The Hydrogen Kinetic Isotope Effects of the Reactions of <Emphasis Type="Italic">n</Emphasis>-Alkanes with Chlorine Atoms in the Gas Phase

The stable-hydrogen kinetic isotope effects (KIEs) for a series of n-alkanes in reaction with chlorine atoms in the gas phase were studied in a 25-L PTFE reaction chamber at 298 K. The time dependence of both the stable hydrogen isotope ratios and the concentrations was determined using a gas chromatography pyrolysis isotope ratio mass spectrometry (GC-P-IRMS) system. The following KIE values, in per mil (‰), were obtained: 39.6 ± 2.7 (n-butane), 28.2 ± 0.9 (n-pentane), 24.6 ± 1.0 (n-hexane), 24.0 ± 1.2 (n-heptane), 17.9 ± 3.3 (n-octane), 15.1 ± 0.7 (n-nonane), and 14.9 ± 1.8 (n-decane). The errors given are the ±1σ standard errors. These measured values were used to derive structure–reactivity relationship (SRRs), which allow for the calculation of the KIEs for the reaction of n-alkanes with Cl atoms. The results of the calculations agree with the measurements within few per mil or better. The site specific stable hydrogen isotope fractionation effects for methyl groups are approximately a factor of 3 larger than those for methylene group, a finding which is qualitatively similar to site-specific stable hydrogen isotope effects reported in literature for reactions of alkanes with the OH radical. Because n-alkanes with close to natural isotope ratios (i.e. neither artificially labeled, nor enriched or depleted) were used, the KIE data are directly applicable to atmospheric studies. Based on these KIE values, the impact of Cl-atom reactions of the stable hydrogen isotope ratio on alkanes are estimated for different levels of Cl-atom concentrations. On average in the troposphere, the impact of Cl-atom reactions of the stable hydrogen isotope ratio of n-alkanes will be small. However, in regions of the troposphere with high concentrations of Cl atoms, such as the tropospheric ozone depletion episodes during polar sunrise, the impact of Cl-atom reactions is substantial. An erratum to this article is available at .     

Laboratory measurements of the <Superscript>12</Superscript>C/<Superscript>13</Superscript>C kinetic isotope effects in the gas-phase reactions of unsaturated hydrocarbons with Cl atoms at 298 ± 3 K

The carbon kinetic isotope effects (KIEs) in the reactions of several unsaturated hydrocarbons with chlorine atoms were measured at room temperature and ambient pressure using gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). All measured KIEs, defined as the ratio of the rate constants for the unlabeled and labeled hydrocarbon reaction k ₁₂/k ₁₃, are greater than unity or normal KIEs. The KIEs, reported in per mil according to ^Cl ɛ = (k ₁₂/k ₁₃−1) × 1000‰ with the number of experimental determinations in parenthesis, are as follows: ethene, 5.65 ± 0.34 (1); propene, 5.56 ± 0.18 (2); 1-butene, 5.93 ± 1.16 (1); 1-pentene, 4.86 ± 0.63 (1); cyclopentene, 3.75 ± 0.14 (1); toluene, 2.89 ± 0.31 (2); ethylbenzene, 2.17 ± 0.17 (2); o-xylene, 1.85 ± 0.54 (2). To our knowledge, these are the first reported KIE measurements for reactions of unsaturated NMHC with Cl atoms. Relative rate constants were determined concurrently to the KIE measurements. For the reactions of cyclopentene and ethylbenzene with Cl atoms, no rate constant has been reported in refereed literature. Our measured rate constants are: cyclopentene (7.32 ± 0.88) relative to propene (2.68 ± 0.32); ethylbenzene (1.15 ± 0.04) relative to o-xylene (1.35 ± 0.21), all × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The KIEs in reactions of aromatic hydrocarbons with Cl atoms are similar to previously reported KIEs in Cl-reactions of alkanes with the same numbers of carbon atoms. Unlike the KIEs for previously studied gas-phase hydrocarbon reactions, the KIEs for alkene–Cl reactions do not exhibit a simple inverse dependence on carbon number. This can be explained by competing contributions of normal and inverse isotope effects of individual steps in the reaction mechanism. Implications for the symmetries of the transition state structures in these reactions and the potential relevance of Cl-atom reactions on stable carbon isotope ratios of atmospheric NMHC are discussed.     

Laboratory Determination of the Carbon Kinetic Isotope Effects (KIEs) for Reactions of Methyl Halides with Various Nucleophiles in Solution

Large carbon kinetic isotope effects (KIEs) were measured for reactions of methyl bromide (MeBr), methyl chloride (MeCl), and methyl iodide (MeI) with various nucleophiles at 287 and 306 K in aqueous solutions. Rates of reaction of MeBr and MeI with H₂O (neutral hydrolysis) or Cl⁻ (halide substitution) were consistent with previous measurements. Hydrolysis rates increased with increasing temperature or pH (base hydrolysis). KIEs for hydrolysis were 51 ± 6%₀ for MeBr and 38 ± 8%₀ for MeI. Rates of halide substitution increased with increasing temperature and greater reactivity of the attacking nucleophile, with the fastest reaction being that of MeI with Br⁻. KIEs for halide substitution were independent of temperature but varied with the reactant methyl halide and the attacking nucleophile. KIEs were similar for MeBr substitution with Cl⁻ and MeCl substitution with Br⁻ (57 ± 5 and 60 ± 9%₀, respectively). The KIE for halide exchange of MeI was lower overall (33 ± 8%₀) and was greater for substitution with Br⁻ (46 ± 6%₀) than with Cl⁻ (29 ± 6%₀).     

Characterization of carbon monoxide,methane and nonmethane hydrocarbons in emerging cities of Saudi Arabia and Pakistan and in Singapore

We investigate the composition of 63 C₂-C₁₀ nonmethane hydrocarbons (NMHCs), methane (CH₄) and carbon monoxide (CO), in Jeddah, Mecca, and Madina (Saudi Arabia), in Lahore, (Pakistan), and in Singapore. We established a database with which to compare and contrast NMHCs in regions where ambient levels and emissions are poorly characterized, but where conditions are favorable to the formation of tropospheric ozone, and where measurements are essential for improving emission inventories and modeling. This dataset will also serve as a base for further analysis of air pollution in Western Saudi Arabia including, but not limited to, the estimation of urban emissions and long range pollution transport from these regions. The measured species showed enhanced levels in all Saudi Arabian cities compared to the local background but were generally much lower than in Lahore. In Madina, vehicle exhaust was the dominant NMHC source, as indicated by enhanced levels of combustion products and by the good correlation between NMHCs and CO, while in Jeddah and Mecca a combination of sources needs to be considered. Very high NMHC levels were measured in Lahore, and elevated levels of CH₄ in Lahore were attributed to natural gas. When we compared our results with 2010 emissions from the MACCity global inventory, we found discrepancies in the relative contribution of NMHCs between the measurements and the inventory. In all cities, alkenes (especially ethene and propene) dominated the hydroxyl radical (OH) reactivity (k _OH) because of their great abundance and their relatively fast reaction rates with OH.     

Isotopic Composition of Non-Methane Hydrocarbons in Emissions from Biomass Burning

The stable carbon isotope ratios of nonmethane hydrocarbons (NMHC) and methyl chloride emitted from biomass burning were determined by analyzing seven whole air samples collected during different phases of the burning process as part of a laboratory study of wood burning. The average of the stable carbon isotope ratios of emitted alkanes, alkenes and aromatic compounds is identical to that of the burnt fuel; more than 50% of the values are within a range of ±1.5 of thecomposition of the burnt fuel wood. Thus for the majority of NMHC emitted from biomass burning stable carbon isotope ratio of the burnt fuel a good first order approximation for the isotopic composition of the emissions. Of the more than twenty compounds we studied, only methyl chloride and ethyne differed in stable carbon isotope ratios by more than a few per mil from the composition of the fuel. Ethyne is enriched in ¹³C by approximately 20–30, and most of the variability can beexplained by a dependence on flame temperature. The ¹³C values decreaseby 0.019 /K (±0.0053/K) with increasing temperature. Methyl chloride is highly depleted in ¹³C, on average by25. However the results cover a wide range of nearly 30. Specifically, in two measurements with wood from Eucalyptus (Eucalyptus delegatensis) as fuel we observed the emission of extremely light methyl chloride (–68.5and–65.5). This coincides with higher than average emission ratiosfor methyl chloride (15.5 × 10^–5 and 18 ×10^–5 mol CH₃Cl/mol CO₂). These high emission ratios are consistent with the highchlorine content of the burnt fuel, although, due to the limited number of measurements, it would be premature to generalize these findings. The limited number of observations also prevents any conclusion on a systematic dependence between chlorine content of the fuel, emission ratios and stable carbon isotope ratio of methyl chloride emissions. However, our results show that a detailed understanding of the emissions of methyl chloride from chloride rich fuels is important for understanding its global budget. It is also evident that the usefulness of stable carbon isotope ratios to constrain the global budget of methyl chloride will be complicated by the very large variability of the stable carbon isotope ratio of biomass burning emissions. Nevertheless, ultimately the large fractionation may provide additional constraints for the contribution of biomass burning emissions to the atmospheric budget of methyl chloride.     

The Stable Carbon Isotope Ratio of Biogenic Emissions of Isoprene and the Potential Use of Stable Isotope Ratio Measurements to Study Photochemical Processing of Isoprene in the Atmosphere

A technique was developed that allows the determination of the stable carbon isotope ratio of isoprene in air. The method was used for a limited number of ambient measurements as well as laboratory studies of isoprene emitted from Velvet Bean (Mucana pruriens L. var. utilis), including the light and temperature dependence. The mean stable carbon isotope ratio ( ¹³C) of isoprene emitted from Velvet Bean (Mucana pruriens L. var. utilis) for all our measurements is –27.7 ± 2.0 (standard deviation for 23 data points). Our results indicate a small dependence of the stable carbon isotope ratios on leaf temperature and photosynthetic photon flux density (PPFD). The light dependence is 0.0026 ± 0.0012/( mol of photons m^–2 s^–1) for the studied range from 400 to 1700 mol of photons m^–2 s^–1. The temperature dependence is 0.16 ± 0.09/K. On average, the emitted isoprene is 2.6 ± 0.9 lighter than the leaf carbon. An uncertainty analysis of the possibility to use stable carbon isotope ratio measurements of isoprene for estimates of its mean photochemical age suggests that meaningful results can be obtained. This is supported by the results of a small number of measurements of the stable carbon isotope composition of ambient isoprene at different locations. The results range from approximately –29 to –16. They are consistent with vegetation emissions of isoprene that is slightly depleted in ¹³C relative to the plant material and enrichment of ¹³C in the atmosphere due to isotope fractionation associated with the reaction with OH-radicals. The stable carbon isotope ratio of ambient isoprene at locations directly influenced by isoprene emissions is very close to the values we found in our emission studies, whereas at sites located remote from isoprene emitting vegetation we find substantial enrichment of ¹³C. This suggests that stable carbon isotope ratio measurements will be a valuable, quantitative method to determine the extent of photochemical processing of isoprene in ambient air.     

Ab initio study on carbon Kinetic Isotope Effect (KIE) in the reaction of CH4+Cl•

Recent studies suggest that the destruction of methane by Cl in the marine boundary layer could be accounted for as another major sink besides the methane destruction by OH. High level ab initio molecular orbital calculations have been carried out to study the CH₄+Cl reaction, the carbon Kinetic Isotope Effect (KIE) is calculated using Conventional Transition-State Theory (CTST) plus Wigner and Eckart semiclassical tunneling corrections. The calculated KIE is around 1.026 at 300 K and has a small temperature variation. This is by far the largest KIE among different processes involving atmospheric methane destruction (e.g., OH, soil). A calculated mass balance of atmospheric methane including the KIE for the CH₄+Cl reaction is found to favor those methane budgets with enhanced biological methane sources, which have relatively lighter carbon isotope composition.     

A smog chamber for studies of the reactions of terpenes and alkanes with ozone and OH

The design and performance of a smog chamber for the study of photochemical reactions under simulated environmental conditions is described. The chamber is thermostated for aerosol experiments, and it comprises a gas chromatographic sample enrichment system suitable for monitoring hydrocarbons at the ppbv level. By irradiating NO _x /alkane-mixtures rate constants for the reaction of OH radicals with n-alkanes are determined from n-pentane to n-hexadecane to be (k±2)/10^–12 cm³ s^–1=4.29±0.16, 6.2±0.6, 7.52 (reference value), 8.8±0.3, 10.2±0.3, 11.7±0.4, 13.7±0.3, 15.1±0.5, 17.5±0.6, 19.3±0.7, 22.3±1.0, and 25.0±1.3, respectively at 312 K. Rate constants, (k±2)/10^–17 cm³ s^–1, for the reaction of ozone with trans-2-butene (21.2±1.0), cis-3-methylpentene-(2) (47.2±1.7), cyclopentene (62.4±3.5), cyclohexene (7.8±0.5), cycloheptene (28.3±1.5), -pinene (8.6±1.3), and -pinene (1.4±0.2) are determined in the dark at 297 K using cis-2-butene (13.0) as reference standard.     

Relative-rate study of the gas-phase reaction of hydroxy radicals with difunctional organic nitrates at 298 K and atmospheric pressure

Rate coefficients for the reactions of difunctional nitrates with atmospherically important OH radicals are not currently available in the literature. This study represents the first determination of rate coefficients for a number of C(3) and C(4) carbonyl nitrates and dinitrates with OH radicals in a 38 l glass reaction chamber at 1000 mbar total pressure of synthetic air by 298±2 K using a relative kinetic technique.The following rate coefficients (in units of 10^-12 cm³ molecule^-1 s^-1) were obtained: 1,2-propandiol dinitrate, <0.31; 1,2-butandiol dinitrate, 1.70±0.32; 2,3-butandiol dinitrate, 1.07±0.26; -nitrooxyacetone, <0.43; 1-nitrooxy-2-butanone, 0.91±0.16; 3-nitrooxy-2-butanone, 1.27±0.14; 1,4-dinitrooxy-2-butene, 15.10±1.45; 3,4-dinitrooxy-1-butene, 10.10±0.50.The possible importance of reaction of OH as an atmospheric sink for the compounds compared to other loss processes is considered.     

Absolute Rate Coefficients for the Gas-Phase Reactions of NO3 Radical with a Series of Monoterpenes at T = 298 to 433 K

The aim of this work is to study the reactivity of some naturally emitted terpenes, 2-carene, sabinene, myrcene, -phellandrene, d-limonene, terpinolene and -terpinene, towards NO3 radical to evaluate the importance of these reactions in the atmosphere and their atmospheric impact. The experiments with these monoterpenes have been carried out under second-order kinetic conditions over the range of temperature 298–433 K, using a discharge flow system and monitoring the NO3 radical by Laser Induced Fluorescence (LIF). This work is the first temperature dependence study for the reactions of the nitrate radical with the above-mentioned monoterpenes. The measured rate constants at 298 K for the reaction of NO3 with such terpenes are as follows: 2-carene, 16.6 ± 1.8, sabinene 10.7 ± 1.6, myrcene 12.8 ± 1.1, -phellandrene 42 ± 10, d-limonene 9.4 ± 0.9, terpinolene 52 ± 9 and -terpinene 24 ± 7, in units of 10-12 cm3 molecule-1 s-1. The proposed Arrhenius expressions, for the reactions of NO3 with 2-carene, sabinene, myrcene and -phellandrene are, respectively k1 = (1.4 ± 0.7) × 10-12 exp[(741 ± 190/T)] (cm3 molecule-1 s-1), k2=(2.3 ± 1.3) × 10-10 exp[–(940 ± 200/T)] (cm3 molecule-1 s-1), k3 = (2.2 ± 0.2) × 10-12 exp[(523 ± 35/T)] (cm3 molecule1 s-1) and k4 = (1.9 ± 1.3) × 10-9 exp[–(1158 ± 270/T)] (cm3 molecule-1 s-1). A decrease in the rate constants when raising the temperature has also been found for the reaction of d-limonene with NO3 while an increase in the rate constant with temperature has been observed for the reactions of terpinolene and -terpinene with NO3. Tropospheric half-lives for these terpenes have been calculated at night and during the day for typical NO3 and OH concentrations showing that both radicals provide an effective tropospheric sink for these compounds and that the night-time reaction with NO3 radical can be an important, if not dominant, loss process for these naturally emitted organics and for NO3 radicals.     

Structure-Reactivity Based Estimation of the Rate Constants for Hydroxyl Radical Reactions with Hydrocarbons

The reaction with the OH radical constitutes the singlemost important removal process for most organiccompounds found in the atmosphere. Efforts to measurethe OH radical rate constants of all troposphericconstituents remain incomplete due to the largevariety of primary emitted compounds and theirtropospheric degradation products.Based on the measured rate constants of 250molecules with the OH radical, a structure-activityrelationship (SAR) for OH reactions has beendeveloped. The molecules used in the dataset includemost classes of tropospheric compounds (includingalkanes, alkenes, and oxygenated hydrocarbons), withthe exception of aromatic and halogen-containingcompounds. Using a new parameterization of themolecular structure, the overall agreement betweenmeasured values and those estimated using the SARdeveloped in this study is usually very good, with10% of the molecules showing deviations larger than50%. In particular, the estimated rate constants ofethers and ketones are in better agreement withexperimental data than with previous SARs (Kwok andAtkinson, Atmos. Environ. 29, 1685–1695,1995). Rate constants of organic nitrates werenot well described by the SAR used in thisstudy. The basic assumption that the additive rateconstant for a chemical group is only influenced byneighbouring functional groups did not allow a goodparameterization for the rate constants of organicnitrates. The use of a second parameter to alter thereactivity of C-H bonds in -position to thefunctional group resulted in markedly better agreementbetween calculated and measured rate constants, butwas not extended due to the limited set of data. This indicates that strong electron withdrawing groups(e.g., nitrate groups) might influence the reactivityof C-H bonds that are not directly adjacent.     

Kinetics of the reactions of hydroxyl radicals with aliphatic ethers studied under simulated atmospheric conditions: Temperature dependences of the rate coefficients

Rate coefficients have been measured for the reactions of hydroxyl radicals with five aliphatic ethers over the temperature range 242–328 K. Competitive studies were carried out in an atmospheric flow reactor in which the hydroxyl radicals were generated by the photolysis of methyl nitrite in the presence of air containing nitric oxide. The reaction of OH with 2,3-dimethyl-butane was used as the reference reaction and the following Arrhenius parameters have been obtained for the reactions: OH+RORproducts:

Chlorine-hydrocarbon photochemistry in the marine troposphere and lower stratosphere

A one-dimensional photochemical model was used to explore the role of chlorine atoms in oxidizing methane and other nonmethane hydrocarbons (NMHCs) in the marine troposphere and lower stratosphere. Where appropriate, the model predictions were compared with available measurements. Cl atoms are predicted to be present in the marine troposphere at concentrations of approximately 10³ cm^-3, mostly as a consequence of the reaction of OH with HCl released from sea spray. Despite this low abundance, our results indicate that 20 to 40% of NMHC oxidation in the troposphere (0–10 km) and 40 to 90% of NMHC oxidation in the lower stratosphere (10–20 km) is caused by Cl atoms. At 15 km, NMHC-Cl reactions account for nearly 80% of the PAN produced.The model was also used to test the longstanding hypothesis that NOCl is an intermediate to HCl formation from sea salt aerosols. It was found that the NOCl concentration required (10 ppt) would be incompatible with field observations of reactive nitrogen and ozone abundance. Chlorine nitrate (ClONO₂) and methyl nitrate (CH₃ONO₂) were shown to be minor components of the total NO _y abundance. Heterogeneous reactions that might enhance photolysis of halocarbons or convert ClONO₂ to HOCl or Cl₂ were determined to be relatively unimportant sources of Cl atoms. Specific and reliable measurements of HCl and other reactive chlorine species are needed to better assess their role in tropospheric chemistry.     

The atmospheric chemistry of dimethylsulfoxide (DMSO) kinetics and mechanism of the OH+DMSO reaction

We have employed a pulsed laser photolysis-pulsed laser induced fluorescence technique to study the kinetics and mechanism of the reaction of OH with dimethylsulfoxide and its deuterated analogue. A rate coefficient of (1.0±0.3)×10^-10 cm³ molecule^-1 s^-1 was obtained ar room temperature. The rate coefficient was independent of pressure over the range 25–700 Torr, showed no dependence on the nature of the buffer gas and showed no kinetic isotope effect. A limited study of the temperature dependence indicated that the reaction displays a negative activation energy. The gas phase ultraviolet absorption spectrum was obtained at room temperature and showed a strong absorption feature in the far ultraviolet. The absolute absorption cross-section at 205 nm, the absorption peak, is (1.0±0.3)×10^-17 cm², where the large uncertainty results from experimental difficulties associated with the low vapor pressure and stickiness of DMSO.     

A Temperature-Dependent Study of the Ozonolysis of Propene

The ozonolysis of propene has been investigated in a temperature controlled reaction chamber at 295, 260, and 230 K. Experiments were performed using a total zero air pressure of 760 Torr (STP) and propene/ozone reactant mixing ratios ranging from 2.3 to 23 ppmv. An analysis of FTIR spectra collected at the conclusion of each reaction revealed that methane was formed with a yield of 0.14 ± 0.03 (precision) for all the temperatures investigated.In addition, the yield of HCHO decreased from 0.67 ± 0.04 to 0.43± 0.03 upon cooling from 295 to 230 K, whereas the yield of HCOOH increased from 0.11 ± 0.02 to 0.53 ± 0.04. Experiments were also performedusing an excess of cyclohexane (to scavenge OH) and it was found that the formaldehyde yield was 0.79 ± 0.05 and 0.61 ± 0.04 at 295 and260 K, respectively. Finally, to more fully understand the reaction energies involved in product formation, we have performed molecular orbital calculations of heats of formation of reactants, stable intermediates, and products. Three conclusions can be made of this work. First, the reaction CH₂OO + Aldehyde Secondary Ozonide HCOOH + Aldehyde is not an important mechanism in formic acid production. Second, the decomposition of the primary ozonide products (e.g., C₂ radical species) appears to occur, in part, by a thermal mechanism (e.g., thermalized to chamber temperature). Third, ab initio resultscombined with experiment reveal no correlation between reaction exothermicity and products formed (e.g., kinetically dictated product formation occurs). The abinitio database is provided nevertheless as a starting point for transition state calculations to be performed in the future. Finally, since formaldehyde yield decreases by at most 35% with decreasing temperature and formic acid is relatively unreactive in the atmosphere, our results suggest that temperature-dependent HCHO yield will constitute only a minor perturbation to HO_x formation in the middle troposphere.     

Products and Mechanism of the Gas Phase Reaction of Ozone with β-Pinene

Gas phase ozonolysis of -pinene was performedin a 570 l static reactor at 730 Torr and 296 K insynthetic air and the products were analysed by acombination of gas phase FTIR spectroscopy, HPLC andIC analyses of gas phase and aerosol samples,respectively. The reaction mechanism was investigatedby adding HCHO, HCOOH and H₂O as Criegeeintermediate scavenger and cyclohexane as OH radicalscavenger. Main identified products (yields inparentheses) in the presence of cyclohexane as OHradical scavenger were HCHO (0.65 ± 0.04),nopinone (0.16 ± 0.04), 3-hydroxy-nopinone (0.15± 0.05), CO₂ (0.20 ± 0.04), CO (0.030± 0.002), HCOOH (0.020 ± 0.002), the secondaryozonide of -pinene (0.16 ± 0.05), andcis-pinic acid (0.02 ± 0.01). The decompositionof the primary ozonide was found to yieldpredominantly the excited C₉-Criegee intermediateand HCHO (0.84 ± 0.04) and to a minor extent theexcited CH₂OO intermediate and nopinone (0.16± 0.04). Roughly 40% of the excitedC₉-Criegee intermediate becomes stabilised andcould be shown to react with HCHO, HCOOH and H₂O. The atmospherically important reaction of thestabilised C₉-Criegee intermediate with H₂Owas found to result in a nopinone increase of (0.35± 0.05) and in the formation of H₂O₂(0.24 ± 0.03). Based on the observed products,the unimolecular decomposition/isomerisationchannels of the C₉-Criegee intermediate arediscussed in terms of the hydroperoxide and esterchannels. Subsequent reactions of the nopinonylradical, formed in the hydroperoxide channel, lead tomajor products like 3-hydroxy-nopinone but also tominor products like cis-pinic acid. A mechanismfor the formation of this dicarboxylic acid isproposed and its possible role in aerosol formationprocesses discussed.     

Autoxidation of N(III), S(IV), and other Species in Frozen Solution – A Possible Pathway for Enhanced Chemical Transformation in Freezing Systems

The chemistry of glycolaldehyde (hydroxyacetaldehyde) relevant to the troposphere has been investigated using UV absorption spectrometry and FTIR absorption spectrometry in an environmental chamber. Quantitative UV absorption spectra have been obtained for the first time. The UV spectrum peaks at 277 nm with a maximum cross section of (5.5± 0.7)×10^–20 cm² molecule^–1. Studies of the ultraviolet photolysis of glycolaldehyde ( = 285 ± 25 nm) indicated that the overall quantum yield is > 0.5 in one bar of air, with the major products being CH₂OH and HCO radicals. Rate coefficients for the reactions of Cl atoms and OH radicals with glycolaldehyde have been determined to be (7.6± 1.5)×10^–11 and (1.1± 0.3)×10^–11 cm³ molecule^–1 s^–1, respectively, in good agreement with the only previous study. The lifetime of glycolaldehyde in the atmosphere is about 1.0 day for reaction with OH, and > 2.5 days for photolysis, although both wet and dry deposition should also be considered in future modeling studies.     

Products and mechanisms of the gas phase reactions of NO3 with CH3SCH3, CD3SCD3, CH3SH and CH3SSCH3

Products and mechanisms of the reaction between the nitrate radical (NO₃) and three of the most abundant reduced organic sulphur compounds in the atmosphere (CH₃SCH₃, CH₃SH and CH₃SSCH₃), have been studied in a 480 L reaction chamber using in situ FT-IR and ion chromatography as analytical techniques. In the three reactions, methanesulphonic acid was found to be the most abundant sulphur containing product. In addition the stable products SO₂, H₂SO₄, CH₂O, and CH₃ONO₂ were identified and quantified and thionitric acid-S-methyl ester (CH₃SNO₂) was observed in the i.r. spectrum from all of the three reactions. Deuterated dimethylsulphide (CD₃SCD₃) showed an isotope effect on the reaction Deuterated dimethylsulphide (CD₃SCD₃) showed an isotope effect on the reaction rate constant (k_H/k_D) of 3.8±0.6, indicating that hydrogen abstraction is the first step in the NO₃+CH₃SCH₃ reaction, probably after the formation of an inital adduct.Based on the products and intermediates identified, reaction mechanisms are proposed for the three reactions.     

The Gas Phase Reaction of Unsaturated Oxygenates with Ozone: Carbonyl Products and Comparison with the Alkene-Ozone Reaction

Carbonyl products have been identified and their formation yields measured in the gas phase reaction of ozone with unsaturated oxygenates in experiments carried out at ambient T, p = 1 atm. of purified humid air (RH = 50%) and with sufficient cyclohexane added to scavenge the hydroxyl radical. The compounds studied are the esters methyl acrylate, vinyl acetate and cis-3-hexenyl acetate, the carbonyl crotonaldehyde, the hydroxy-substituted diene linalool, the ether ethylvinyl ether and the keto-ether trans-4-methoxy-3-buten-2-one. The alkene 1-pentene was included for comparison. The nature and formation yields of the carbonyl products from this study and those measured in earlier work under the same conditions are compared to those of alkenes and are supportive of a reaction mechanism that is similar to that for the reaction of ozone with alkenes, i.e. O₃ + R₁R₂C=CR₃X (R₁COR₂ + R₃XCOO) + (1 – )(R₃COX + R₁R₂COO), where R_i are the alkyl substituents, X is the oxygen-containing substituent (–CHO for aldehydes; –C(O)R for ketones; –C(O)OR and –OC(O)R for esters; –OH and hydroxyalkyl for alcohols; and –OR for ethers), R₁COR₂ is the primary carbonyl, R₃COX is the other primary product and R₁R₂COO and R₃XCOO are the carbonyl oxide biradicals. The biradicals lead to carbonyls in reactions that are also analogous to those involved in carbonyl formation from biradicals in the ozone-alkene reaction. These features make it possible to predict the nature and formation yields of the major carbonyl products of the reaction of ozone with unsaturated oxygenates that may be components of biogenic emissions.     

Atmospheric chemistry of the monoterpene reaction products nopinone,camphenilone, and 4-acetyl-1-methylcyclohexene

Rate constants for the gas-phase reactions of OH radicals with nopinone (6,6-dimethylbicyclo[3.1.1]heptan-2-one) and camphenilone (3,3-dimethylbicyclo[2.2.1]heptan-2-one) and for the reactions of 4-acetyl-1-methylcyclohexene with OH and NO₃ radicals and O₃ have been measured at 296±2 K. The rate constants (cm³ molecule^–1 s^–1 units) obtained were, for reaction with the OH radical: nopinone, (1.43±0.37)×10^–11; camphenilone, (5.15±1.44)×10^–12; and 4-acetyl-1-methylcyclohexene, (1.29±0.33)×10^–10; for reaction with the NO₃ radical: 4-acetyl-1-methylcyclohexene, (1.05±0.38)×10^–11; and for reaction with O₃: 4-acetyl-1-methylcyclohexene, (1.50±0.53)×10^–16. These data are used to calculate the tropospheric lifetimes of these monoterpene atmospheric reaction products.