Reaction of HO with hydroxyacetone (HOCH2C(O)CH3): rate coefficients (233-363 K) and mechanism

Absolute rate coefficients for the title reaction, HO + HOCH(2)C(O)CH(3)--> products (R1) were measured over the temperature range 233-363 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. The rate coefficient displays a slight negative temperature dependence, which is described by: k(1)(233-363 K) = (2.15 +/- 0.30) x 10(-12) exp{(305 +/- 10)/T} cm(3) molecule(-1) s(-1), with a value of (5.95 +/- 0.50) x 10(-12) cm(3) molecule(-1) s(-1) at room temperature. 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 hydroxyacetone throughout the troposphere, resulting in formation of methylglyoxal at all atmospheric temperatures. As part of this work, the rate coefficient for reaction of O((3)P) with HOCH(2)C(O)CH(3) (R4) was measured at 358 K: k(4)(358 K) = (6.4 +/- 1.0) x 10(-14) cm(3) molecule(-1) s(-1) and the absorption cross section of HOCH(2)C(O)CH(3) at 184.9 nm was determined to be (5.4 +/- 0.1) x 10(-18) cm(2) molecule(-1).     

Rate coefficients for the OH + HC(O)C(O)H (glyoxal) reaction between 210 and 390 K

Rate coefficients, k1(T), over the temperature range of 210-390 K are reported for the gas-phase reaction OH + HC(O)C(O)H (glyoxal) --> products at pressures between 45 and 300 Torr (He, N2). Rate coefficients were determined under pseudo-first-order conditions in OH using pulsed laser photolysis production of OH radicals coupled with OH detection by laser-induced fluorescence. The rate coefficients obtained were independent of pressure and bath gas. The room-temperature rate coefficient, k1(296 K), was determined to be (9.15 +/- 0.8) x 10-12 cm3 molecule-1 s-1. k1(T) shows a negative temperature dependence with a slight deviation from Arrhenius behavior that is reproduced over the temperature range included in this study by k1(T) = [(6.6 +/- 0.6) x 10-18]T2[exp([820 +/- 30]/T)] cm3 molecule-1 s-1. For atmospheric modeling purposes, a fit to an Arrhenius expression over the temperature range included in this study that is most relevant to the atmosphere, 210-296 K, yields k1(T) = (2.8 +/- 0.7) x 10-12 exp[(340 +/- 50)/T] cm3 molecule-1 s-1 and reproduces the rate coefficient data very well. The quoted uncertainties in k1(T) are at the 95% confidence level (2sigma) and include estimated systematic errors. Comparison of the present results with the single previous determination of k1(296 K) and a discussion of the reaction mechanism and non-Arrhenius temperature dependence are presented.     

Kinetics of the O + HCNO reaction

The kinetics of the O + HCNO reaction were investigated by a relative rate technique using infrared diode laser absorption spectroscopy. Laser photolysis (355 nm) of NO2 was used to produce O atoms, followed by O atom reactions with CS2, NO2, and HCNO, and infrared detection of OCS product from the O + CS2 reaction. Analysis of the experiment data yields a rate constant of k1= (9.84 +/- 3.52) x 10-12 exp[(-195 +/- 120)/T)] (cm3 molecule-1 s-1) over the temperature range 298-375 K, with a value of k1 = (5.32 +/- 0.40) x 10-12 cm3 molecule-1 s-1 at 298 K. Infrared detection of product species indicates that CO producing channels, probably CO + NO + H, dominate the reaction.     

Room temperature and shock tube study of the reaction HCO+O2 using the photolysis of glyoxal as an efficient HCO source

The rate of the reaction 1, HCO+O2-->HO2+CO, has been determined (i) at room temperature using a slow flow reactor setup (20 mbarH2+HCO+CO, into additional HCO radicals. The rate constants of reaction 4 were determined from unperturbed photolysis experiments to be k4(295 K)=(3.6+/-0.3)x10(10) cm3 mol-1 s-1 and k4(769-1107 K)=5.4x10(13)exp(-18 kJ mol-1/RT) cm3 mol-1 s-1(Delta log k4=+/-0.12).     

Kinetics of the reaction of SH and SD with NO2

The rate constants for the reactions of NO2 with SH and SD were measured between 250 and 360 K to be 2.8 x 10(-11) exp{(270+/-40)/T(K)} and 2.6x10(-11) exp{(285+/-20)/T(K)} cm3 molecule-1 s-1, respectively. SH(SD) radicals were generated by pulsed laser photolysis of H2S(D2S) or CH3SH and detected via pulsed laser-induced fluorescence. The laser-induced fluorescence excitation spectrum of SH was found to be contaminated by the presence of the SO radical. This contamination is suggested as a possible reason for differences among some of the reported values of k1 in the literature. The title reaction influences the atmospheric lifetime of the SH radical when NO2 is greater than 100 pptv, but the revised value of k1 does not significantly alter our current understanding of SH oxidation in the atmosphere.     

Kinetics and mechanism of the reaction of chlorine atoms with n-pentanal

The kinetics and mechanism of the reaction Cl + CH3(CH2)3CHO was investigated using absolute (PLP-LIF) and relative rate techniques in 8 Torr of argon or 800-950 Torr of N2 at 295 +/- 2 K. The absolute rate experiments gave k[Cl+CH3(CH2)3CHO] = (2.31 +/- 0.35) x 10(-10) in 8 Torr of argon, while relative rate experiments gave k[Cl+CH3(CH2)3CHO] = (2.24 +/- 0.20) x 10(-10) cm3 molecule(-1) s(-1) in 800-950 Torr of N2. Additional relative rate experiments gave k[Cl+CH3(CH2)3C(O)Cl] = (8.74 +/- 1.38) x 10(-11) cm3 molecule-1 s(-1) in 700 Torr of N2. Smog chamber Fourier transform infrared (FTIR) techniques indicated that the acyl-forming channel accounts for 42 +/- 3% of the reaction. The results are discussed with respect to the literature data and the importance of long range (greater than or equal to two carbon atoms along the aliphatic chain) effects in determining the reactivity of organic molecules toward chlorine atoms.     

A kinetic and product study of the Cl + HO2 reaction

Absolute rate data and product branching ratios for the reactions Cl + HO2 --> HCl + O2 (k1a) and Cl + HO2 --> OH + ClO (k1b) have been measured from 226 to 336 K at a total pressure of 1 Torr of helium using the discharge flow resonance fluorescence technique coupled with infrared diode laser spectroscopy. For kinetic measurements, pseudo-first-order conditions were used with both reagents in excess in separate experiments. HO2 was produced by two methods: through the termolecular reaction of H atoms with O2 and also by the reaction of F atoms with H2O2. Cl atoms were produced by a microwave discharge of Cl2 in He. HO2 radicals were converted to OH radicals prior to detection by resonance fluorescence at 308 nm. Cl atoms were detected directly at 138 nm also by resonance fluorescence. Measurement of the consumption of HO2 in excess Cl yielded k1a and measurement of the consumption of Cl in excess HO2 yielded the total rate coefficient, k1. Values of k1a and k1 derived from kinetic experiments expressed in Arrhenius form are (1.6 +/- 0.2) x 10(-11) exp[(249 +/- 34)/T] and (2.8 +/- 0.1) x 10(-11) exp[(123 +/- 15)/T] cm3 molecule(-1) s(-1), respectively. As the expression for k1 is only weakly temperature dependent, we report a temperature-independent value of k1 = (4.5 +/- 0.4) x 10(-11) cm3 molecule(-1) s(-1). Additionally, an Arrhenius expression for k1b can also be derived: k1b = (7.7 +/- 0.8) x 10(-11) exp[-(708 +/- 29)/T] cm3 molecule(-1) s(-1). These expressions for k1a and k1b are valid for 226 K < or = T < or = 336 and 256 K < or = T < or = 296 K, respectively. The cited errors are at the level of a single standard deviation. For the product measurements, an excess of Cl was added to known concentrations of HO2 and the reaction was allowed to reach completion. HCl product concentrations were determined by IR absorption yielding the ratio k1a/k1 over the temperature range 236 K < or = T < or = 296 K. OH product concentrations were determined by resonance fluorescence giving rise to the ratio k1b/k1 over the temperature range 226 K < or = T < or = 336 K. Both of these ratios were subsequently converted to absolute numbers. Values of k1a and k1b from the product experiments expressed in Arrhenius form are (1.5 +/- 0.1) x 10(-11) exp[(222 +/- 17)/T] and (10.6 +/- 1.5) x 10(-11) exp[-(733 +/- 41)/T] cm3 molecule(-1) s(-1), respectively. These expressions for k1a and k1b are valid for 256 K < or = T < or = 296 and 226 K < or = T < or = 336 K, respectively. A combination of the kinetic and product data results in the following Arrhenius expressions for k1a and k1b of (1.4 +/- 0.3) x 10(-11) exp[(269 +/- 58)/T] and (12.7 +/- 4.1) x 10(-11) exp[-(801 +/- 94)/T] cm3 molecule(-1) s(-1), respectively. Numerical simulations were used to check for interferences from secondary chemistry in both the kinetic and product experiments and also to quantify the losses incurred during the conversion process HO2 --> OH for detection purposes.     

The gas phase tropospheric removal of fluoroaldehydes (CxF2x+1CHO, x = 3, 4, 6)

The rate coefficient of the OH reaction with the perfluoroaldehydes C(3)F(7)CHO and C(4)F(9)CHO have been determined in the temperature range 252-373 K using the pulsed laser photolysis-laser induced fluorescence (PLP-LIF) method: k(C(3)F(7)CHO+OH) = (2.0 +/- 0.6) x 10(-12) exp[-(369 +/- 90)/T] and k(C(4)F(9)CHO+OH) = (2.0 +/- 0.5) x 10(-12) exp[-(356 +/- 70)/T] cm(3) molecule(-1) s(-1), corresponding to (5.8 +/- 0.6) x 10(-13) and (6.1 +/- 0.5) x 10(-13) cm(3) molecule(-1) s(-1), respectively, at 298 K. The UV absorption cross sections of these two aldehydes and CF(3)(CF(2))(5)CH(2)CHO have been measured over the range 230-390 nm at 298 K and also at 328 K for CF(3)(CF(2))(5)CH(2)CHO. The obtained results for C(3)F(7)CHO and C(4)F(9)CHO are in good agreement with two recent determinations but the maximum value of the absorption cross section for CF(3)(CF(2))(5)CH(2)CHO is over a factor of two lower than the single one recently published. The photolysis rates of C(3)F(7)CHO, C(4)F(9)CHO and CF(3)(CF(2))(5)CHO have been measured under sunlight conditions in the EUPHORE simulation chamber in Valencia (Spain) at the beginning of June. The photolysis rates were, respectively, J(obs) = (1.3 +/- 0.6) x 10(-5), (1.9 +/- 0.8) x 10(-5) and (0.6 +/- 0.3) x 10(-5) s(-1). From the J(obs) measurements and calculated photolysis rate J(calc), assuming a quantum yield of unity across the atmospheric range of absorption of the aldehydes, quantum yields J(obs)/J(calc) = (0.023 +/- 0.012), (0.029 +/- 0.015) and (0.046 +/- 0.028) were derived for the photodissociation of C(3)F(7)CHO, C(4)F(9)CHO and CF(3)(CF(2))(5)CHO, respectively. The atmospheric implication of the data obtained in this work is discussed. The main conclusion is that the major atmospheric removal pathway for fluoroaldehydes will be photolysis, which under low NO(x) conditions, may be a source of fluorinated carboxylic acids in the troposphere.     

An experimental and theoretical study of the reactions OIO+NO and OIO+OH

The kinetics of the reaction OIO+NO were studied by pulsed laser photolysis/time-resolved cavity ring-down spectroscopy, yielding k(235-320 K)=7.6(+4.0)(-3.1) x 10(-13) exp[(607+/-128)/T] cm3 molecule-1 s-1. Quantum calculations on the OIO+NO potential-energy surface show that the reactants form a weakly bound OIONO intermediate, which then dissociates to the products IO+NO2. Rice-Ramsberger-Kassel-Markus (RRKM) calculations on this surface are in good accord with the experimental result. The most stable potential product, IONO2, cannot form because of the significant rearrangement of OIONO that would be required. The reaction OIO+OH was then investigated by quantum calculations of the relevant stationary points on its potential-energy surface. The very stable HOIO2 molecule can form by direct recombination, but the bimolecular reaction channels to HO2+IO and HOI+O2 are closed because of significant energy barriers. RRKM calculations of the HOIO2 recombination rate coefficient yield krec,0=1.5x10(-27) (T/300 K)(-3.93) cm6 molecule-2 s-1, krec,infinity=5.5x10(-10) exp(46/T) cm3 molecule-1 s-1, and Fc=0.30. The rate coefficients of both reactions are fast enough around 290 K and 1 atm pressure for these reactions to play a potentially important role in the gas phase and aerosol chemistry in the marine boundary layer of the atmosphere.     

Rate coefficients and equilibrium constant for the CH2CHO + O2 reaction system

The kinetics of the CH2CHO + O2 reaction was experimentally studied in two quasi-static reactors and a discharge flow-reactor at temperatures ranging from 298 to 660 K and pressures between 1 mbar and 46 bar with helium as the bath gas. The CH2CHO radicals were produced by the laser-flash photolysis of ethyl vinyl ether at 193 nm and by the reaction F + CH3CHO, respectively. Laser-induced fluorescence excited at 337 or 347.4 nm was used to monitor the CH2CHO concentration. The reaction proceeded via reversible complex formation with subsequent isomerization and fast decomposition: CH2CHO + O2 <= => O2CH2CHO --> HO2CH2CO --> products. The rate coefficients for the first and second steps were determined (k1, k-1, k2) and analyzed by a master equation with specific rate coefficients from the Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Molecular and transition-state parameters were obtained from quantum chemical calculations. A third-law analysis led to the following thermodynamic parameters for the first step: Delta(R)S degrees 300K(1) = -144 J K(-1) mol(-1) (1 bar) and Delta(R)H degrees 300K(1) = (-101 +/- 4) kJ mol(-1). From the falloff analysis, the following temperature dependencies for the low- and high-pressure limiting rate coefficients were obtained: k1(0) = 5.14 x 10(-14) exp(210 K/T) cm(-3) s(-1); k1(infinity) = 1.7 x 10(-12) exp(-520 K/T) cm(-3) s(-1); and k2(infinity) = 1.3 x 10(12) exp[-(82 +/- 4) kJ mol(-1)/RT] s(-1). Readily applicable analytical representations for the pressure and temperature dependence of k1 were derived to be used in kinetic modeling.     

Investigation of the radical product channel of the CH3COO2 + HO2 reaction in the gas phase

The reaction of CH(3)C(O)O(2) with HO(2) has been investigated at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl(2)/CH(3)CHO/CH(3)OH/air mixtures. The branching ratio for the reaction channel forming CH(3)C(O)O, OH and O(2) (reaction ) has been determined from experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The dependence of the phenol yield on benzene concentration was found to be consistent with its formation from the OH-initiated oxidation of benzene, thereby confirming the presence of OH radicals in the system. The dependence of the phenol yield on the initial peroxy radical precursor reagent concentration ratio, [CH(3)OH](0)/[CH(3)CHO](0), is consistent with OH formation resulting mainly from the reaction of CH(3)C(O)O(2) with HO(2) in the early stages of the experiments, such that the limiting yield of phenol at high benzene concentrations is well-correlated with that of CH(3)C(O)OOH, a well-established product of the CH(3)C(O)O(2) + HO(2) reaction (via channel (3a)). However, a delayed source of phenol was also identified, which is attributed mainly to an analogous OH-forming channel of the reaction of HO(2) with HOCH(2)O(2) (reaction ), formed from the reaction of HO(2) with product HCHO. This was investigated in additional series of experiments in which Cl(2)/CH(3)OH/benzene/air and Cl(2)/HCHO/benzene/air mixtures were photolysed. The various reaction systems were fully characterised by simulations using a detailed chemical mechanism. This allowed the following branching ratios to be determined: CH(3)C(O)O(2) + HO(2)--> CH(3)C(O)OOH + O(2), k(3a)/k(3) = 0.38 +/- 0.13; --> CH(3)C(O)OH + O(3), k(3b)/k(3) = 0.12 +/- 0.04; --> CH(3)C(O)O + OH + O(2), k(3c)/k(3) = 0.43 +/- 0.10: HOCH(2)O(2) + HO(2)--> HCOOH + H(2)O + O(2), k(17b)/k(17) = 0.30 +/- 0.06; --> HOCH(2)O + OH + O(2), k(17c)/k(17) = 0.20 +/- 0.05. The results therefore provide strong evidence for significant participation of the radical-forming channels of these reactions, with the branching ratio for the title reaction being in good agreement with the value reported in one previous study. As part of this work, the kinetics of the reaction of Cl atoms with phenol (reaction (14)) have also been investigated. The rate coefficient was determined relative to the rate coefficient for the reaction of Cl with CH(3)OH, during the photolysis of mixtures of Cl(2), phenol and CH(3)OH, in either N(2) or air at 296 K and 760 Torr. A value of k(14) = (1.92 +/- 0.17) x 10(-10) cm(3) molecule(-1) s(-1) was determined from the experiments in N(2), in agreement with the literature. In air, the apparent rate coefficient was about a factor of two lower, which is interpreted in terms of regeneration of phenol from the product phenoxy radical, C(6)H(5)O, possibly via its reaction with HO(2).     

A kinetic and mechanistic study of the reactions of OH radicals and Cl atoms with 3,3,3-trifluoropropanol under atmospheric conditions

Product distribution studies of the OH radical and Cl atom initiated oxidation of CF3CH2CH2OH in air at 1 atm and 298 +/- 5 K have been carried out in laboratory and outdoor atmospheric simulation chambers in the presence and absence of NOx. The results show that CF3CH2CHO is the only primary product and that the aldehyde is fairly rapidly removed from the system. In the absence of NOx the major degradation product of CF3CH2CHO is CF3CHO, and the combined yields of the two aldehydes formed from CF3CH2CH2OH are close to unity (0.95 +/- 0.05). In the presence of NOx small amounts of CF3CH2C(O)O2NO2 were also observed (<15%). At longer reaction times CF3CHO is removed from the system to give mainly CF2O. The laser photolysis-laser induced fluorescence technique was used to determine values of k(OH + CF3CH2CH2OH) = (0.89 +/- 0.03) x 10(-12) and k(OH + CF3CH2CHO) = (2.96 +/- 0.04) x 10(-12) cm3 molecule(-1) s(-1). A relative rate method has been employed to measure the rate coefficients k(OH + CF3CH2CH2OH) = (1.08 +/- 0.05) x 10(-12), k(OH + C6F13CH2CH2OH) = (0.79 +/- 0.08) x 10(-12), k(Cl + CF3CH2CH2OH) = (22.4 +/- 0.4) x 10(-12), and k(Cl + CF3CH2CHO) = (25.7 +/- 0.4) x 10(-12) cm3 molecule(-1) s(-1). The results from this investigation are discussed in terms of the possible importance of emissions of fluorinated alcohols as a source of fluorinated carboxylic acids in the environment.     

Atmospheric chemistry of propionaldehyde: kinetics and mechanisms of reactions with OH radicals and Cl atoms, UV spectrum, and self-reaction kinetics of CH3CH2C(O)O2 radicals at 298 K

The kinetics and mechanism of the reactions of Cl atoms and OH radicals with CH3CH2CHO were investigated at room temperature using two complementary techniques: flash photolysis/UV absorption and continuous photolysis/FTIR smog chamber. Reaction with Cl atoms proceeds predominantly by abstraction of the aldehydic hydrogen atom to form acyl radicals. FTIR measurements indicated that the acyl forming channel accounts for (88 +/- 5)%, while UV measurements indicated that the acyl forming channel accounts for (88 +/- 3)%. Relative rate methods were used to measure: k(Cl + CH3CH2CHO) = (1.20 +/- 0.23) x 10(-10); k(OH + CH3CH2CHO) = (1.82 +/- 0.23) x 10(-11); and k(Cl + CH3CH2C(O)Cl) = (1.64 +/- 0.22) x 10(-12) cm3 molecule(-1) s(-1). The UV spectrum of CH3CH2C(O)O2, rate constant for self-reaction, and rate constant for cross-reaction with CH3CH2O2 were determined: sigma(207 nm) = (6.71 +/- 0.19) x 10(-18) cm2 molecule(-1), k(CH3CH2C(O)O2 + CH3CH2C(O)O2) = (1.68 +/- 0.08) x 10(-11), and k(CH3CH2C(O)O2 + CH3CH2O2) = (1.20 +/- 0.06) x 10(-11) cm3 molecule(-1) s(-1), where quoted uncertainties only represent 2sigma statistical errors. The infrared spectrum of C2H5C(O)O2NO2 was recorded, and products of the Cl-initiated oxidation of CH3CH2CHO in the presence of O2 with, and without, NO(x) were identified. Results are discussed with respect to the atmospheric chemistry of propionaldehyde.     

Wide temperature range (T = 295 K and 770-1305 K) study of the kinetics of the reactions HCO + NO and HCO + NO2 using frequency modulation spectroscopy

The rate constants for , HCO + NO --> HNO + CO, and , HCO + NO(2)--> products, have been measured at temperatures between 770 K < T < 1305 K behind reflected shock waves and, for the purpose of a consistency check, in a slow flow reactor at room temperature. HCO radicals were generated by 193 nm excimer laser photolysis of diluted gas mixtures containing glyoxal, (CHO)(2), and NO or NO(2) in argon and were monitored using frequency modulation (FM) absorption spectroscopy. Kinetic simulations based on a comprehensive reaction mechanism showed that the rate constants for the title reactions could be sensitively extracted from the measured HCO profiles. The determined high temperature rate constants are k(1)(769-1307 K) = (7.1 +/- 2.7) x 10(12) cm(3) mol(-1) s(-1) and k(2)(804-1186 K) = (3.3 +/- 1.8) x 10(13) cm(3) mol(-1) s(-1). The room temperature values were found to be in very good agreement with existing literature data and show that both reactions are essentially temperature independent. The weak temperature dependence of can be explained by the interplay of a dominating direct abstraction pathway and a complex-forming mechanism. Both pathways yield the products HNO + CO. In contrast to , no evidence for a significant contribution of a direct high temperature abstraction channel was found for . Here, the observed temperature independent overall rate constant can be described by a complex-forming mechanism with several product channels. Detailed information on the strongly temperature dependent channel branching ratios is provided. Moreover, the high temperature rate constant of , OH + (CHO)(2), has been determined to be k(7) approximately 1.1 x 10(13) cm(3) mol(-1) s(-1).     

A temperature-dependent relative-rate study of the OH initiated oxidation of n-butane: the kinetics of the reactions of the 1- and 2-butoxy radicals

The kinetics of the reactions of 1-and 2-butoxy radicals have been studied using a slow-flow photochemical reactor with GC-FID detection of reactants and products. Branching ratios between decomposition, CH3CH(O*)CH2CH3 --> CH3CHO + C2H5, reaction (7), and reaction with oxygen, CH3CH(O*)CH2CH3+ O2 --> CH3C(O)C2H5+ HO2, reaction (6), for the 2-butoxy radical and between isomerization, CH3CH2CH2CH2O* --> CH2CH2CH2CH2OH, reaction (9), and reaction with oxygen, CH3CH2CH2CH2O* + O2 --> C3H7CHO + HO2, reaction (8), for the 1-butoxy radical were measured as a function of oxygen concentration at atmospheric pressure over the temperature range 250-318 K. Evidence for the formation of a small fraction of chemically activated alkoxy radicals generated from the photolysis of alkyl nitrite precursors and from the exothermic reaction of 2-butyl peroxy radicals with NO was observed. The temperature dependence of the rate constant ratios for a thermalized system is given by k7/k6= 5.4 x 10(26) exp[(-47.4 +/- 2.8 kJ mol(-1))/RT] molecule cm(-3) and k9/k8= 1.98 x 10(23) exp[(-22.6 +/- 3.9 kJ mol(-1))/RT] molecule cm(-3). The results agree well with the available experimental literature data at ambient temperature but the temperature dependence of the rate constant ratios is weaker than in current recommendations.     

Rate coefficients for the reaction of OH with (E)-2-pentenal, (E)-2-hexenal, and (E)-2-heptenal

Rate coefficients for the gas-phase reaction of the OH radical with (E)-2-pentenal (CH(3)CH(2)CH[double bond]CHCHO), (E)-2-hexenal (CH(3)(CH(2))(2)CH[double bond]CHCHO), and (E)-2-heptenal (CH(3)(CH(2))(3)CH[double bond]CHCHO), a series of unsaturated aldehydes, over the temperature range 244-374 K at pressures between 23 and 150 Torr (He, N(2)) are reported. Rate coefficients were measured under pseudo-first-order conditions in OH with OH radicals produced via pulsed laser photolysis of HNO(3) or H(2)O(2) at 248 nm and detected by pulsed laser-induced fluorescence. The rate coefficients were independent of pressure and the room temperature rate coefficients and Arrhenius expressions obtained are (cm(3) molecule(-1) s(-1) units): k(1)(297 K)=(4.3 +/- 0.6)x 10(-11), k(1)(T)=(7.9 +/- 1.2)x 10(-12) exp[(510 +/- 20)/T]; k(2)(297 K)=(4.4 +/- 0.5)x 10(-11), k(2)(T)=(7.5 +/- 1.1)x 10(-12) exp[(520 +/- 30)/T]; and k(3)(297 K)=(4.4 +/- 0.7)x 10(-11), k(3)(T)=(9.7 +/- 1.5)x 10(-12) exp[(450 +/- 20)/T] for (E)-2-pentenal, (E)-2-hexenal and (E)-2-heptenal, respectively. The quoted uncertainties are 2sigma(95% confidence level) and include estimated systematic errors. Rate coefficients are compared with previously published room temperature values and the discrepancies are discussed. The atmospheric degradation of unsaturated aldehydes is also discussed.     

Reflected shock tube studies of high-temperature rate constants for OH + NO2 --> HO2 + NO and OH + HO2 --> H2O + O2

The motivation for the present study comes from the preceding paper where it is suggested that accepted rate constants for OH + NO2 --> NO + HO2 are high by approximately 2. This conclusion was based on a reevaluation of heats of formation for HO2, OH, NO, and NO2 using the Active Thermochemical Table (ATcT) approach. The present experiments were performed in C2H5I/NO2 mixtures, using the reflected shock tube technique and OH-radical electronic absorption detection (at 308 nm) and using a multipass optical system. Time-dependent profile decays were fitted with a 23-step mechanism, but only OH + NO2, OH + HO2, both HO2 and NO2 dissociations, and the atom molecule reactions, O + NO2 and O + C2H4, contributed to the decay profile. Since all of the reactions except the first two are known with good accuracy, the profiles were fitted by varying only OH + NO2 and OH + HO2. The new ATcT approach was used to evaluate equilibrium constants so that back reactions were accurately taken into account. The combined rate constant from the present work and earlier work by Glaenzer and Troe (GT) is k(OH+NO2) = 2.25 x 10(-11) exp(-3831 K/T) cm3 molecule(-1) s(-1), which is a factor of 2 lower than the extrapolated direct value from Howard but agrees well with NO + HO2 --> OH + NO2 transformed with the updated equilibrium constants. Also, the rate constant for OH + HO2 suitable for combustion modeling applications over the T range (1200-1700 K) is (5 +/- 3) x 10(-11) cm3 molecule(-1) s(-1). Finally, simulating previous experimental results of GT using our updated mechanism, we suggest a constant rate for k(HO2+NO2) = (2.2 +/- 0.7) x 10(-11) cm3 molecule(-1) s(-1) over the T range 1350-1760 K.     

Rate coefficients for the reaction of OH with a series of unsaturated alcohols between 263 and 371 K

Rate coefficients for the gas-phase reactions of OH radicals with four unsaturated alcohols, 3-methyl-3-buten-1-ol (k1), 2-buten-1-ol (k2), 2-methyl-2-propen-1-ol (k3) and 3-buten-1-ol (k4), were measured using two different techniques, a conventional relative rate method and the pulsed laser photolysis-laser induced fluorescence technique. The Arrhenius rate coefficients (in units of cm(3) molecule(-1) s(-1)) over the temperature range 263-371 K were determined from the kinetic data obtained as k1 = (5.5 +/- 1.0) x 10(-12) exp [(836 +/- 54)/T]; k2 = (6.9 +/- 0.9) x 10(-12) exp [(744 +/- 40)/T]; k3 = (10 +/- 1) x 10(-12) exp [(652 +/- 27)/T]; and k4 = (4.0 +/- 0.4) x 10(-12) exp [(783 +/- 32)/T]. At 298 K, the rate coefficients obtained by the two methods for each of the alcohols studied were in good agreement. The results are presented and compared with those obtained previously for the same and related reactions of OH radicals. Reactivity factors for substituent groups containing the hydroxyl group are determined. The atmospheric implications for the studied alcohols are considered briefly.     

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.     

Kinetics of the reactions of CH2Br and CH2I radicals with molecular oxygen at atmospheric temperatures

The kinetics of the reactions of CH2Br and CH2I radicals with O2 have been studied in direct measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated by pulsed laser photolysis of appropriate precursors at 193 or 248 nm. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of O2 being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH2I + O2 reaction in the range 0.2-15 x 10(17) cm(-3) of He at 298 K. In this same density range the CH2Br + O2 reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH2I + O2 reaction is found to depend on temperature as k(CH2I + O2)=(1.39 +/- 0.01)x 10(-12)(T/300 K)(-1.55 +/- 0.06) cm3 s(-1)(220-450 K). Obtained primary products of this reaction are I atom and IO radical and the yield of I-atom is significant. The rate coefficient and temperature dependence of the CH2Br + O2 reaction in the third-body region is k(CH2Br + O2+ He)=(1.2 +/- 0.2)x 10(-30)(T/300 K)(-4.8 +/- 0.3) cm6 s(-1)(241-363 K), which was obtained by fitting the complete data set simultaneously to a Troe expression with the F(cent) value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about +/-25%.