Rate constants for reactions of OH radicals with some Br-containing haloalkanes

Rate constants have been measured for the gas-phase reactions of hydroxyl radical with two halons and three of their proposed substitutes and also with CHClBr-CF₃ using the discharge-flow-EPR technique over the temperature range 298–460 K. The following Arrhenius expressions have been derived (units are 10^–13 cm³ molecule^–1 s^–1): (9.3 _–0.9 ^+1.0 ) exp{–(1326±33)/T} for CHF₂Br; (7.2 _–0.6 ^+0.7 ) exp{–(1111±32)/T} for CHFBrCF₃; (8.5 _–0.8 ^+0.9 ) exp{–(1113±35)/T} for CH₂BrCF₃; (12.8 _–1.2 ^+1.5 ) exp{–(995±38)/T} for CHClBrCF₃. The rate constants at 298 K have been estimated to be <2×10^–17 cm³ molecule^–1 s^–1 for CF₃Br and CF₂Br—CF₂Br. The atmospheric lifetimes due to hydroxyl attack have been estimated to be 5.5, 3.3, 2.8, and 1.2 years for CHF₂Br, CHFBr—CF₃, CH₂Br—CF₃ and CHClBr—CF₃, respectively.     

Kinetics of the Gas-Phase Reactions of OH Radicals, NO3 Radicals and O3 with Three C7-Carbonyls Formed From The Atmospheric Reactions of Myrcene, Ocimene and Terpinolene

Rate constants for the gas-phase reactions of OH radicals, NO₃ radicals and O₃ with the C₇-carbonyl compounds 4-methylenehex-5-enal [CH₂=CHC(=CH₂)CH₂CH₂CHO], (3Z)- and (3E)-4-methylhexa-3,5-dienal [CH₂=CHC(CH₃)=CHCH₂CHO] and 4-methylcyclohex-3-en-1-one, which are products of the atmospheric degradations of myrcene, Z- and E-ocimene and terpinolene, respectively, have been measured at 296 ± 2 K and atmospheric pressure of air using relative rate methods. The rate constants obtained (in cm³ molecule^–1 s^–1 units) were: for 4-methylenehex-5-enal, (1.55 ± 0.15) × 10^–10, (4.75 ± 0.35) × 10^–13 and (1.46 ± 0.12) × 10^–17 for the OH radical, NO₃ radical and O₃ reactions, respectively; for (3Z)-4-methylhexa-3,5-dienal: (1.61 ± 0.35) × 10^–10, (2.17 ± 0.30) × 10^–12, and (4.13 ± 0.81) × 10^–17 for the OH radical, NO₃ radical and O₃ reactions, respectively; for (3E)-4-methylhexa-3,5-dienal: (2.52 ± 0.65) × 10^–10, (1.75 ± 0.27) × 10^–12, and (5.36 ± 0.28) × 10^–17 for the OH radical, NO₃ radical and O₃ reactions, respectively; and for 4-methylcyclohex-3-en-1-one: (1.10 ± 0.19) × 10^–10, (1.81 ± 0.35) × 10^–12, and (6.98 ± 0.40) × 10^–17 for the OH radical, NO₃ radical and O₃ reactions, respectively. These carbonyl compounds are all reactive in the troposphere, with daytime reaction with the OH radical and nighttime reaction with the NO₃ radical being predicted to dominate as loss processes and with estimated lifetimes of about an hour or less.     

Rate constants for the reactions of the NO3 radical with HCOOH/HCOO− and CH3COOH/CH3COO− in aqueous solution between 278 and 328 K

The kinetics of the aqueous phase reactions of NO₃ radicals with HCOOH/HCOO^– and CH₃COOH/CH₃COO^– have been investigated using a laser photolysis/long-path laser absorption technique. NO₃ was produced via excimer laser photolysis of peroxodisulfate anions (S₂O ₈ ^2– ) at 351 nm followed by the reactions of sulfate radicals (SO ₄ ^– ) with excess nitrate. The time-resolved detection of NO₃ was achieved by long-path laser absorption at 632.8 nm. For the reactions of NO₃ with formic acid (1) and formate (2) rate coefficients ofk ₁=(3.3±1.0)×10⁵ l mol^–1 s^–1 andk ₂=(5.0±0.4)×10⁷ l mol^–1 s^–1 were found atT=298 K andI=0.19 mol/l. The following Arrhenius expressions were derived:k ₁(T)=(3.4±0.3)×10¹⁰ exp[–(3400±600)/T] l mol^–1 s^–1 andk ₂(T)=(8.2±0.8)×10¹⁰ exp[–(2200±700)/T] l mol^–1 s^–1. The rate coefficients for the reactions of NO₃ with acetic acid (3) and acetate (4) atT=298 K andI=0.19 mol/l were determined as:k ₃=(1.3±0.3)×10⁴ l mol^–1 s^–1 andk ₄=(2.3±0.4)×10⁶ l mol^–1 s^–1. The temperature dependences for these reactions are described by:k ₃(T)=(4.9±0.5)×10⁹ exp[–(3800±700)/T] l mol^–1 s^–1 andk ₄(T)=(1.0±0.2)×10¹² exp[–(3800±1200)/T] l mol^–1 s^–1. The differences in reactivity of the anions HCOO^– and CH₃COO^– compared to their corresponding acids HCOOH and CH₃COOH are explained by the higher reactivity of NO₃ in charge transfer processes compared to H atom abstraction. From a comparison of NO₃ reactions with various droplets constituents it is concluded that the reaction of NO₃ with HCOO^– may present a dominant loss reaction of NO₃ in atmospheric droplets.     

FT-IR Study of the Kinetics and Products of the Reactions of Dimethylsulphide, Dimethylsulphoxide and Dimethylsulphone with Br and BrO

Kinetics and products of the gas-phase reactions of dimethylsulphide (DMS), dimethylsulphoxide (DMSO) and dimethylsulphone (DMSO₂) with Br atoms and BrO radicals in air have beeninvestigated using on-line Fourier Transform Infrared Spectroscopy (FT-IR) as analytical technique at 740 ± 5 Torr total pressure and at 296 ± 3 K in a480 L reaction chamber. Using a relative rate method for determining the rate constants; the following values (expressed in cm³molecule^–1 s^–1) were found: k_DMS+Br = (4.9 ±1.0) ×10^–14, k_DMSO + Br < 6 × 10^–14,k_{DMSO 2} + Br 1 × 10^–15,k_DMSO + BrO = (1.0 ± 0.3) × 10^–14 andk_{DMSO 2} + BrO 3 × 10^–15 (allvalues are given with one on the experimental data). DMSO, SO₂, COS, CH₃SBr andCH₃SO₂Br were identified as the main sulphur containing products of the oxidation of DMS by Br atoms. From the reaction between DMSO and Br atoms, DMSO₂and CH₃SO₂Br were the only sulphur containing products thatwere identified. DMSO, DMSO₂ and SO₂ were identified as themain sulphur containing products of the reaction between DMS and BrO.DMSO₂ was found to be the only product of the reaction between DMSO and BrO. For the reactions of DMSO₂ with Br and BrO no products were identified because the reactions were too slow.The implications of these results for atmospheric chemistry are discussed.     

FTIR studies of reactions between the nitrate radical and haloethenes

Products and mechanisms for the gas-phase reactions of NO₃ radicals with CH₂=CHCl, CH₂=CCl₂, CHCl=CCl₂,cis-CHCl=CHCl andtrans-CHCl=CHCl in air have been studied. The experiments were carried out at 295±2 K and 740±5 Torr in a 480-L Teflon-coated reaction chamber and at 295±2 K and 760±5 Torr in a 250-L stainless steel reactor. NO₃ was generated by the thermal dissociation of N₂O₅. Experiments with¹⁵NO₃ and CD₂CDCl have also been performed. The initially formed nitrate peroxynitrates decay into carbonyl compounds, nitrates, HCl and ClNO₂. In adidtion, there are indications of nitrooxy acid chlorides being produced. The reactions with CH₂=CCl₂ and CHCl=CCl₂ are more complex due to release of chlorine atoms which eventually lead to formation of chloroacid chlorides.A general reaction mechanism is proposed and the observed concentration-time profiles of reactants and products are simulated for each compound. The rate constants for the initial step of NO₃ addition to the chloroethenes are determined as: (2.6±0.5, 9.4±0.9, 2.0±0.4 and 1.4±0.4) × 10^–16 cm³ molecule^–1 s^–1 for CH₂=CHCl, CH₂=CCl₂, CHCl=CCl₂ andcis-CHCl=CHCl, respectively.     

Determination of rate constants for reactions of some hydrohaloalkanes with OH radicals and their atmospheric lifetimes

Rate constants have been measured for the gas-phase reactions of hydroxyl radical with partly halogenated alkanes using the discharge-flow-EPR technique over the temperature range 298–460 K. The following Arrhenius expressions have been derived (units 10^–13 cm³ molecule^–1 s^–1): (8.1 _–1.2 ^+1.5 ) exp{–(1516±53)/T} for CHF₂Cl (HCFC-22); (10.3 _–1.5 ^+1.8 ) exp{–(1588±52)/T} for CH₂FCF₃ (HFC-134a); (11.3 _–1.6 ^+2.1 ) exp{–(918±52)/T} for CHCl₂CF₂Cl (HCFC-122); (9.2 _–2.0 ^+2.5 ) exp{–(1281±85)/T} for CHFClCF₂Cl (HCFC-123a).The atmospheric lifetimes for the substances have been estimated to be 12.6, 12.9, 1.05, and 4.8 years, respectively, and the accuracy of the estimates is discussed.     

Kinetics and Products of the Gas-Phase Reaction of OH Radicals with 5-Hydroxy-2-Pentanone at 296± 2 K

The 1,4-hydroxycarbonyl 5-hydroxy-2-pentanone is an important product of the gas-phase reaction of OH radicals with n-pentane in the presence of NO. We have used a relative rate method with 4-methyl-2-pentanone as the reference compound to measure the rate constant for the reaction of OH radicals with 5-hydroxy-2-pentanone at 296 ± 2 K. The carbonyls were sampled by on-fiber derivatization using a Solid Phase Micro Extraction (SPME) fiber coated with O> -(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride with subsequent thermal desorption of the oxime derivatives and quantification by gas chromatography with flame ionization detection. For comparison, the reference compound was also analyzed following sample collection onto a Tenax adsorbent cartridge. Products of the reaction were investigated using coated-fiber SPME sampling with gas chromatography-mass spectrometry analysis as well as by using in situ atmospheric pressure ionization mass spectrometry. A rate constant for the reaction of OH radicals with 5-hydroxy-2-pentanone of (1.6 ± 0.4) × 10^–11 cm³ molecule^–1 s^–1 was obtained at 296 ± 2 K. Two dicarbonyl products, of molecular weight 86 and 100, were observed and are attributed to CH₃C(O)CH₂CHO and CH₃C(O)CH₂CH₂CHO, respectively. Reaction schemes leading to these products are presented.     

Kinetic Studies of OH and O3 Reactions with Allyl and Isopropenyl Acetate

Rate coefficients have been measured for the gas phasereactions of hydroxyl (OH) radicals and ozone with twounsaturated esters, allyl acetate(CH₃C(O)OCH₂CH=CH₂) and isopropenylacetate (CH₃C(O)OC(CH₃)=CH₂). The OHexperiments were carried out using the pulsed laserphotolysis – laser induced fluorescence technique overthe temperature range 243–372 K and the kinetic dataused to derive the following Arrhenius expressions (inunits of cm³ molecule^-1 s^-1): allylacetate, k ₁ = (2.33 ± 0.27) ×10^-12 exp[(732 ± 34)/T]; and isopropenyl acetate,k ₂ = (4.52 ± 0.62) × 10^-12exp[(809 ± 39)/T]. At 298 K, the rate coefficients obtained (inunits of 10^-12 cm³ molecule^-1 s^-1)are: k ₁ = (27.1 ± 3.0) and k ₂= (69.6± 9.4). The relative rate technique has been usedto determine rate coefficients for the reaction ofozone with the acetates. Using methyl vinyl ketone asthe reference compound and a value of4.8 × 10^-18 cm³ molecule^-1s^-1 asthe rate coefficient for its reaction with O₃,the following rate coefficients were derived at 298 ± 4 K (in units of10^-18 cm³molecule^-1 s^-1): allyl acetate, (2.4 ± 0.7) andisopropenyl acetate (0.7 ± 0.2). Theresults are discussed in terms of structure-activityrelationships and used to derive atmospheric lifetimesfor the acetates.     

Fourier transform infrared studies of the reaction of Cl atoms with PAN,PPN, CH3OOH,HCOOH, CH3COCH3 and CH3COC2H5 at 295±2 K

The relative rate technique has been used to measure rate constants for the reaction of chlorine atoms with peroxyacetylnitrate (PAN), peroxypropionylnitrate (PPN), methylhydroperoxide, formic acid, acetone and butanone. Decay rates of these organic species were measured relative to one or more of the following reference compounds; ethene, ethane, chloroethane, chloromethane, and methane. Using rate constants of 9.29×10^–11, 5.7×10^–11, 8.04×10^–12, 4.9×10^–13, and 1.0×10^–13 cm³ molecule^–1 sec^–1 for the reaction of Cl atoms with ethene, ethane, chloroethane, chloromethane, and methane respectively, the following rate constants were derived, in units of cm³ molecule^–1 s^–1: PAN, <7×10^–15; PPN, (1.14±0.12)×10^–12; HCOOH, (2.00±0.25)×10^–13; CH₃OOH, (5.70±0.23)×10^–11; CH₃COCH₃, (2.37±0.12)×10^–12; and CH₃COC₂H₅, (4.13±0.57)×10^–11. Quoted errors represent 2 and do not include possible systematic errors due to errors in the reference rate constants. Experiments were performed at 295±2 K and 700 torr total pressure of nitrogen or synthetic air. The results are discussed with respect to the previous literature data and to the modelling of nonmethane hydrocarbon oxidation in the atmosphere.In recent discussions with Dr. R. A. Cox of Harwell Laboratory, UKAEA, we learnt of a preliminary value for the rate constant of the reaction of Cl with acetone of (2.5±1.0)×10^–12 cm³ molecule^–1 sec^–1 measured by R. A. Cox, M. E. Jenkin, and G. D. Hayman using molecular modulation techniques. This value is in good agreement with our results.     

Identification of Regional Sources of Methyl Bromide and Methyl Iodide from AGAGE Observations at Cape Grim,Tasmania

There are large uncertainties in identifying and quantifying the globally significant sources and sinks of methyl bromide (CH₃Br) and methyl iodide (CH₃I). Long-term, quasi-continuous observations can provide valuable information about their regional sources, which may be significant in the global context. We report 3 years of in situobservations of these trace gases from the AGAGE (Advanced Global Atmospheric Gas Experiment) program at Cape Grim, Tasmania (41 °S, 145 °E). The average background levels of CH₃Br and CH₃I during March 1998–March 2001 were 8.05 and 1.39 ppt (dry air mole fractions expressed in parts per 10¹²), respectively. The CH₃Br background data showed little seasonal variability. Trajectory analyses reveal that air masses showing elevated CH₃Br levels at Cape Grim have had significant contact with coastal-terrestrial and/or coastal-seawater and/or urban source regions. The CH₃I background data showed a seasonal cycle with a 3-year average amplitude of 0.47 ppt and maximum concentrations in summer, suggesting that the Southern Ocean is a significant source.Trajectory analyses reveal that air masses showing highly elevated CH₃I levels at Cape Grim have had significant contact with coastal-terrestrial and/or coastal-seawater regions and/or the open-ocean regions of Bass Strait and the Tasman Sea.     

Henry's law coefficients of formic and acetic acids

The Henry's law constants, K _H, of dilute aqueous formic and acetic acids were determined experimentally as a function of concentration and temperature using a new counterflow packed-column technique. K _H was found to be (8.9±1.3)×10³ and (4.1±0.4)×10³ M atm^-1 at 25°C for HCOOH and CH₃COOH, respectively. The reaction enthalpies, H, were found to be –51±2 kJ mol^-1 and –52±1 kJ mol^-1 for formic and acetic acid, respectively. These are in good agreement with calculated thermochemical values.Whereas the K _H values are in reasonably good agreement with certain other experimentally determined values, K _H (HCOOH) is two to three times higher than calculated thermochemical values while K _H (CH₃COOH) is lower than the two calculated values.The best experimental values appear to be (11±2)×10³ M atm^-1 and (7±3)×10³ M atm^-1 for HCOOH and CH₃COOH, respectively.     

A discharge flow mass-spectrometric study of the reaction between the NO3 radical and isoprene

The kinetics and mechanism of the reactionNO₃+CH₂=C(CH₃)–CH=CH₂productswere studied in two laboratories at 298 K in the pressure range 0.7–3 torr using the discharge-flow mass-spectrometric method. The rate constant obtained under pseudo-first-order conditions with excess of either NO₃ or isoprene was: k ₁=(7.8±0.6)×10^–13 cm³ molecule^–1 s^–1. The product analysis indicated that the primary addition of NO₃ occurred on both -bonds of the isprene molecule.     

AGAGE Observations of Methyl Bromide and Methyl Chloride at Mace Head, Ireland, and Cape Grim, Tasmania, 1998–2001

In situ AGAGE GC-MS measurements of methyl bromide (CH₃Br) and methyl chloride (CH₃Cl) at Mace Head, Ireland and Cape Grim, Tasmania (1998–2001) reveal a complex pattern of sources. At Mace Head both gases have well-defined seasonal cycles with similar average annual decreases of 3.0% yr⁻¹ (CH₃Br) and 2.6% yr⁻¹ (CH₃Cl), and mean northern hemisphere baseline mole fractions of 10.37 ± 0.05 ppt and 535.7 ± 2.2 ppt, respectively. We have used a Lagrangian dispersion model and local meteorological data to segregate the Mace Head observations into different source regions, and interpret the results in terms of the known sources and sinks of these two key halocarbons. At Cape Grim CH₃Br and CH₃Cl also show annual decreases in their baseline mixing ratios of 2.5% yr⁻¹ and 1.5% yr⁻¹, respectively. Mean baseline mole fractions were 7.94 ± 0.03 ppt (CH₃Br) and 541.3 ± 1.1 ppt (CH₃Cl). Although CH₃Cl has astrong seasonal cycle there is no well-defined seasonal cycle in the Cape Grim CH₃Br record. The fact that both gases are steadily decreasing in the atmosphere at both locations implies that a change has occurred which is affecting a common, major source of both gases (possibly biomass burning) and/or their major sink process (destruction by hydroxyl radical).     

Henry's law coefficients for aqueous solutions of acetone,acetaldehyde and acetonitrile,and equilibrium constants for the addition compounds of acetone and acetaldehyde with bisulfite

Vapor phase concentrations of acetone, acetaldehyde and acetonitrile over their aqueous solutions were measured to determine Henry's law partition coefficients for these compounds in the temperature range 5–40 °C. The results are for acetone: ln(H ₁/atm)=–(5286±100)T+(18.4±0.3); acetaldehyde: ln(H ₁/atm)=–(5671±22)/T+(20.4±0.1); and acetonitrile: ln(H ₁/atm)=–(4106±101)/T+(13.8±0.3). Artificial seawater of 3.5% salinity in place of deiionized water raisesH ₁ by about 15%. A similar technique has been used to measure the equilibrium constants for the addition compounds of acetone and acetaldehyde with bisulfite in aqueous solution. The results are ln(K ₁/M ^–1)=(4972±318)/T–(11.2±1.1) and ln(K ₁/M ^–1)=(6240±427)/T–(8.1±1.3), respectively. The results are compared and partly combined with other data in the literature to provide an average representation.     

Further studies of the temperature dependence of the rate coefficients for the reactions of OH with a series of ethers under simulated atmospheric conditions

The following temperature-dependent rate coefficients (k/cm³ molecule^–1 s^–1) of the reactions of hydroxyl radicals with aliphatic ethers have been determined over the temperature range 247–373 K by a competitive flow technique: diethyl ether,k _OH=5.2×10^–12 exp[(262±150)/T]; methyln-butyl ether,k _OH=5.4×10^–12 exp[(309±150)/T]; ethyln-butyl ether,k _OH=7.3×10^–12 exp[(335±150)/T]; di-n-butyl ether,k _OH=5.5×10^–12 exp[(502±150)/T] and di-n-pentyl ether,k _OH=8.5×10^–12 exp[(417±150)/T]. The data have been measured relative to the rate coefficientk(OH + 2,3-dimethylbutane)=6.2×10^–12 cm³ molecule^–1 s^–1 independent of temperature.Previous discrepancies in the room-temperature rate coefficients for the OH reactions with ethyln-butyl ether and di-n-butyl ether, obtained in the flow and static experiments of Bennett and Kerr (J. Atmos. Chem. 8, 87–94, 1989;10, 29–38, 1990) compared with those of Wallingtonet al. (Int. J. Chem. Kinet. 20, 541–547, 1988;21, 993–1001, 1989) and of Nelsonet al. (Int. J. Chem. Kinet. 22, 1111–1126, 1990) have been resolved. The results are considered in relation to the available literature data and evaluated rate expressions are deduced where possible. The data are also discussed in terms of structure-activity relationships.     

An Experimental Study on the Temperature Dependence for the Gas-Phase Reactions of NO3 Radical with a Series of Aliphatic Aldehydes

The absolute rate constants for the gas-phasereactions of the NO₃ radical with a series ofaldehydes such as acetaldehyde, propanal, butanal,pentanal, hexanal and, heptanal were measured overthe temperature range 298–433 K, using a dischargeflow system and monitoring the NO₃ radical byLaser Induced Fluorescence (LIF).The measured rate constants at 298 K for thereaction of NO₃, in units of 10^–14 cm³molecule^–1 s^–1, were as follows:acetaldehyde 0.32 ± 0.04, propanal 0.60 ± 0.06, butanal 1.46± 0.16, pentanal 1.75 ±0.06, hexanal 1.83 ± 0.36, and heptanal 2.37 ±0.42. The proposed Arrhenius expressions arek₁ = (6.2 ± 7.5) × 10^–11 exp[–(2826 ± 866)/T] (cm³ molecule^–1s^–1),k₂ = (1.7 ± 1.0) × 10^–11 exp[–(2250 ± 192)/T] (cm³ molecule^–1s¹), k₃ =(7.6 ± 9.8) × 10¹¹ exp[–(2466 ± 505)/T] (cm³ molecule^–1s^–1),k₄ = (2.8 ± 1.4) × 10^–11 exp[–(2189 ± 156)/T] (cm³ molecule^–1s^–1), k₅ = (7.0 ± 1.8) ×10^–11 exp [–(2382 ± 998)/T](cm³ molecule^–1 s^–1), andk₆ = (7.8 ± 1.0) × 10^–11 exp[–(2406 ± 481)/T](cm³ molecule^–1 s^–1).Tropospheric lifetimes for these aldehydes werecalculated at night and during the day for typicalNO₃ and OH average concentrations and showed thatboth radicals provide an effective tropospheric sinkfor these compounds and that the night-time reactionwith the NO₃ radical can be an important, if notdominant, loss process for these emitted organics andfor NO₃ radicals.     

Regional Sources of Methyl Chloride, Chloroform and Dichloromethane Identified from AGAGE Observations at Cape Grim, Tasmania, 1998–2000

There are large uncertainties in identifying and quantifying the natural and anthropogenic sources of chloromethanes – methyl chloride (CH₃Cl), chloroform (CHCl₃) and dichloromethane (CH₂Cl₂), which are responsible for about 15% of the total chlorine in the stratosphere. We report two years of in situ observations of these species from the AGAGE (Advanced Global Atmospheric Gas Experiment) program at Cape Grim, Tasmania (41° S, 145° E). The average background levels of CH₃Cl, CHCl₃ and CH₂Cl₂ during 1998–2000 were 551± 8, 6.3± 0.2 and 8.9± 0.2 ppt (dry air mole fractions expressed in parts per 10¹²) respectively, with a two-year average amplitude of the seasonal cycles in background air of 25, 1.1 and 1.5 ppt respectively. The CH₃Cl and CHCl₃ records at Cape Grim show clear episodes of elevated mixing ratios up to 1300 ppt and 55 ppt respectively, which are highly correlated, suggesting common source(s). Trajectory analyses show that the sources of CH₃Cl and CHCl₃ that are responsible for these elevated observations are located in coastal-terrestrial and/or coastal-seawater regions in Tasmania and the south-eastern Australian mainland. Elevated levels of CH₂Cl₂ (up to 70 ppt above background) are associated mainly with emissions from the Melbourne/Port Phillip region, a large urban/industrial complex (population 3.5 million) 300 km north of Cape Grim.Now at the Centre for Atmospheric ChemistryNow at School of Environmental Sciences     

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.     

The Henry's Law Constants of the Haloacetic Acids

Henry's law constants K_H (mol kg^-1 atm^-1) have been measured between 278.15 K and 308.15 K for the following organic acids: CH₂FCOOH (ln(K_H[298.15 K]) = 11.3 ± 0.2), CH₂ClCOOH (11.59 ± 0.14), CH₂BrCOOH (11.94 ± 0.21), CHF₂COOH (10.32 ± 0.10), CHCl₂COOH (11.69 ± 0.11), CHBr₂COOH (12.33 ± 0.29), CBr₃COOH (12.61 ± 0.21), and CClF₂COOH (10.11 ± 0.12). The variation of K_H with temperature was determined for all acids except CH₂FCOOH and CBr₃COOH, with _r H° for the dissolution reaction ranging from –85.2 ± 2.6 to –57.1 ± 2.5 kJ mol^-1, meaning that their solubility is generally more sensitive to temperature than is the case for the simple carboxylic acids. The Henry's law constants show consistent trends with halogen substitution and, together with their high solubility compared to the parent (acetic) acid (ln(K_H[298.15 K]) = 8.61), present a severe test of current predictive models based upon molecular structure. The solubility of haloacetic acids and strong dissociation at normal pH mean that they will partition almost entirely into cloud and fog in the atmosphere (0.05–1.0 g H₂O m^-3), but can reside in both phases for the liquid water contents typical of aerosols (10^-5-10^-4 g H₂O m^-3).     

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.