Uncovering the fundamental chemistry of alkyl + O2 reactions via measurements of product formation

The reactions of alkyl radicals (R) with molecular oxygen (O(2)) are critical components in chemical models of tropospheric chemistry, hydrocarbon flames, and autoignition phenomena. The fundamental kinetics of the R + O(2) reactions is governed by a rich interplay of elementary physical chemistry processes. At low temperatures and moderate pressures, the reactions form stabilized alkylperoxy radicals (RO(2)), which are key chain carriers in the atmospheric oxidation of hydrocarbons. At higher temperatures, thermal dissociation of the alkylperoxy radicals becomes more rapid and the formation of hydroperoxyl radicals (HO(2)) and the conjugate alkenes begins to dominate the reaction. Internal isomerization of the RO(2) radicals to produce hydroperoxyalkyl radicals, often denoted by QOOH, leads to the production of OH and cyclic ether products. More crucially for combustion chemistry, reactions of the ephemeral QOOH species are also thought to be the key to chain branching in autoignition chemistry. Over the past decade, the understanding of these important reactions has changed greatly. A recognition, arising from classical kinetics experiments but firmly established by recent high-level theoretical studies, that HO(2) elimination occurs directly from an alkylperoxy radical without intervening isomerization has helped resolve tenacious controversies regarding HO(2) formation in these reactions. Second, the importance of including formally direct chemical activation pathways, especially for the formation of products but also for the formation of the QOOH species, in kinetic modeling of R + O(2) chemistry has been demonstrated. In addition, it appears that the crucial rate coefficient for the isomerization of RO(2) radicals to QOOH may be significantly larger than previously thought. These reinterpretations of this class of reactions have been supported by comparison of detailed theoretical calculations to new experimental results that monitor the formation of products of hydrocarbon radical oxidation following a pulsed-photolytic initiation. In this article, these recent experiments are discussed and their contributions to improving general models of alkyl + O(2) reactions are highlighted. Finally, several prospects are discussed for extending the experimental investigations to the pivotal questions of QOOH radical chemistry.     

The heterogeneous interaction of HOCl with solid KBr substrates: the catalytic role of adsorbed halogens

The heterogeneous reactivity of HOCl on solid KBr at ambient temperature has been studied using a Knudsen flow reactor. On solid KBr steady-state uptake experiments reveal the formation of Br- and Cl-containing reaction products formed in secondary reactions such as Br(2), BrCl, HOBr, BrOCl, Cl(2) and Cl(2)O with the latter two predominating in the late stages of the reaction. The uptake coefficient gamma spanning a range between 0.15 and 1 x 10(-3) and product yields of HOCl strongly depend on the nature of the solid sample, whether grain, ground grain or thin sprayed film, as well as on sample processing such as pumping and/or heating. Furthermore, the presence of adsorbed halogen species such as Br(2)(a) are crucial for the kinetics of the reaction of HOCl with solid KBr substrates. The presence of surface-adsorbed water (SAW) leads to deactivation of KBr whereas mechanical stress such as grinding leads to the formation of surface defects that become reaction centers. Desorption of SAW at T > 620 K induces high reactivity of the KBr sample at ambient temperature. A reaction mechanism encompassing all significant observations including unusual autocatalytic activity is given as there is no direct reaction of HOCl with solid KBr. It stresses the importance of adsorbed Br-containing species such as Br(2)(a) and HBr(a) that initiate the heterogeneous chemistry of HOCl on solid KBr in the presence of SAW. The role of surface acidity and SAW for the extent of reaction is emphasized.     

Revisiting the reaction of hydroxyl radicals with vicinal diols in water

The carbonyl products of the reactions of hydroxyl radicals with three vicinal diols (ethane-1,2-diol, propane-1,2-diol and butane-2,3-diol) have been identified and quantified. Hydroxyl radicals were produced by γ-radiolysis of N(2)O-saturated aqueous solutions. The reactions result in the formation of alkoxyl radicals (~15%) followed by β-fragmentation, and α-hydroxyl alkyl radicals that undergo H(2)O elimination. The latter process is part of a radical chain reaction at higher diol concentrations.     

p-Nitrobenzenesulfenate esters as precursors for laser flash photolysis studies of alkyl radicals

A series of p-nitrobenzenesulfenate esters was used in laser flash photolysis (LFP) studies to generate alkoxyl radicals that fragmented to give the (2,2-diphenylcyclopropyl)methyl radical. Rate constants for the beta-scission reactions increased as a function of the carbonyl compound produced in the fragmentation reaction in the order CH2O < MeCHO < Me2CO < PhCHO < Ph2CO and increased with increasing solvent polarity. For alkoxyl radicals that fragment to produce benzaldehyde and benzophenone, the beta-scission reactions are faster than 1,5-hydrogen atom abstractions when the incipient carbon radical is as stable as a secondary alkyl radical, and this entry to carbon radicals can be used in LFP kinetic studies.     

Role of O2 + QOOH in low-temperature ignition of propane. 1. Temperature and pressure dependent rate coefficients

The kinetics of the reaction of molecular oxygen with hydroperoxyalkyl radicals have been studied theoretically. These reactions, often referred to as second O(2) addition, or O(2) + QOOH reactions, are believed to be responsible for low-temperature chain branching in hydrocarbon oxidation. The O(2) + propyl system was chosen as a model system. High-level ab initio calculations of the C(3)H(7)O(2) and C(3)H(7)O(4) potential energy surfaces are coupled with RRKM master equation methods to compute the temperature and pressure dependence of the rate coefficients. Variable reaction coordinate transition-state theory is used to characterize the barrierless transition states for the O(2) + QOOH addition reactions as well as subsequent C(3)H(6)O(3) dissociation reactions. A simple kinetic mechanism is developed to illustrate the conditions under which the second O(2) addition increases the number of radicals. The sequential reactions O(2) + QOOH → OOQOOH → OH + keto-hydroperoxide → OH + OH + oxy-radical and the corresponding formally direct (or well skipping) reaction O(2) + QOOH → OH + OH + oxy-radical increase the total number of radicals. Chain branching through this reaction is maximized in the temperature range 600-900 K for pressures between 0.1 and 10 atm. The results confirm that n-propyl is the smallest alkyl radical to exhibit the low-temperature combustion properties of larger alkyl radicals, but n-butyl is perhaps a truer combustion archetype.     

Thermochemistry and reaction paths in the oxidation reaction of benzoyl radical: C6H5C•(═O)

Alkyl substituted aromatics are present in fuels and in the environment because they are major intermediates in the oxidation or combustion of gasoline, jet, and other engine fuels. The major reaction pathways for oxidation of this class of molecules is through loss of a benzyl hydrogen atom on the alkyl group via abstraction reactions. One of the major intermediates in the combustion and atmospheric oxidation of the benzyl radicals is benzaldehyde, which rapidly loses the weakly bound aldehydic hydrogen to form a resonance stabilized benzoyl radical (C6H5C(?)═O). A detailed study of the thermochemistry of intermediates and the oxidation reaction paths of the benzoyl radical with dioxygen is presented in this study. Structures and enthalpies of formation for important stable species, intermediate radicals, and transition state structures resulting from the benzoyl radical +O2 association reaction are reported along with reaction paths and barriers. Enthalpies, ΔfH298(0), are calculated using ab initio (G3MP2B3) and density functional (DFT at B3LYP/6-311G(d,p)) calculations, group additivity (GA), and literature data. Bond energies on the benzoyl and benzoyl-peroxy systems are also reported and compared to hydrocarbon systems. The reaction of benzoyl with O2 has a number of low energy reaction channels that are not currently considered in either atmospheric chemistry or combustion models. The reaction paths include exothermic, chain branching reactions to a number of unsaturated oxygenated hydrocarbon intermediates along with formation of CO2. The initial reaction of the C6H5C(?)═O radical with O2 forms a chemically activated benzoyl peroxy radical with 37 kcal mol(-1) internal energy; this is significantly more energy than the 21 kcal mol(-1) involved in the benzyl or allyl + O2 systems. This deeper well results in a number of chemical activation reaction paths, leading to highly exothermic reactions to phenoxy radical + CO2 products.     

Reactive uptake of NO3 radicals by unsaturated fatty acid particles

The reactive uptake of NO(3) by particles containing four unsaturated fatty acid species was studied for the first time in a flow tube reactor coupled to a chemical ionization mass spectrometer (CIMS). Using a mixed-phase relative rates technique, the reactive uptake coefficients, γ(part), were determined to be (0.27 ± 0.06) for oleic acid (OA), (0.53 ± 0.12) for linoleic acid (LA), (0.72 ± 0.17) for linolenic acid (LNA), and (1.01 ± 0.18) for conjugated linoleic acid (CLA) in N(2) bath gas. In the presence of 20% O(2), their uptake coefficients are (0.31 ± 0.09) for OA, (0.55 ± 0.12) for LA, (0.78 ± 0.17) for LNA, and (1.08 ± 0.17) for CLA. All the uncertainties are 2σ and represent precision only. The products of the NO(3) reactions with these organic particles were studied in the presence and absence of O(2) using CIMS. It was found that the detected yields and compositions of the products were substantially different with or without O(2). Based on the observed products and general gas-phase chemistry of NO(3) radicals, a reaction mechanism is proposed for NO(3) reactions with unsaturated organic particles. The atmospheric implications of the heterogeneous chemical processes involving NO(3) are also discussed.     

Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance

Organic peroxy radicals (often abbreviated RO(2)) play a central role in the chemistry of the Earth's lower atmosphere. Formed in the atmospheric oxidation of essentially every organic species emitted, their chemistry is part of the radical cycles that control the oxidative capacity of the atmosphere and lead to the formation of ozone, organic nitrates, organic acids, particulate matter and other so-called secondary pollutants. In this review, laboratory studies of this peroxy radical chemistry are detailed, as they pertain to the chemistry of the atmosphere. First, a brief discussion of methods used to detect the peroxy radicals in the laboratory is presented. Then, the basic reaction pathways - involving RO(2) unimolecular reactions and bimolecular reactions with atmospheric constituents such as NO, NO(2), NO(3), O(3), halogen oxides, HO(2), and other RO(2) species - are discussed. For each of these reaction pathways, basic reaction rates are presented, along with trends in reactivity with radical structure. Focus is placed on recent advances in detection methods and on recent advances in our understanding of radical cycling processes, particularly pertaining to the complex chemistry associated with the atmospheric oxidation of biogenic hydrocarbons.     

Trajectory calculations of OH radical- and Cl atom-initiated reaction of glyoxal: atmospheric chemistry of the HC(O)CO radical

On-the-fly quasi-classical trajectory calculations using the density functional method were carried out to investigate the dynamics of the HC(O)CO radical, formed by OH radical- and Cl atom-initiated reactions of glyoxal at 298 K. The energy difference between the A' HC(O)CO radical, formed immediately after H atom abstraction, and the most stable A″ HC(O)CO radical is estimated to be 6.0 kcal mol(-1). The surplus energy followed by relaxation from A' HC(O)CO to A″ HC(O)CO goes to internal energy of the nascent HC(O)CO radicals and causes prompt decomposition into HCO + CO. The average internal energy partitioned into the HC(O)CO radical is higher in the OH reaction than in the Cl reaction, in accordance with exothermicity of the reactions. A fraction of the nascent HC(O)CO radicals (91% for the OH reaction and 47% for the Cl reaction) promptly decomposes into HCO and CO within 2.5 ps. The remaining HC(O)CO radicals, which do not undergo prompt decomposition, decompose thermally or add with O(2) in the presence of O(2). I re-evaluated the previous two experiment results of the product yield ratio [CO]/[CO(2)] vs. [O(2)](-1) in the Cl atom-initiated reaction, in light of the reaction mechanism involving prompt decomposition. The two results give 9.5 × 10(6) s(-1) and 1.08 × 10(7) s(-1) for the thermal decomposition rate and 47% and 41% for the fraction of prompt decomposition in the Cl atom-initiated reaction, in good agreement with the present trajectory calculation.     

Rate constants for reactions of alkyl radicals with water and methanol complexes of triethylborane

Reactions of secondary alkyl radicals with triethylborane and several of its complexes were studied. The H-atom transfer reactions from Et3B-OH2 and Et3B-OD2 were suppressed by addition of pyridine to the reaction mixture. Rate constants for reactions of secondary alkyl radicals with triethylborane and its complexes with water, deuterium oxide, methanol, and THF at ambient temperature were determined by radical clock methods. Cyclization of the 1-undecyl-5-hexenyl radical and ring opening of the 1-cyclobutyldodecyl radical were evaluated as clock reactions. The cyclobutylcarbinyl radical ring opening had the appropriate velocity for relatively precise determinations of the ratios of rate constants for H-atom transfer trapping and rearrangement, and these ratios combined with an estimated rate constant for the cyclobutylcarbinyl radical ring opening gave absolute values for the rate constants for the H-atom transfer reactions. For example, the triethylborane-water complex reacts with a secondary alkyl radical in benzene at 20 degrees C with a rate constant of 2 x 10(4) M(-1) s(-1). Variable temperature studies with the Et3B-CH3OH complex in toluene indicate that the hydrogen atom transfer reaction has unusually high entropic demand, which results in substantially more efficient hydrogen atom transfer trapping reactions in competition with radical ring opening and cyclization reactions at reduced temperatures.     

Determination of uptake coefficient for ClO radicals on the surface of mineral salts

The uptake of ClO radicals on KBr, NaCl, and NaBr dry solid films was studied at 1 Torr pressure of helium over the temperature range 290-350 K using a flow tube technique with a modulated molecular beam mass spectrometer as the detection method. A Pyrex tube with the deposited salt sample was introduced into the flow reactor along its axis. The ClO uptake coefficient on the KBr surface did not depend on temperature within the experimental accuracy of ～20%. Chlorine oxide radicals were prepared using the reaction of Cl with ozone. It was found out that the ClO uptake coefficient strongly depended on ozone concentration. The uptake coefficients at T = 293 K and [O(3)] = 4.6 × 10(13) molecules cm(-3) were found to be (9.6 ± 5.7) × 10(-4), (3.7 ± 1.5) × 10(-4), and (12.3 ± 3.6) × 10(-4) for KBr, NaCl, and NaBr, respectively. Bromine-containing species were not observed during the interaction of ClO radicals with KBr film. The results obtained indicate that the ClO loss through heterogeneous interaction with salt surface is not sufficiently rapid to compete with gas-phase self-reaction in the atmosphere.     

Kinetics of the reaction C2H5 + HO2 by time-resolved mass spectrometry

The overall rate constant for the radical-radical reaction C2H5 + HO2 --> products has been determined at room temperature by means of time-resolved mass spectrometry using a laser photolysis/flow reactor combination. Excimer laser photolysis of gas mixtures containing ethane, hydrogen peroxide, and oxalyl chloride was employed to generate controlled concentrations of C2H5 and HO2 radicals by the fast H abstraction reactions of the primary radicals Cl and OH with C2H6 and H2O2, respectively. By careful adjustments of the radical precursor concentrations, the title reaction could be measured under almost pseudo-first-order conditions with the concentration of HO2 in large excess over that of C2H5. From detailed numerical simulations of the measured concentration-time profiles of C2H5 and HO2, the overall rate constant for the reaction was found to be k1(293 K) = (3.1 +/- 1.0) x 10(13) cm3 mol(-1) s(-1). C2H5O could be confirmed as a direct reaction product.     

Kinetics, products, and mechanisms of secondary organic aerosol formation

Secondary organic aerosol (SOA) is formed in the atmosphere when volatile organic compounds (VOCs) emitted from anthropogenic and biogenic sources are oxidized by reactions with OH radicals, O(3), NO(3) radicals, or Cl atoms to form less volatile products that subsequently partition into aerosol particles. Once in particles, these organic compounds can undergo heterogenous/multiphase reactions to form more highly oxidized or oligomeric products. SOA comprises a large fraction of atmospheric aerosol mass and can have significant effects on atmospheric chemistry, visibility, human health, and climate. Previous articles have reviewed the kinetics, products, and mechanisms of atmospheric VOC reactions and the general chemistry and physics involved in SOA formation. In this article we present a detailed review of VOC and heterogeneous/multiphase chemistry as they apply to SOA formation, with a focus on the effects of VOC molecular structure on the kinetics of initial reactions with the major atmospheric oxidants, the subsequent reactions of alkyl, alkyl peroxy, and alkoxy radical intermediates, and the composition of the resulting products. Structural features of reactants and products discussed include compound carbon number; linear, branched, and cyclic configurations; the presence of C[double bond, length as m-dash]C bonds and aromatic rings; and functional groups such as carbonyl, hydroxyl, ester, hydroxperoxy, carboxyl, peroxycarboxyl, nitrate, and peroxynitrate. The intention of this review is to provide atmospheric chemists with sufficient information to understand the dominant pathways by which the major classes of atmospheric VOCs react to form SOA products, and the further reactions of these products in particles. This will allow reasonable predictions to be made, based on molecular structure, about the kinetics, products, and mechanisms of VOC and heterogeneous/multiphase reactions, including the effects of important variables such as VOC, oxidant, and NO(x) concentrations as well as temperature, humidity, and particle acidity. Such knowledge should be useful for interpreting the results of laboratory and field studies and for developing atmospheric chemistry models. A number of recommendations for future research are also presented.     

Absolute rate coefficient determination and reaction mechanism investigation for the reaction of Cl atoms with CH2I2 and the oxidation mechanism of CH2I radicals

The gas-phase reaction of atomic chlorine with diiodomethane was studied over the temperature range 273-363 K with the very low-pressure reactor (VLPR) technique. The reaction takes place in a Knudsen reactor at pressures below 3 mTorr, where the steady-state concentration of both reactants and stable products is continuously measured by electron-impact mass spectrometry. The absolute rate coefficient as a function of temperature was given by k = (4.70 +/- 0.65) x 10-11 exp[-(241 +/- 33)/T] cm3molecule-1s-1, in the low-pressure regime. The quoted uncertainties are given at a 95% level of confidence (2sigma) and include systematic errors. The reaction occurs via two pathways: the abstraction of a hydrogen atom leading to HCl and the abstraction of an iodine atom leading to ICl. The HCl yield was measured to be ca. 55 +/- 10%. The results suggest that the reaction proceeds via the intermediate CH2I2-Cl adduct formation, with a I-Cl bond strength of 51.9 +/- 15 kJ mol-1, calculated at the B3P86/aug-cc-pVTZ-PP level of theory. Furthermore, the oxidation reactions of CHI2 and CH2I radicals were studied by introducing an excess of molecular oxygen in the Knudsen reactor. HCHO and HCOOH were the primary oxidation products indicating that the reactions with O2 proceed via the intermediate peroxy radical formation and the subsequent elimination of either IO radical or I atom. HCHO and HCOOH were also detected by FT-IR, as the reaction products of photolytically generated CH2I radicals with O2 in a static cell, which supports the proposed oxidation mechanism. Since the photolysis of CH2I2 is about 3 orders of magnitude faster than its reactive loss by Cl atoms, the title reaction does not constitute an important tropospheric sink for CH2I2.     

A novel and efficient alkyl radical trap in aqueous medium

A simple water soluble diselenide derivative 1 shows radical scavenger properties towards alkyl and hydroxyl radicals (k₃ (0°C)=6.8×10⁸ M⁻¹ s⁻¹) in Fenton-type chemistry. The reaction rate between produceded alkyl radicals 2 and the diselenide overwhelms self-termination and halogen transfer reactions.     

Heterogeneous reactions of SO2 with HOCl and HOBr on ice surfaces

The heterogeneous reactions of SO2 + HOX (X = Cl or Br) --> products on ice surfaces at low temperature have been investigated in a flow reactor coupled with a differentially pumped quadrupole mass spectrometer. Pseudo-first-order loss of SO2 over the ice surfaces has been measured under the conditions of concurrent HOX flow. The initial uptake coefficient of SO2 reaction with HOX has been determined as a function of HOX surface coverage, theta(HOX), on the ice. The initial uptake coefficients increase as the HOX coverage increases. The uptake coefficient can be expressed as gamma(t) = k(h)theta(HOX), where k(h) is an overall rate constant of SO2 + HOCl, which was determined to be (2.3 +/- 0.6) x 10(-19) and (1.7 +/- 0.5) x 10(-19) molecules(-1) x cm2 at 190 and 210 K, and k(h) of SO2 + HOBr is (6.1 +/- 2.0) x 10(-18) molecules(-1) x cm2 at 190 K. theta( HOX) is in the range 8.1 x 10(13)-9.1 x 10(14) molecules x cm(-2). The kinetic results of the heterogeneous reaction of SO2 + HOX on ice surface are interpreted using the Eley-Rideal mechanism. The activation energy of the heterogeneous reaction of SO2 with HOCl on ice surface was determined to be about -37 +/- 10 kJ/mol in the 190-238 K range.     

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%.     

Thermochemical properties, DeltafH degrees (298), S degrees (298), and Cp degrees (T), for n-butyl and n-pentyl hydroperoxides and the alkyl and peroxy radicals, transition states, and kinetics for intramolecular hydrogen shift reactions of the peroxy radicals

Alkyl radicals in atmospheric and combustion environments undergo a rapid association with molecular oxygen (3O2) to form an alkyl peroxy radical (ROO*). One important reaction of these peroxy radicals is the intramolecular H-shift (intramolecular abstraction) to form a hydroperoxide alkyl radical (R'*COOH), where the hydroperoxide alkyl radical may undergo chemical activation reaction with O2 and result in chain branching at moderate to low temperatures. The thermochemistry and trends in kinetic parameters for the hydrogen shift reactions from each carbon (4-8-member-ring TST's) in n-butyl and n-pentyl peroxy radicals (CCCCOO* and CCCCCOO*) are analyzed using density functional and ab initio calculation methods. Thermochemical properties, DeltafH degrees (298 K), C-H bond energies, S degrees (298 K), and Cp degrees (T) of saturated linear C4 and C5 aliphatic peroxides (ROOH), as well as the corresponding hydroperoxide alkyl radicals (R'*COOH), are determined. DeltafH degrees (298 K) are obtained from isodesmic reactions and the total energies of the CBS-QB3 and B3LYP computational methods. Contributions to the entropy and the heat capacity from translation, vibration, and external rotation are calculated using the rigid-rotor-harmonic-oscillator approximation based on the CBS-QB3 frequencies and structures. The results indicate that pre-exponential factors, A(T), decrease with the increase of the ring size (4-8-member-ring TS, H-atom included). The DeltaH for 4-, 5-, 6-, and 7-member rings in n-butyl (and n-pentyl) peroxy are 40.8 (40.8), 31.4 (31.5), 20.5 (20.0), 22.6-p (19.4) kcal mol(-1), respectively. The DeltaH for the 8-member ring in n-pentylperoxy is 23.8-p kcal mol(-1), All abstractions are from secondary (-CH2-) groups except those marked (-p), which are from primary sites. Enthalpy and barrier values from the B3LYP/6-311++G(2d,p) and BHandHLYP/6-311G(d,p) methods are compared with CBS-QB3 results. The B3LYP results show good agreement with the higher level CBS-QB3 calculation method; the BHandH barriers for the intramolecular peroxy H-shifts are not acceptable.     

Kinetics and products from reaction of Cl radicals with dioctyl sebacate (DOS) particles in O(2): a model for radical-initiated oxidation of organic aerosols

The reaction of Cl radicals with bis (2-ethylhexyl) sebacate (also known as dioctyl sebacate, DOS) particles in the presence of O(2) is studied as a model of radical-initiated oxidation of organic aerosols. The uptake coefficient as measured from the rate of loss of DOS is gamma(DOS) = 1.7 (+/-0.3) indicating that a radical chain is operative. It is observed that nearly all of the detected products, accounting for 86% (+/-12%) of the reacted DOS, remain in the particles indicating that they are not efficiently volatilized. Correspondingly, the particles do not decrease in volume even after 60% of the DOS has reacted; upon further reaction the volume does decrease by up to 20%. Additionally, the mass of a DOS film increases with reaction indicating that the density increases. The two primary products identified are the ketone (38 +/- 10% yield) and alcohol (14 +/- 4% yield) resulting from reactions of alkylperoxy radicals originating from DOS oxidation. The fact that the ketone/alcohol ratio is >1 implies that the Russell mechanism, the typical fate of alkylperoxy radicals in liquids whereby both a ketone and an alcohol are generated, is not the only source of ketones. In fact, the ketone yield demonstrates a Langmuir-Hinshelwood type dependence on the O(2) concentration indicating that 44% (+/-8%) of the ketone is created from the reaction of alkoxy radicals with O(2) at the surface of the particles (at 20% O(2)). While this is a common reaction in the gas phase, it is generally not considered to occur in organic solvents. Furthermore, the appearance of gas-phase H(2)O(2) suggests that peroxy radicals react to form two ketones and H(2)O(2)via the Bennett and Summers mechanism. The absence of aldehyde products, both in the gas phase and in the particles, indicates that beta-scission of the alkoxy radicals is not significant. The results of this study suggest that organic aerosols in the troposphere are efficiently oxidized by gas-phase radicals but that their chemical transformation does not lead to their removal through volatilization.     

Mechanistic modeling of the wall reactions in the pyrolysis of pentachloroethane

The thermal dehydrochlorination C₂HCl₅ → C₂Cl₄ + HCl has been studied in a static system between 565 and 645 K at pressures ranging from 5 to 21 torr. The course of the reaction was followed by measuring the pressure rise in the conditioned quartz reaction vessel and by analyzing the products by gas chromatography. The observed experimental results and data from the literature for flow systems can be explained quantitatively in terms of a radical reaction model involving heterogeneous chain initiation and termination steps. The rate constants have been deduced for reactions of Cl, Cl₂, and C₂HCl₅ over reactor walls covered with a pyrolytic carbon film and for reactions of adsorbed Cl atoms. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 322–330, 2002