OH Reaction Rate Research Articles

Indirect Photolysis of Organic Compounds: Prediction of OH Reaction Rate Constants through Molecular Orbital Calculations

The MOOH approach is a perturbational molecular orbital method to predict rate constants of indirect photolysis of organic compounds through reaction with OH radicals. It employs the semiempirical AM1 scheme as the underlying quantum chemical model. The original method introduced by Klamt has been reparametrized using an up-to-date set of 675 compounds with experimental rate constants and outperforms the prominent Atkinson increment scheme for this training set as well as for an extended set of 805 compounds, yielding an overall root-mean-square error of 0.32 log units. The discussion includes detailed comparative analyses of the model performances for individual compound classes. The present model calibration refers mainly to monofunctional compounds but performs already reasonably well for multifunctional compounds. For predictive applications, both the Atkinson scheme and the alternative, independent AM1-MOOH model can be used as components of a consensus modeling approach, arriving at increased confidence in cases where the different models agree.

Monte Carlo Simulations of Site-Specific Radical Attack to DNA Bases

An atomistic biophysical model permitting the calculation of initial attacks to a 38-bp representation of B-DNA base moieties by water radicals is presented. This model is based on a previous radiation damage model developed by Aydogan et al. (Radiat. Res. 157, 38-44, 2002). Absolute efficiencies for radical attack to the 38-bp DNA molecule are calculated to be 41, 0.8 and 15% for hydroxyl radical ((.)OH), hydrogen radical (H(.)), and hydrated electron (e(aq))(,) respectively. Among the nucleobases, guanine is found to have the highest percentage (.)OH attack probability at 36%. Adenine, cytosine and thymine moieties have initial attack probabilities of 24, 18 and 22%, respectively. A systematic study is performed to investigate (.)OH attack probabilities at each specified attack site in four molecular models including free bases, single nucleotides, single base pairs, and the central eight base pairs of the 38-bp DNA molecule. Cytosine is the free base moiety for which the closest agreement is observed between the model prediction and the experimental data. The initial (.)OH attack probabilities for cytosine as the free base are calculated to be 72 and 28%, while experimental data are reported at 87 and 13% for the C5 and C6 positions on the base, respectively. In this study, we incorporated atomic charges to scale the site-specific (.)OH reaction rates at the individual atomic positions on the pyrimidine and purine bases. Future updates to the RIDNA model will include the use of electron densities to scale the reaction rates. With respect to reactions of the aqueous electron with DNA, a comparison of the initial distribution of electron attack sites calculated in this study and experimental results suggests an extremely rapid and extensive redistribution of the e(-)(aq) after their initial reactions with DNA.

OH reaction rate constants and UV absorption cross-sections of unsaturated esters

Absolute rate coefficients have been determined for the gas-phase reactions of hydroxyl radicals with methyl acrylate ( k 1), methyl methacrylate ( k 2) and ethyl acrylate ( k 3). Experiments were performed using two different techniques, the relative rate method and the pulsed laser photolysis-laser induced fluorescence technique. The kinetic data obtained were used to derive the following Arrhenius expressions in the temperature range 253–374 K (in units of cm 3 molecule −1 s −1): k 1 = ( 2.0 ± 0.8 ) × 1 0 - 12 exp [ ( 553 ± 51 ) / T ] , k 2 = ( 2.5 ± 0.8 ) × 1 0 - 12 exp [ ( 821 ± 55 ) / T ] , k 3 = ( 2.3 ± 0.8 ) × 1 0 - 12 exp [ ( 580 ± 65 ) / T ] . At 298 K, the reaction rate constants obtained by the two methods were in good agreement. In addition, the UV absorption spectra for the three unsaturated esters have been determined at (298±2) K and the absorption cross-sections in the wavelength region 215–298 nm were reported. The results are presented, discussed and used to estimate the atmospheric lifetimes for the studied esters.

Atmospheric Fate of Dichlorvos: Photolysis and OH-Initiated Oxidation Studies

The OH-initiated oxidation of dichlorvos (a widely used insecticide) has been investigated under atmospheric conditions at the large outdoor European photoreactor (EUPHORE) in Valencia, Spain. The rate constant of OH reaction with dichlorvos, k, was measured by using a conventional relative rate technique where 1,3,5-trimethylbenzene (TMB) and cyclohexane were taken as references. With the use of the rate constants of 5.67 x 10(-11) and of 6.97 x 10(-12) cm3 molecule(-1) s(-1) for the reactions OH + TMB and OH + cyclohexane, respectively, the resulting value of the OH reaction rate constant with dichlorvos was derived to be k = (2.6 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1). The tropospheric lifetime of dichlorvos with respect to reaction with OH radical has been estimated to be around 11 h. The major carbon-containing products observed for the OH reaction with dichlorvos in air under sunlight condition were phosgene and carbon monoxide. The formation of a very stable toxic primary product such as phosgene associated with the relatively short lifetime of dichlorvos may make the use of this pesticide even more toxic for humans when released into the atmosphere.

Study of the Carbon-13 and Deuterium Kinetic Isotope Effects in the Cl and OH Reactions of CH4 and CH3Cl

Relative rate experiments have been carried out for three isotopologues of chloromethane and their reactions with Cl atoms and OH radicals. The OH and Cl reaction rates of CH2DCl and CHD2Cl were measured by long-path FTIR spectroscopy relative to CH3Cl at 298+/-2 K and 1013+/-10 hPa in purified air. The FTIR spectra were fitted using a nonlinear least squares spectral fitting method including measured high-resolution infrared spectra as references. The relative reaction rates defined by alpha=klight/kheavy were determined to be kOH+CH3Cl/kOH+CH2DCl=1.41+/-0.05, kOH+CH3Cl/kOH+CHD2Cl=2.03+/-0.05, kCl+CH3Cl/kCl+CH2DCl=1.42+/-0.04, and kCl+CH3Cl/kCl+CHD2Cl=2.27+/-0.04. The carbon-13 and deuterium kinetic isotope effects in the OH and Cl reactions of CH3Cl were investigated further using variational transition state theory, and the results were compared to similar calculations performed for the CH4+OH/Cl reaction systems. The calculations show that the order of magnitude difference for the carbon-13 kinetic isotope effect in the OH reaction of CH3Cl compared to CH4 reported by Gola et al. (Atmos. Chem. Phys. 2005, 5, 2395) can be explained by the lower barrier to internal rotation of the OH radical in the transition state of the CH4+OH reaction than in the CH3Cl+OH reaction. The deuterium kinetic isotope effects can be explained in terms of combined variational effects and tunneling.

Kinetics of the OH and Cl reactions with N-methylformamide, N, N-dimethylformamide and N, N-dimethylacetamide

A relative kinetic technique was used in combination with long-path Fourier transform infrared spectroscopy to study the kinetics of the reactions of N-methylformamide (HC(O)NHCH 3), N, N-dimethylformamide (HC(O)N(CH 3) 2) and N, N-dimethylacetamide (CH 3C(O)N(CH 3) 2) with OH radicals and Cl atoms at 298 ± 2 K and atmospheric pressure 1000 ± 10 mbar of air. The measured OH and Cl reaction rate coefficients (in units of cm 3 molecule −1 s −1) were, respectively, (0.86 ± 0.24) × 10 −11 and (0.97 ± 0.17) × 10 −10 for N-methylformamide, (1.4 ± 0.3) × 10 −11 and (1.9 ± 0.3) × 10 −10 for N, N-dimethylformamide, (1.9 ± 0.3) × 10 −11 and (2.5 ± 0.4) × 10 −10 for N, N-dimethylacetamide. The results are presented and discussed in terms of structure–activity relationships and the atmospheric impact of the amides.

Atmospheric Chemistry of CHF2CHO: Study of the IR and UV−Vis Absorption Cross Sections, Photolysis, and OH-, Cl-, and NO3-Initiated Oxidation

The infrared and ultraviolet-visible absorption cross sections, effective quantum yield of photolysis, and OH, Cl, and NO3 reaction rate coefficients of CHF2CHO are reported. Relative rate measurements at 298 +/- 2 K and 1013 +/- 10 hPa gave kOH = (1.8 +/- 0.4) x 10(-12) cm3 molecule(-1) s(-1) (propane as reference compound), kCl = (1.24 +/- 0.13) x 10(-11) cm3 molecule(-1) s(-1) (ethane as reference compound), and kNO3 = (5.9 +/- 1.7) x 10(-17) cm3 molecule(-1) s(-1) (trans-dichloroethene as reference compound). The photolysis of CHF2CHO has been investigated under pseudonatural tropospheric conditions in the European simulation chamber, Valencia, Spain (EUPHORE), and an effective quantum yield of photolysis equal to 0.30 +/- 0.05 over the wavelength range 290-500 nm has been extracted. The tropospheric lifetime of CHF2CHO is estimated to be around 1 day and is determined by photolysis. The observed photolysis rates of CH3CHO, CHF2CHO, and CF3CHO are discussed on the basis of results from quantum chemical calculations.

Study of the OH and Cl-Initiated Oxidation, IR Absorption Cross-Section, Radiative Forcing, and Global Warming Potential of Four C4-Hydrofluoroethers

Infrared absorption cross-sections and OH and Cl reaction rate coefficients for four C4-hydrofluoroethers (CF3)2CHOCH3, CF3CH2OCH2CF3, CF3CF2CH2OCH3, and CHF2CF2CH2OCH3 are reported. Relative rate measurements at 298 K and 1013 hPa of OH and Cl reaction rate coefficients give k(OH+(CF3)2CHOCH3) = (1.27+/-0.13) x 10(-13), k(OH+CF3CH2OCH2CF3) = (1.51+/-0.24) x 10(-13), k(OH+CF3CF2CH2OCH3) = (6.42+/-0.33) x 10(-13), k(OH+CHF2CF2CH2OCH3) = (8.7 +/-0.5) x 10(-13), k(Cl+(CF3)2CHOCH3) = (8.4+/-1.3) x 10(-12), k(Cl+CF3CH2OCH2CF3) = (6.5+/-1.7) x 10(-13), k(Cl+CF3CF2CH2OCH3) = (4.0+/-0.8) x 10(-11), and k(Cl+CHF2CF2CH2OCH3) = (2.65+/-0.17) x 10(-11) cm3 molecule(-1) s(-1). The primary products of the OH and Cl reactions with the fluorinated ethers have been identified as esters, and OH and Cl reaction rate coefficients for one of these, CF3CH2OCHO, are reported: k(OH+CF3CH2OCHO) = (7.7+/-0.9) x 10(-14) and kCl+CF3CH2OCHO) = (6.3+/-1.9) x 10(-14) cm3 molecule(-1) s(-1) The rate coefficient for the Cl-atom reaction with CHF2CH2F is derived as k(Cl+CHF2CH2F) = (3.0+/-0.9) x 10(-14) cm3 molecule(-1) s(-1) at 298 K. The error limits include 3sigma from the statistical data analyses as well as the errors in the rate coefficients of the reference compounds employed. The tropospheric lifetimes of the hydrofluoroethers are estimated to be short tauOH((CF3)2CHOCH3) approximately 100 days, tauOH(CF3CH2OCH2CF3) approximately 80 days, tauOH(CF3CF2CH2OCH3) approximately 20 days, and tauOH(CHF2CF2CH2OCH3) approximately 14 days, and their global warming potentials are small compared to CFC-11.

A study of the IR and UV-Vis absorption cross-sections, photolysis and OH-initiated oxidation of CF3CHO and CF3CH2CHO

Infrared and ultraviolet-visible absorption cross-sections, effective quantum yields of photolysis and OH reaction rate coefficients for CF3CHO and CF3CH2CHO are reported. Relative rate measurements at 298(2) K and 1013(10) hPa, give k(OH + CF3CHO)/k(OH + CH3CH3) = 2.00(13), k(OH + CF3CH2CHO)/k(OH + CH3CH2OH) = 1.21(5) and k(OH + CF3CH2CHO)/k(OH + HC(O)OC2H5) = 3.51(9) (2σ). The effective quantum yield of photolysis was measured under pseudo-natural conditions in the European simulation chamber, Valencia, Spain (EUPHORE). Over the wavelength range 290–400 nm, the effective quantum yields of photolysis for CF3CHO and CF3CH2CHO are less than 2 × 10−2 and 4 × 10−2, respectively. The tropospheric lifetimes are estimated to be: τOH(CF3CHO) ∼ 26 days; τphotol(CF3CHO) > 27 days; τOH(CF3CH2CHO) ∼ 4 days; τphotol(CF3CH2CHO) > 15 days.

Direct measurements of urban OH reactivity during Nashville SOS in summer 1999.

Emissions of volatile chemicals control the hydroxyl radical (OH), the atmosphere's main cleansing agent, and thus the production of secondary pollutants. Accounting for all of these chemicals can be difficult, especially in environments with mixed urban and forest emissions. The first direct measurements of the atmospheric OH reactivity, the inverse of the OH lifetime, were made as part of the Southern Oxidant Study (SOS) at Cornelia Fort Airpark in Nashville, TN in summer 1999. Measured OH reactivity was typically 11 s(-1). Measured OH reactivity was 1.4 times larger than OH reactivity calculated from the sum of the products of measured chemical concentrations and their OH reaction rate coefficients. This difference is statistically significant at the 1sigma uncertainty level of both the measurements and the calculations but not the 2sigma uncertainty level. Measured OH reactivity was 1.3 times larger than the OH reactivity from a model that uses measured ambient concentrations of volatile organic compounds (VOCs), NO, NO2, SO2, and CO. However, it was within approximately 10% of the OH reactivity from a model that includes hydrocarbon measurements made in a Nashville tunnel and scaled to the ambient CO at Cornelia Fort Airpark. These comparisons indicate that 30% of the OH reactivity in Nashville may come from short-lived highly reactive VOCs that are not usually measured in field intensive studies or by US EPA's Photochemical Assessment Monitoring Stations.

Reaction-rate, mixture-fraction, and temperature imaging in turbulent methane/air jet flames

Instantaneous two-dimensional measurements of reaction rate, mixture fraction, and temperature are demonstrated in turbulent partially premixed methane/air jet flames. The forward reaction rate of the reaction CO OH ⇒ CO2 H is measured by simultaneous OH laser-induced fluorescence (LIF) and two-photon CO LIF. The product of the two LIF signals is shown to be proportional to the reaction rate. Temperature and fuel concentration are measured using polarized and depolarized Rayleigh scattering. A three-scalar technique for determining mixture fraction is investigated using a combination of polarized Rayleigh scattering, fuel concentration, and CO LIF. Measurements of these three quantities are coupled with previous detailed multiscalar point measurements to obtain the most probable value of the mixture fraction at each point in the imaged plane. This technique offers improvements over two-scalar methods, which suffer from decreased sensitivity around the stoichiometric contour and biases in fuel-rich regions due to parent fuel loss. Simultaneous reaction-rate, mixture-fraction, and temperature imaging is demonstrated in laminar (Re 1100) and turbulent (Re 22,400) CH4/air (1/3 by volume) jet flames. The turbulent jet flame is the subject of multiple numerical modeling efforts. A primary objective for developing these imaging diagnostics is to provide measurements of fundamental quantities that are needed to accurately model interactions between turbulent flows and flames.

Atmospheric degradation and global warming potentials of three perfluoroalkenes

The vapour phase reactions of perfluoropropene, CF 3CFCF 2, and perfluorobuta-1,3-diene, CF 2CFCFCF 2, with OH, NO 3 and O 3 were studied at 298±4 K and 740±5 Torr using long-path FT-IR detection. The reactions with ozone are very slow, k CF 3CFCF 2+O 3 =(6.2±1.5)×10 −22 and k CF 2CFCFCF 2+O 3 =(6.5±0.2)×10 −21 cm 3 molecules −1 s −1, and upper limits of 3×10 −15 cm 3 molecules −1 s −1 are reported for the NO 3 reaction rate coefficients. The OH reaction rate coefficients were determined as k CF 3CFCF 2+OH =(2.6±0.7)×10 −12 and k CF 2CFCFCF 2+OH =(1.1±0.3)×10 −11 cm 3 molecules −1 s −1; perfluoropropene gave a nearly quantitative yield of CF 3CFO and CF 2O as organic products, while perfluorobuta-1,3-diene gave from 130% to 170% of CF 2O. A chemistry transport model was applied to calculate the atmospheric distributions and lifetimes of the perfluoroalkenes; the global and yearly averaged lifetimes were calculated as 1.9 day for C 2F 4 and C 4F 6 and 6 days for C 3F 6. Quantitative infrared cross-sections of perfluoroethene, perfluoropropene, and perfluorobuta-1,3-diene have been obtained at 298 K in the region 100–2600 cm −1. Radiative forcing calculations have been performed for these gases assuming either constant vertical profiles or the distribution derived from the chemistry transport model. The results show that the Global Warming Potentials are totally negligible for these compounds.

Reactivity of certain organophosphorus insecticides toward hydroxyl radicals at elevated air temperatures.

Methods were developed to determine OH reaction rates for medium-weight organophosphorus pesticides in the gas phase. A 57-L chamber was constructed that utilized xenon arc irradiation (>290 nm) to photolyze the OH precursor, methyl nitrite. Experiments were performed at elevated temperatures ranging from 60 to 80 degrees C to minimize wall sorption. Solid-phase microextraction (SPME) sampling of the gas phase was employed to assess the simultaneous rate of loss of the pesticides in relation to the rate of loss of two reference substances with known OH rate constants. An internal gas-phase standard (hexachlorobenzene), relatively stable to hydroxyl radicals, was used to assess other processes, which included dilution and wall sorption. The relative reaction rates of the organophosphorus insecticides, diazinon and chlorpyrifos, when compared to reference compounds, were unaffected by various air temperatures between 60 and 80 degrees C. Although both insecticides were expected to react at similar rates on the basis of structural activity model predictions, diazinon reacted 3 times more rapidly than chlorpyrifos and gave estimated environmental OH half-lives of 0.5 and 1.4 h, respectively. The degree of sorption onto the chamber walls was minimal and similar for each of the compounds examined. Experimental gas-phase determinations at elevated temperatures may provide important information that can be used when in the assessment of the potential of airborne pesticide risks to nontarget and ecologically sensitive areas.

Henry's Law Constants of Some β-, γ-, and δ-Hydroxy Alkyl Nitrates of Atmospheric Interest

Organic nitrates form via the photodegradation of hydrocarbons in the troposphere in the presence of NO and NO2. This process competes with the chemical cycle leading to ozone production since it sequesters both nitrogen oxides and organic radicals. Hydroxy nitrates form via the atmospheric reactions of alkanes and alkenes and are thought to be an important nitrogen oxides reservoir. In this study, new synthetic methods to produce β-, γ-, and δ-hydroxy nitrates of atmospheric interest were developed. NMR and IR spectroscopies were used to characterize these compounds. Henry's law coefficients of C4 and C5 hydroxy nitrates at 291 ± 2 K were measured using a dynamic equilibrium system. The solubility decreases with the organic chain length and increases with increasing distance between the nitrooxy and hydroxy groups. Due to their large Henry's law coefficients these species will partition into droplets in the presence of clouds and fogs. Measurements of the OH reaction and photolysis rate coefficients are needed for an accurate assessment of the atmospheric lifetimes of these compounds.

Rate Constants for the Reactions of OH Radicals with CH3OCF2CHF2, CHF2OCH2CF2CHF2, CHF2OCH2CF2CF3, and CF3CH2OCF2CHF2 over the Temperature Range 250−430 K

The OH reaction rate constants have been measured for CH3OCF2CHF2, CHF2OCH2CF2CHF2, CHF2OCH2CF2CF3, and CF3CH2OCF2CHF2 over the temperature range 250−430 K. Kinetic measurements have been carried out using the flash photolysis, laser photolysis, and discharge flow methods, combined respectively with the laser-induced fluorescence technique to monitor the OH radical concentrations. The influence of impurities contained in the samples of fluorinated ethers has been investigated by means of sample purification using gas chromatography. No sizable effect of impurities was found on the measured rate constants of these fluorinated ethers, if the purified samples were used in the kinetic measurements. The Arrhenius rate constants have been determined from the respective kinetic data as k(CH3OCF2CHF2) = 2.60 × 10-12 exp[−(1420 ± 170)/T], k(CHF2OCH2CF2CHF2) = 2.49 × 10-12 exp[−(1500 ± 170)/T], k(CHF2OCH2CF2CF3) = 2.14 × 10-12 exp[−(1580 ± 170)/T], and k(CF3CH2OCF2CHF2) = 1.49 × 10-12 exp[−(1520 ± 170)/T] cm3 molec...

Direct measurement of the total reaction rate of OH in the atmosphere

A new method to investigate atmospheric chemical kinetics is presented. It is based on the pump and probe principle. Preliminary measurements are described on the decay rate of OH in the planetary boundary layer its concentration has been raised by laser flash photolysis of background ozone and subsequent reaction of the O(1D) with atmospheric water vapor. The measured OH disappearance kinetics are compared with model calculations yielding information on the fast reaction of OH. The presented point measurements can be extended to a range resolved differential absorption LIDAR technique.

Measurement of the OH Reaction Rate Constants for CF3CH2OH, CF3CF2CH2OH, and CF3CH(OH)CF3

Fully halogenated chlorofluorocarbons (CFCs) cause depletion of the stratospheric ozone layer as well as global warming. Partially fluorinated alcohols are important in relevance with potential substitutes of CFCs. The OH reaction rate constants have been measured for CF{sub 3}CH{sub 2}OH, CF{sub 3}CF{sub 2}CH{sub 2}OH, and CF{sub 3}CH(OH)CF{sub 3} over the temperature range 250--430K. Kinetic measurements have been carried out using the discharge flow, laser photolysis, and flash photolysis methods, combined respectively with the laser-induced fluorescence technique to monitor the OH radical concentrations. The influence of impurities contained in the sample of CF{sub 3}CF{sub 2}CH{sub 2}OH has been investigated by means of sample purification using gas chromatography. No sizable effect of impurities was found on the measured rate constants of these three fluorinated alcohols. The Arrhenius rate constants have been determined from the respective kinetic data as {kappa}(CF{sub 3}CH{sub 2}OH) = (2.00 {+-} 0.37) {times} 10{sup {minus}12} exp[{minus}(890 {+-} 60)/T], {kappa}(CF{sub 3}CF{sub 2}CH{sub 2}OH) = (1.40 {+-} 0.27) {times} 10{sup {minus}12} exp[{minus}(780 {+-} 60)/T], and {kappa}(CF{sub 3}CH(OH)CF{sub 3}) = (6.99 {+-} 1.56) {times} 10{sup {minus}13} exp[{minus}(990 {+-} 70)/T] cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. A method of predicting the OH reaction rate constants for fluorinated alcohols, hydrofluorocarbons, alkanes, and alcohols hasmore » been proposed.« less

Ambient levels of gas phase pollutants in Porto Alegre, Brazil

Air samples have been collected using electropolished canisters in downtown Porto Alegre, Brazil, where ethanol is used as a vehicle fuel and methyl-tert-butyl ether (MTBE) is used as a vehicle fuel additive. The 150 volatile organic compounds (VOC) identified by GC-FID and GC-MS included 46 alkanes, 30 alkenes, 22 aromatics, 17 carbonyls, 3 alcohols, 8 bicyclic aromatics, 11 halogenated hydrocarbons and 13 other compounds. The most abundant VOC on a mass concentration basis (after CO 2, CH 4 and CO) included acetylene, MTBE, ethanol, the alkanes propane, n-butane, n-pentane, isopentane, n-hexane, 2-methylpentane and indane, the alkenes ethylene and propene, and the aromatics benzene, toluene, ethylbenzene and ( m+ p) xylene. During the ca. one-year period studied, 20 March, 1996–16 April, 1997, ambient concentrations of VOC correlated well with those of carbon monoxide, for which vehicle exhaust emissions account for ca. 99% of total emissions in Porto Alegre. Two VOC photochemical reactivity rankings are presented: one involves reaction with OH (product of VOC concentration and VOC–OH reaction rate constant) and the other involves production of ozone (product of VOC concentration and VOC maximum incremental reactivity coefficient). Reaction with OH is dominated by CO followed by 2-methyl-2-butene and by several other alkenes. Ozone production is dominated by ethylene and CO (about equal contribution) followed by several alkenes, alkylbenzenes and aldehydes. The two fuel oxygenates, ethanol and MTBE, play only a minor role as photochemical precursors (reaction with OH and production of ozone) in the atmosphere of Porto Alegre.

Ambient Concentrations, Sources, Emission Rates, and Photochemical Reactivity of C2−C10 Hydrocarbons in Porto Alegre, Brazil

The combination of vehicle fuels used in Porto Alegre, Brazil, is unique in the world (17%, 9%, and 74% of the vehicles run on ethanol, diesel, and a mixture of 85% gasoline and 15% MTBE, respectively). As a result, air pollution control strategies for Porto Alegre, where vehicles account for ca. 99% and ca. 86% of total city wide CO and hydrocarbons emissions, respectively, must make use of air quality data specific to the Porto Alegre area. In this study, ambient concentrations of 66 C2−C10 hydrocarbons have been measured for ca. 1 year (March 1996−April 1997) at downtown Porto Alegre locations. On the average, the 10 most abundant hydrocarbons on a mass concentra tion basis were, in decreasing order, as follows: acetylene, ethylene, propane, toluene, isopentane, (m+p)-xylene, propene, n-butane, benzene, and n-pentane. Scatterplots of ambient concentrations showed a high degree of correlation among hydrocarbon classes (e.g., alkenes, isobutene vs 1-pentene), between hydrocarbons (alkanes, alkenes, and aromatics) and acetylene, and between hydrocarbons and carbon monoxide. Mid-1996 hydrocarbon emission rates (e.g., 3274 ± 828 and 1418 ± 369 t per year for acetylene and isopentane, respectively) are estimated from regression analysis of ambient hydrocarbon vs ambient CO. Reactivity rankings are presented, one involving reaction with OH (using hydrocarbon−OH reaction rate constants) and the other involving the production of ozone (using maximum incremental reactivity coefficients). Production of ozone is dominated by the following alkenes and aromatics (in order of decreasing importance): ethylene, (m+p)-xylene (calculated for m-xylene), propene, 1,2,4-trimethylbenzene, toluene, trans-2-pentene, 2-methyl-2-butene, isobutene, p-ethyltoluene, and o-xylene. Reac tion with OH is dominated by these nine alkenes (in order of decreasing importance): ethylene, 2-methyl-2-butene, propene, isobutene, trans-2-pentene, 2-methyl-1-butene, cyclopentene, trans-2-butene, and 1,3-butadiene. If carbon monoxide, ethanol, and MTBE are included along with the 66 hydrocarbons, the reactivity rankings indicate a major role for CO (ranked second after ethylene for ozone forma tion and ranked first for reaction with OH), a modest role for ethanol (ranked 16 for ozone formation and 18 for reaction with OH) and a minor role for MTBE (ranked 30 for ozone formation and 28 for reaction with OH).

OH Reaction Kinetics of Gas-Phase α- and γ-Hexachlorocyclohexane and Hexachlorobenzene

Rate constants for the gas-phase reactions of the hydroxyl radical (OH) with α- and γ-hexachlorocyclohexane (α- and γ-HCH) and hexachlorobenzene (HCB) were measured over the temperature range 346−386 K. The experiments were carried out in He diluent gas at approximately 1 atm, in a 160-mL quartz chamber. OH was produced by the photolysis of ozone in the presence of H2O, and reactants in the chamber were monitored by online mass spectrometry. The rate constants measured at elevated tem peratures were extrapolated by the Arrhenius equation, and OH reaction rate constants at 298 K were estimated for each compound. These rate constants at 298 K (in units of 10-13 cm3 s-1) were α-HCH, 1.4; γ-HCH, 1.9; and HCB, 0.27. Atmospheric lifetimes based on OH reactions (τOH) were also estimated (in units of days): α-HCH, 120; γ-HCH, 96; and HCB, 940. These relatively high atmospheric lifetimes indicate that these compounds can be transported great distances through the atmosphere before removal processes associated wit...