldehyde (GAld) is a C₂ water-soluble aldehyde produced during the atmospheric oxidation of isoprene and many other species and is commonly found in cloudwater. Previous work has established that glycolaldehyde evaporates more readily from drying aerosol droplets containing ammonium sulfate (AS) than does glyoxal, methylglyoxal, or hydroxyacetone, which implies that it does not oligomerize as quickly as these other species. Here, we report NMR measurements of glycolaldehyde’s aqueous-phase reactions with AS, methylamine, and glycine. Reaction rate constants are smaller than those of respective glyoxal and methylglyoxal reactions in the pH range of 3–6. In follow-up cloud chamber experiments, deliquesced glycine and AS seed particles were found to take up glycolaldehyde and methylamine and form brown carbon. At very high relative humidity, these changes were more than 2 orders of magnitude faster than predicted by our bulk liquid NMR kinetics measurements, suggesting that reactions involving surface-active species at crowded air–water interfaces may play an important role. The high-resolution liquid chromatography–electrospray ionization–mass spectrometric analysis of filter extracts of unprocessed AS + GAld seed particles identified sugar-like C₆ and C₁₂ GAld oligomers, including proposed product 3-deoxyglucosone, with and without modification by reactions with ammonia to diimine and imidazole forms. Chamber exposure to methylamine gas, cloud processing, and simulated sunlight increased the incorporation of both ammonia and methylamine into oligomers. Many C₄–C₁₆ imidazole derivatives were detected in an extract of chamber-exposed aerosol along with a predominance of N-derivatized C₆ and C₁₂ glycolaldehyde oligomers, suggesting that GAld is capable of forming brown carbon SOA.

Enhanced formation of secondary organic aerosol from photochemical oxidation during the COVID-19 lockdown in a background site in Northwest China

The COVID-19 pandemic has drastically affected the economic and social activities, leading to large reductions in anthropogenic emissions on a global scale. Despite the reduction of primary emissions during the lockdown period, heavy haze pollution was observed unexpectedly in megacities in North and East China. In this study, we conducted online measurements of organic aerosol in a background site before and during the lockdown in Guanzhong basin, Northwest China. The oxygenated organic aerosol (OOA) increased from 24% of total OA (3.2 ± 1.6 μg m^-3) before lockdown to 54% of total OA (4.5 ± 1.3 μg m^-3) during lockdown, likely due to substantial decrease of NO_x emissions during lockdown which resulted in large increase of O₃ and thus atmospheric oxidizing capacity. OOA showed higher mass concentrations and fractional contributions during lockdown than before lockdown, and increased with the increase of O_x in both periods. In comparison, aqueous secondary organic aerosol (aqSOA) showed high mass concentrations and fractional contributions in both polluted periods before and during lockdown with the increase of aerosol liquid water content (ALWC). The increase of aqSOA under high ALWC conditions is very likely the reason of pollution events during lockdown. Combined with trajectory analysis, the absence of Guanzhong cluster in polluted period during lockdown may play a key role in the OA variations between two polluted periods. In addition, when comparing the clusters from the same transmission directions between before lockdown and during lockdown, the OA fractions showed similar variations during lockdown in all clusters, suggesting the OA variations are widespread in northwest China.

Impact of e-Liquid Composition, Coil Temperature, and Puff Topography on the Aerosol Chemistry of Electronic Cigarettes

E-cigarette aerosol is a complex mixture of gases and particles with a composition that is dependent on the e-liquid formulation, puffing regimen, and device operational parameters. This work investigated mainstream aerosols from a third generation device, as a function of coil temperature (315-510 °F, or 157-266 °C), puff duration (2-4 s), and the ratio of propylene glycol (PG) to vegetable glycerin (VG) in e-liquid (100:0-0:100). Targeted and untargeted analyses using liquid chromatography high-resolution mass spectrometry, gas chromatography, in situ chemical ionization mass spectrometry, and gravimetry were used for chemical characterizations. PG and VG were found to be the major constituents (>99%) in both phases of the aerosol. Most e-cigarette components were observed to be volatile or semivolatile under the conditions tested. PG was found almost entirely in the gas phase, while VG had a sizable particle component. Nicotine was only observed in the particle phase. The production of aerosol mass and carbonyl degradation products dramatically increased with higher coil temperature and puff duration, but decreased with increasing VG fraction in the e-liquid. An exception is acrolein, which increased with increasing VG. The formation of carbonyls was dominated by the heat-induced dehydration mechanism in the temperature range studied, yet radical reactions also played an important role. The findings from this study identified open questions regarding both pathways. The vaping process consumed PG significantly faster than VG under all tested conditions, suggesting that e-liquids become more enriched in VG and the exposure to acrolein significantly increases as vaping continues. It can be estimated that a 30:70 initial ratio of PG:VG in the e-liquid becomes almost entirely VG when 60-70% of e-liquid remains during the vaping process at 375 °F (191 °C). This work underscores the need for further research on the puffing lifecycle of e-cigarettes.

Toward Replacing Ethylene Oxide in a Sustainable World: Glycolaldehyde as a Bio-Based C2 Platform Molecule

Fossil-based platform molecules such as ethylene and ethylene oxide currently serve as the primary feedstock for the C₂ -based chemical industry. However, in the search for a more sustainable chemical industry, fossil-based resources may preferentially be replaced by renewable alternatives, provided there is realistic economic feasibility. This Review compares and critically discusses several production routes toward bio-based structural analogues of ethylene oxide and the required adaptations for their implementation in state-of-the-art C₂ -based chemical processes. For example, glycolaldehyde, a structural analogue obtainable from carbohydrates by atom-economic retro-aldol reactions, may replace ethylene oxide's leading role. This alternative chemical route may not only allow the carbon footprint of conventional chemicals production to be lowered, but the introduction of a bio-based pathway may also contribute to safer production processes. Where possible, challenges, drawbacks, and prospects are highlighted.

Application of High-Resolution Mass Spectrometry and a Theoretical Model to the Quantification of Multifunctional Carbonyls and Organic Acids in e-Cigarette Aerosol

Electronic (e-) cigarette aerosol (particle and gas) is a complex mixture of chemicals, of which the profile is highly dependent on device operating parameters and e-liquid flavor formulation. The thermal degradation of the e-liquid solvents propylene glycol and glycerol often generates multifunctional carbonyls that are challenging to quantify because of unavailability of standards. We developed a theoretical method to calculate the relative electrospray ionization sensitivities of hydrazones of organic acids and carbonyls with 2,4-dinitrophenylhydrazine based on their gas-phase basicities (ΔG_{deprotonation}). This method enabled quantification by high-performance liquid chromatography-high-resolution mass spectrometry HPLC-HRMS in the absence of chemical standards. Accurate mass and tandem multistage MS (MSⁿ) were used for structure identification of vaping products. We quantified five simple carbonyls, six hydroxycarbonyls, four dicarbonyls, three acids, and one phenolic carbonyl in the e-cigarette aerosol with Classic Tobacco flavor. Our results suggest that hydroxycarbonyls, such as hydroxyacetone, lactaldehyde, and dihydroxyacetone can be significant components in e-cigarette aerosols but have received less attention in the literature and have poorly understood health effects. The data support the radical-mediated e-liquid thermal degradation scheme that has been previously proposed and emphasize the need for more research on the chemistry and toxicology of the complex product formation in e-cigarette aerosols.

Insights into the Reactions of Hydroxyl Radical with Diolefins from Atmospheric to Combustion Environments

Hydroxyl radicals and olefins are quite important from a combustion and an atmospheric chemistry standpoint. Large amounts of olefinic compounds are emitted into the earth's atmosphere from both biogenic and anthropogenic sources. Olefins make a significant share in transportation fuels (e.g., up to 20% by volume in gasoline), and they appear as important intermediates during hydrocarbon oxidation. As olefins inhibit low-temperature heat release, they have caught some attention for their applicability in future advanced combustion engine technology. Despite their importance, the literature data for the reactions of olefins are quite scarce. In this work, we have measured the rate coefficients for the reaction of hydroxyl radicals (OH) with several diolefins, namely 1,3-butadiene, cis-1,3-pentadiene, trans-1,3-pentadiene, and 1,4-pentadiene, over a wide range of experimental conditions ( T = 294-468 K and p ∼ 53 mbar; T = 881-1348 K and p ∼ 1-2.5 bar). We obtained the low- T data in a flow reactor using laser flash photolysis and laser-induced fluorescence (LPFR/LIF), and the high- T data were obtained with a shock tube and UV laser-absorption (ST/LA). At low temperatures, we observed differences in the reactivity of cis- and trans-1,3-pentadiene, but these molecules exhibited similar reactivity at high temperatures. Similar to monoolefins + OH reactions, we observed negative temperature dependence for dienes + OH reactions at low temperatures-revealing that OH-addition channels prevail at low temperatures. Except for the 1,4-pentadiene + OH reaction, which shows evidence of significant H-abstraction reactions even at low-temperatures, other diolefins studied here almost exclusively undergo addition reaction with OH radicals at the low-temperature end of our experiments; whereas the reactions mainly switch to hydrogen abstraction at high temperatures. These reactions show complex Arrhenius behavior as a result of many possible chemical pathways in such a convoluted system.

Long-term (2001-2013) observations of water-soluble dicarboxylic acids and related compounds over the western North Pacific: trends, seasonality and source apportionment

To better understand the impact of East Asian pollutants on the molecular composition of marine organic aerosols, we conducted long-term (2001-2013) observations of water-soluble dicarboxylic acids and related compounds in total suspended particulate samples collected at Chichijima Island in the western North Pacific (WNP). Seasonal variations of all the diacids and related compounds showed maxima in winter and spring and minima in summer, except for azelaic acid (C₉), which maximized in summer to autumn. The overall annual concentrations of the total diacids, ω-oxoacids and α-dicarbonyls showed an increase during 2001-2013. We found a significant (p < 0.05) decadal increase in the inter-annual trends of pyruvic and glyoxylic (p > 0.05) acids, and methylglyoxal (MeGly). In contrast, phthalic acid (p < 0.05) and glyoxal (Gly) showed a decrease in their trends. We also found a significant decrease in the trend of the Gly/MeGly mass ratios. These results demonstrate that the enhanced concentrations of diacids over the WNP are majorly attributed to the aqueous-phase photooxidation of biogenic volatile organic compounds from East Asia followed by long-range atmospheric transport. Further, positive matrix factorization analysis showed a biogenic photochemical contribution (42%) was the dominant source of oxalic acid in the WNP.

Real-Time Studies of Iron Oxalate-Mediated Oxidation of Glycolaldehyde as a Model for Photochemical Aging of Aqueous Tropospheric Aerosols

The complexation of iron(III) with oxalic acid in aqueous solution yields a strongly absorbing chromophore that undergoes efficient photodissociation to give iron(II) and the carbon dioxide anion radical. Importantly, iron(III) oxalate complexes absorb near-UV radiation (λ > 350 nm), providing a potentially powerful source of oxidants in aqueous tropospheric chemistry. Although this photochemical system has been studied extensively, the mechanistic details associated with its role in the oxidation of dissolved organic matter within aqueous aerosol remain largely unknown. This study utilizes glycolaldehyde as a model organic species to examine the oxidation pathways and evolution of organic aerosol initiated by the photodissociation of aqueous iron(III) oxalate complexes. Hanging droplets (radius 1 mm) containing iron(III), oxalic acid, glycolaldehyde, and ammonium sulfate (pH ∼3) are exposed to irradiation at 365 nm and sampled at discrete time points utilizing field-induced droplet ionization mass spectrometry (FIDI-MS). Glycolaldehyde is found to undergo rapid oxidation to form glyoxal, glycolic acid, and glyoxylic acid, but the formation of high molecular weight oligomers is not observed. For comparison, particle-phase experiments conducted in a laboratory chamber explore the reactive uptake of gas-phase glycolaldehyde onto aqueous seed aerosol containing iron and oxalic acid. The presence of iron oxalate in seed aerosol is found to inhibit aerosol growth. These results suggest that photodissociation of iron(III) oxalate can lead to the formation of volatile oxidation products in tropospheric aqueous aerosols.

A Theoretical Study of the Photoisomerization of Glycolaldehyde and Subsequent OH Radical-Initiated Oxidation of 1,2-Ethenediol

It has recently been discovered that carbonyl compounds can undergo UV-induced isomerization to their enol counterparts under atmospheric conditions. This study investigates the photoisomerization of glycolaldehyde (HOCH2CHO) to 1,2-ethenediol (HOCH═CHOH) and the subsequent (•)OH-initiated oxidation chemistry of the latter using quantum chemical calculations and stochastic master equation simulations. The keto-enol tautomerization of glycolaldehyde to 1,2-ethenediol is associated with a barrier of 66 kcal mol(-1) and involves a double-hydrogen shift mechanism to give the lower-energy Z isomer. This barrier lies below the energy of the UV/vis absorption band of glycolaldehyde and is also considerably below the energy of the products resulting from photolytic decomposition. The subsequent atmospheric oxidation of 1,2-ethenediol by (•)OH is initiated by addition of the radical to the π system to give the (•)CH(OH)CH(OH)2 radical, which is subsequently trapped by O2 to form the peroxyl radical (•)O2CH(OH)CH(OH)2. According to kinetic simulations, collisional deactivation of the latter is negligible and cannot compete with rapid fragmentation reactions, which lead to (i) formation of glyoxal hydrate [CH(OH)2CHO] and HO2(•) through an α-hydroxyl mechanism (96%) and (ii) two molecules of formic acid with release of (•)OH through a β-hydroxyl pathway (4%). Phenomenological rate coefficients for these two reaction channels were obtained for use in atmospheric chemical modeling. At tropospheric (•)OH concentrations, the lifetime of 1,2-ethenediol toward reaction with (•)OH is calculated to be 68 h.

Ethylene glycol emissions from on-road vehicles

Ethylene glycol (HOCH2CH2OH), used as engine coolant for most on-road vehicles, is an intermediate volatility organic compound (IVOC) with a high Henry's law coefficient. We present measurements of ethylene glycol (EG) vapor in the Caldecott Tunnel near San Francisco, using a proton transfer reaction mass spectrometer (PTR-MS). Ethylene glycol was detected at mass-to-charge ratio 45, usually interpreted as solely coming from acetaldehyde. EG concentrations in bore 1 of the Caldecott Tunnel, which has a 4% uphill grade, were characterized by infrequent (approximately once per day) events with concentrations exceeding 10 times the average concentration, likely from vehicles with malfunctioning engine coolant systems. Limited measurements in tunnels near Houston and Boston are not conclusive regarding the presence of EG in sampled air. Previous PTR-MS measurements in urban areas may have overestimated acetaldehyde concentrations at times due to this interference by ethylene glycol. Estimates of EG emission rates from the Caldecott Tunnel data are unrealistically high, suggesting that the Caldecott data are not representative of emissions on a national or global scale. EG emissions are potentially important because they can lead to the formation of secondary organic aerosol following oxidation in the atmospheric aqueous phase.

Reactive uptake and photo-Fenton oxidation of glycolaldehyde in aerosol liquid water

The reactive uptake and aqueous oxidation of glycolaldehyde were examined in a photochemical flow reactor using hydrated ammonium sulfate (AS) seed aerosols at RH = 80%. The glycolaldehyde that partitioned into the aerosol liquid water was oxidized via two mechanisms that may produce aqueous OH: hydrogen peroxide photolysis (H2O2 + hν) and the photo-Fenton reaction (Fe (II) + H2O2 + hν). The uptake of 80 (±10) ppb glycolaldehyde produced 2-4 wt % organic aerosol mass in the dark (kH* = (2.09-4.17) × 10(6) M atm(-1)), and the presence of an OH source increased the aqueous uptake by a factor of 4. Although the uptake was similar in both OH-aging mechanisms, photo-Fenton significantly increased the degree of oxidation (O/C = 0.9) of the aerosols compared to H2O2 photolysis (O/C = 0.5). Aerosol organics oxidized by photo-Fenton and H2O2 photolysis resemble ambient "aged" and "fresh" OA, respectively, after the equivalent of 2 h atmospheric aging. No uptake or changes in particle composition occurred on dry seed aerosol. This work illustrates that photo-Fenton chemistry efficiently forms highly oxidized organic mass in aerosol liquid water, providing a possible mechanism to bridge the gap between bulk-phase experiments and ambient particles.

Atmospheric production of glycolaldehyde under hazy prebiotic conditions

The early Earth's atmosphere, with extremely low levels of molecular oxygen and an appreciable abiotic flux of methane, could have been a source of organic compounds necessary for prebiotic chemistry. Here, we investigate the formation of a key RNA precursor, glycolaldehyde (2-hydroxyacetaldehyde, or GA) using a 1-dimensional photochemical model. Maximum atmospheric production of GA occurs when the CH4:CO2 ratio is close to 0.02. The total atmospheric production rate of GA remains small, only 1 × 10(7) mol yr(-1). Somewhat greater amounts of GA production, up to 2 × 10(8) mol yr(-1), could have been provided by the formose reaction or by direct delivery from space. Even with these additional production mechanisms, open ocean GA concentrations would have remained at or below ~1 μM, much smaller than the 1-2 M concentrations required for prebiotic synthesis routes like those proposed by Powner et al. (Nature 459:239-242, 2009). Additional production or concentration mechanisms for GA, or alternative formation mechanisms for RNA, are needed, if this was indeed how life originated on the early Earth.

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.

Product study of the OH radical and Cl atom initiated oxidation of 1,3-dioxane

The products of the hydroxyl (OH) radical and chlorine (Cl) atom initiated oxidation of 1,3-dioxane are determined under various reaction conditions in a 50 L teflon reaction chamber using FTIR spectroscopy for analysis. The major products detected in all experiments are (2-oxoethoxy)methyl formate, formic acid and methylene glycol diformate with average molar yields of 0.50±0.05, 0.41±0.02 and 0.03±0.01 respectively for the OH initiated oxidation in the presence of NO(x). The yields of these products do not vary significantly with O(2) partial pressure or oxidising agent (OH or Cl). However, the yield of formic acid decreased by at least a factor of two in the absence of NO(x). The results of these experiments are used to elucidate a simplified gas-phase atmospheric degradation scheme for 1,3-dioxane and also provide valuable information on the atmospheric fate of the cyclic and linear alkoxy radicals produced in these and similar reactions. The available experimental data suggests that the relative importance of the competing pathways (reaction with O(2) and ring opening by C-C or C-O bond fission) is a strong function of the ring strain in the cycloalkoxy radicals.

Photooxidation of 2-methyl-3-Buten-2-ol (MBO) as a potential source of secondary organic aerosol

2-Methyl-3-buten-2-ol (MBO) is an important biogenic hydrocarbon emitted in large quantities by pine forests. Atmospheric photooxidation of MBO is known to lead to oxygenated compounds, such as glycolaldehyde, which is the precursor to glyoxal. Recent studies have shown that the reactive uptake of glyoxal onto aqueous particles can lead to formation of secondary organic aerosol (SOA). In this work, MBO photooxidation under high- and low-NO(x) conditions was performed in dual laboratory chambers to quantify the yield of glyoxal and investigate the potential for SOA formation. The yields of glycolaldehyde and 2-hydroxy-2-methylpropanal (HMPR), fragmentation products of MBO photooxidation, were observed to be lower at lower NO(x) concentrations. Overall, the glyoxal yield from MBO photooxidation was 25% under high-NO(x) and 4% under low-NO(x) conditions. In the presence of wet ammonium sulfate seed and under high-NO(x) conditions, glyoxal uptake and SOA formation were not observed conclusively, due to relatively low (< 30 ppb) glyoxal concentrations. Slight aerosol formation was observed under low-NO(x) and dry conditions, with aerosol mass yields on the order of 0.1%. The small amount of SOA was not related to glyoxal uptake, but is likely a result of reactions similar to those that generate isoprene SOA under low-NO(x) conditions. The difference in aerosol yields between MBO and isoprene photooxidation under low-NO(x) conditions is consistent with the difference in vapor pressures between triols (from MBO) and tetrols (from isoprene). Despite its structural similarity to isoprene, photooxidation of MBO is not expected to make a significant contribution to SOA formation.

Conformational flexibility in hydrated sugars: the glycolaldehyde-water complex

Conformational flexibility in the smallest hydrated sugar, the glycolaldehyde-water complex, has been investigated in the gas phase by means of a combination of a microwave Fourier transform spectroscopy experiment in a supersonic molecular beam and ab initio quantum chemistry calculations. The water molecule inserts into glycolaldehyde using H-bonding selectivity: the two lowest energy conformations are stabilized by two intermolecular hydrogen bonds, and the next two by one intra- plus one intermolecular hydrogen bond. A dynamical flexibility associated with the two lowest energy conformations has been experimentally observed and accurately modeled with a two-dimensional potential energy surface involving the hydroxyl group and the free OH water group coordinates. The conclusions drawn from the role played in the conformational flexibility by the hydroxyl and carbonyl groups are extended to other carbohydrates and biomolecules.

Kinetic parameters for gas-phase reactions: experimental and theoretical challenges

This article aims to illustrate the added value provided to experimental kinetics investigations by complementary theoretical kinetics studies, using as examples (i) reactions of two major hydrocarbon flame radicals, HCCO and C(2)H, and (ii) reactions of several oxygenated organic compounds with hydroxyl radicals of interest to atmospheric chemistry. The first part, on HCCO and C(2)H kinetics, does not attempt to give an extensive literature review, but rather addresses some major experimental techniques, mainly specific ones, that have allowed a great part of the available reactivity databases on these two species to be established. For several key reactions, it is shown how potential energy surfaces and statistical rate predictions based thereon have provided insight into the molecular mechanisms and have allowed estimates of product distributions as well as reliable extrapolations of experimental rate coefficients and branching ratios to higher temperatures. The second part addresses current issues in atmospheric chemistry relating mainly to hydroxyl radical reactions with oxygenated organics, and focuses on the experimental characterization of the often unusual temperature dependence of their rate coefficients and on the theoretical rationalization thereof, through the formation of hydrogen-bonded pre-reactive complexes and resulting tunnelling-enhanced H-abstraction. Finally, the development of general structure-activity relationships for OH reactions with organics, H-abstractions as well as OH-additions for unsaturated compounds, is briefly discussed.

Reaction of HO with Glycolaldehyde, HOCH2CHO: Rate Coefficients (240−362 K) and Mechanism

Absolute rate coefficients for the title reaction, HO+HOCH2CHO-->products (R1), were measured over the temperature range 240-362 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. Within experimental error, the rate coefficient, k1, is independent of temperature over the range covered and is given by k1(240-362 K)=(8.0+/-0.8)x10(-12) cm3 molecule-1 s-1. The effects of the hydroxy substituent and hydrogen bonding on the rate coefficient are discussed based on theoretical calculations. The present results, which extend the database on the title reaction to a range of temperatures, indicate that R1 is the dominant loss process for HOCH2CHO throughout the troposphere. As part of this work, the absorption cross-section of HOCH2CHO at 184.9 nm was determined to be (3.85+/-0.2)x10(-18) cm2 molecule-1, and the quantum yield of HO formation from the photolysis of HOCH2CHO at 248 nm was found to be (7.0+/-1.5)x10(-2).

Mechanism of the OH-Initiated Oxidation of Glycolaldehyde over the Temperature Range 233−296 K

The mechanism of the gas-phase OH-initiated oxidation of glycolaldehyde (HOCH(2)CHO) was studied in the 233-296 K temperature range using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. In the presence of O2, formaldehyde, CO2, formic acid, and glyoxal were observed at room temperature with the yields of 80, 34, 18, and 14%, respectively. Decrease of temperature to 233 K led to significant changes in the yields of the stable products: those of formaldehyde and glyoxal decreased to 50 and 4%, respectively, whereas that of formic acid increased to 52%. It was also found that the OH + glycolaldehyde + O2 reaction proceeds with considerable reformation of OH radicals (by 25% at 296 K). The observed product yields are explained by a mechanism including formation of short-lived intermediate adducts of the primary radicals with O2. The implication of the obtained results for the HOx budget in the upper troposphere is discussed.