Formation Of Peroxy Radicals Research Articles

A process-analysis based study of the ozone weekend effect

We have used the 3D photochemical model CMAQ to simulate the ozone weekend effect, a phenomenon in which urban areas can have higher ozone concentrations on weekends than on weekdays even though NO x emissions are usually lower on weekends. A simulation containing a weekend is compared to hypothetical simulations in which the anthropogenic emissions for the weekend have been replaced by weekday emissions. The simulations are identical in all other respects. Process analysis is used to explain the results. We find that the weekend effect can be decomposed into an ozone titration component and an odd oxygen component, each contributing about half of the excess weekend ozone. The titration component simply requires that there be lower weekend NO x emissions. The odd oxygen component additionally requires that on weekends there be a higher rate of OH + (VOC or CO) reactions, brought about by one or more of lower nitric acid formation, higher OH formation from O 3 photolysis, and higher VOC emissions. This causes higher weekend peroxy radical formation. The odd oxygen component also requires sufficiently high NO concentration even on the lower-NO x weekends to propagate this higher rate of peroxy formation back to higher weekend OH formation.

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.

Diurnal variation of non-methane hydrocarbons in the subantarctic atmosphere

Environmental context. The ocean surface is known to be supersaturated in some non-methane hydrocarbons and particularly alkenes. This oceanic source, though small on a global scale, can be a dominant component of the background atmosphere in remote areas. Attempts have been made to quantify this source, in order to estimate its magnitude in the budgets of these gases in the water column and the atmosphere. A main difficulty is to determine the production processes involved under the effects of plankton activity and solar and UV radiation penetration in the water column. Abstract. Non-methane hydrocarbons (NMHCs) play a key role in the photochemistry of the remote atmosphere. They are oxidised by OH radicals and subsequently lead to a net formation of peroxy radicals, which have a crucial role in the budget of tropospheric ozone. Whereas in polluted areas, the effect of light hydrocarbons results in a net formation of ozone, in non- or low-polluted areas, the self-reaction of peroxy radicals dominates and leads eventually to ozone destruction, which in turn acts significantly on the OH budget. In remote environments, the origin of the NMHC background level is clearly attributed to a marine production, but a great uncertainty still exists about its geographical, seasonal and diurnal variability. Here, we present in situ measurements of NMHCs and particularly of alkenes in subantarctic areas, which show very systematic diurnal trends in agreement with an origin clearly dependent on photochemical processes on the surface seawater. The diurnal variability of alkene atmospheric mixing ratios appears strongly related to solar irradiance at the ocean surface. The magnitude of this marine source is deduced from a simple 1-D model of the alkene budget in the marine boundary layer. It appears that the required source must be approximately one order of magnitude greater than the source deduced from concentration measured at 1-m depth, and consistent with probable high concentration gradients close to the ocean surface.

Ion beam effects on electrical characteristics of proton conductive polymer

Effect of MeV ion irradiation on the perfluorosulphonic polymer membrane was studied concerning with electrical characteristics. The DC electrical conductivity in dry condition increased of three orders of magnitude as a consequence of irradiation with MeV H and He ion beam at a fluence of 2×1013ions/cm2. The ion induced improvement of the conductivity was independent of the penetrating depth of the ions. The electron and gamma-ray irradiation showed similar enhancement of the conductivity when the same energy was deposited in the membrane. The optical absorption in the ultra-violet region increased in the ion irradiated membrane due to the formation of fluorocarbon and peroxy radicals, while the up-take of the water molecules were not found by the absorption in the infrared region. The ion irradiation may promote the dissociation of the water molecules at the surface and also improve the transport property of protons in the membrane.

Isothermal frontal polymerization: Confirmation of the isothermal nature of the process and the effect of oxygen and polymer seed molecular weight on front propagation

AbstractIsothermal frontal polymerization is a directional polymerization that utilizes the Norish‐Trommsdorff (gel) effect to produce optical gradient materials. When a solution of methyl methacrylate and thermal initiator contacts a polymer seed (a small piece of poly(methyl methacrylate), a viscous region is formed in which the polymerization rate is faster than in the bulk solution. We obtained definitive evidence of the isothermal nature of the process by placing thermocouples above the propagating front. Using the optical technique of laser line deflection (Weiner's method), we studied the front propagation to determine the induction period, and the maximum distance propagated as a function of the molecular weight of the seed. We determined that the polymer seed must have a minimum molecular weight to initiate a front. We also determined that oxygen would act as a bulk polymerization inhibitor and increase the front propagation distance, but after purging the monomer–initiator solution with oxygen for several hours, the distance was shortened. We ascribed this behavior to the formation of peroxy radicals from the slow decomposition of the initiator and subsequent reaction with oxygen. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3601–3608, 2006

Electron Paramagnetic Resonance Spectroscopy Studies of Oxidative Degradation of an Active Pharmaceutical Ingredient and Quantitative Analysis of the Organic Radical Intermediates Using Partial Least-Squares Regression

Electron paramagnetic resonance (EPR) spectroscopy was used to study the radical species formed during the oxidation of an active pharmaceutical ingredient in the solid state. It was found that the extent of radical generation correlated to the formation of an oxidative degradation product. Multifrequency EPR and electron nuclear double resonance spectroscopy gave additional information on the identity of the organic radical species involved in the oxidation process, and a mechanism was proposed for the degradation, involving the formation of both carbon-centered and peroxy radicals. The multivariate analysis technique of partial least-squares (PLS) regression was then used to determine the extent of oxidation of the active pharmaceutical ingredient from the EPR spectra. The suitability of this approach was demonstrated from its application to a series of standards. The conventional approach for the quantitative analysis of EPR spectra is to measure the peak height or to perform double integration of the spectral region containing the signal of interest. Both of these methods have intrinsic errors associated with them, particularly for weak EPR signals with a poor signal-to-noise ratio or a sloping background response. The results obtained showed that greatly improved quantitation was obtained using the PLS regression approach.

Preparation, Testing and Characterization of Doped TiO2 Active in the Peroxidation of Biomolecules under Visible Light

Doped TiO2 samples using different preparative procedures were synthesized using either urea or thiourea leading to N- or S-doped TiO2. Photocatalytic peroxidation and oxidation (mineralization) of phosphatidylethanolamine (PE) lipid with doped TiO2 were carried out under light irradiation lambda > 410 nm. The formation of conjugated double bonds in PE molecules was followed to detect the formation of peroxy radicals (peroxidation index) under light excitation (lambda > 410 nm) when doped TiO2 was used. The kinetics of CO2 production was monitored during the mineralization of PE. Colored TiO2 powders were studied in detail by different and complementary physicochemical techniques. The band gap energies of colored TiO2 were determined by diffuse reflectance spectroscopy (DRS). The visible absorption shoulder of TiO2 was observed to follow Urbach's law. The variation of the transient decay after 354 nm laser pulse excitation does not correlate with the different N- and S-TiO2 doping levels introduced by the addition of urea or thiourea. This suggests that the states (recombination centers or traps) introduced by the doping are not effective in varying the decay kinetics within the nanosecond and microsecond time scale. Elemental analysis shows comparable amounts of S- and N-doping of TiO2 when thiourea is used as dopant. X-ray diffraction reveals no rutile in S-TiO2 samples heated to 600 degrees C, suggesting that the addition of sulfur precludes rutilization during sample crystallization. X-ray photoelectron spectroscopy (XPS) of the S-TiO2 samples confirms the preferential localization of S on the 20 topmost layers of S-TiO2 upon calcination at 500 degrees C for 2 h.

Quantum Chemical and Master Equation Simulations of the Oxidation and Isomerization of Vinoxy Radicals

The vinoxy radical, a common intermediate in gas-phase alkene ozonolysis, reacts with O2 to form a chemically activated alpha-oxoperoxy species. We report CBS-QB3 energetics for O2 addition to the parent (*CH2CHO, 1a), 1-methylvinoxy (*CH2COCH3, 1b), and 2-methylvinoxy (CH3*CHCHO, 1c) radicals. CBS-QB3 predictions for peroxy radical formation agree with experimental data, while the G2 method systematically overestimates peroxy radical stability. RRKM/master equation simulations based on CBS-QB3 data are used to estimate the competition between prompt isomerization and thermalization for the peroxy radicals derived from 1a, 1b, and 1c. The lowest energy isomerization pathway for radicals 4a and 4c (derived from 1a and 1c, respectively) is a 1,4-shift of the acyl hydrogen requiring 19-20 kcal/mol. The resulting hydroperoxyacyl radical decomposes quantitatively to form *OH. The lowest energy isomerization pathway for radical 4b (derived from 1b) is a 1,5-shift of a methyl hydrogen requiring 26 kcal/mol. About 25% of 4a, but only approximately 5% of 4c, isomerizes promptly at 1 atm pressure. Isomerization of 4b is negligible at all pressures studied.

The oxidation of benzene under conditions encountered in waste incinerators

The oxidation of benzene was studied as a function of residence time ( τ res = 0 – 2.5 s ), temperature (850–960 K), and oxygen concentration ( O 2 = 0.2 – 2.3 % ) in a heated laminar flow reactor at atmospheric pressure. Nitrogen, doped with 350 ppm benzene, was injected downstream of the burned gas from a near stoichiometric flame of methane + air. Gas samples were taken at different heights up the reactor and analyzed using GC-FID/TCD and HPLC techniques. Phenol and partially oxidized hydrocarbons such as acetaldehyde, formaldehyde, and acrolein were found with concentrations up to 50 ppm. At relatively low temperatures, the conversion of benzene was observed to proceed considerably more slowly at higher oxygen concentrations. Measured concentration profiles were modeled using detailed reaction schemes. A modified mechanism for the oxidation of benzene called BenWas was constructed from the mechanism of Zhang and McKinnon (Combust. Sci. Technol. 107 (1995) 261) by incorporating a submechanism for benzoquinone (OC 6H 4O) and by updating and enlarging the reaction scheme of cyclopentadiene (C 5H 6). The agreement between observed and predicted concentration profiles, e.g., of phenol (C 6H 5OH), acetylene (C 2H 2), and carbon monoxide (CO), was considerably improved by the use of the BenWas mechanism for rich and lean conditions, mainly due to the introduction of an additional pathway for phenyl oxidation (C 6H 5 + O 2 = OC 6H 4O + H) and due to the changed kinetics of the oxidation of cyclopentadienyl (C 5H 5) in C 5H 5 + O 2 = C 5H 4O + OH. The measured retardation of benzene oxidation with higher amounts of oxygen can be explained by the formation and reactions of peroxy radicals.

A semi-empirical correlation for the rate coefficients for cross- and self-reactions of peroxy radicals in the gas-phase

Abstract Peroxy radicals play an important role as reaction intermediates in the atmospheric oxidation of volatile organic compounds (VOCs). The rate coefficients for the self-reactions vary by up to six orders of magnitude (e.g. 3.0×10 −17 cm 3 molecule −1 s −1 for t -C 4 H 9 O 2 and 1.5×10 −11 cm 3 molecule −1 s −1 for CH 3 C(O)O 2 ) with, up to now, no clear pattern. This work represents the first rationalisation of peroxy radical self-reaction trends in reactivity. A correlation between the logarithm of the rate coefficient of peroxy radical self-reactions with the stabilisation energy, the difference in the enthalpy of formation of peroxy radicals with that of the tetraoxide adduct {Δ H f (RO 4 R)−2Δ H f (RO 2 )}, is presented. The equation, log 10 k =−{Δ H f (RO 4 R)−2Δ H f (RO 2 )+235.95}/13.32 (where energy is in kJ mol −1 ) is given to predict the room temperature rate coefficients of peroxy radical self-reactions. The correlation was extended to predict rate coefficients for the cross-reactions of peroxy radicals with CH 3 O 2 . Furthermore, favourable comparisons are made between the predicted rate coefficients and very recent studies of complex peroxy radical systems.

An electron paramagnetic resonance study of PP and PP/SBS blends irradiated with gamma rays

Electron paramagnetic resonance (EPR) measurements of Polypropylene (PP) and its blends with a Styrene–Butadiene–Styrene (SBS) copolymer in 10, 20, 30 and 40 wt.% of SBS were carried out. The samples were irradiated in the 0⩽D⩽100 kGy range at a dose rate of 4.8 kGy/h. Typical spectra indicative of the formation of peroxy and alkyl radicals were obtained. The dynamics of formation and recombination of radicals in these samples could be explained using a mixed zero and first order generation–recombination fit of the total spin concentration as a function of the integral dose. The time dependence of the spin concentration was studied assuming a mixed first order fit in the decay process. The parameters obtained from the fitting are interpreted in terms of the rate of formation of free radicals at the irradiation time and in terms of the decay time of the total free radical concentration.

Reactions of One-Electron-Oxidized Methionine with Oxygen: An ab Initio Study

A one-electron oxidation of a methionine residue is thought to be a key step in the neurotoxicity of the beta amyloid peptide of Alzheimer's disease. The chemistry of the radical cation of N-formylmethioninamide (11•+) and two model systems, dimethyl sulfide (1•+) and ethyl methyl sulfide (6•+), in the presence of oxygen have been studied by B3LYP/6-31G(d) and CBS-RAD calculations. The stable form of 11•+ has a three-electron bond between the sulfur radical cation and the carbonyl oxygen atom of the i − 1 residue. The radical cation may lose a proton from the methyl or methylene groups flanking the oxidized sulfur. Both 11•+ and the resultant C-centered radicals may add oxygen to form peroxy radicals. The calculations indicate that unlike C-centered radicals the sulfur radical cation does not form a covalent bond to oxygen but rather forms a loose ion-induced dipole complex with an S−O separation of about 2.7 Å, and is bound by about 13 kJ mol-1 (on the basis of 1•+ + O2). Direct intramolecular abstraction of an H atom from the αC site is unlikely. It is endothermic by more than 20 kJ mol-1 and involves a high barrier (ΔG = 79 kJ mol-1). The α-to-S C-centered radicals will add oxygen to form peroxy radicals. The OH BDEs of the parent hydroperoxides are in the range of 352−355 kJ mol-1, similar to SH BDEs (360 kJ mol-1) and αC−H BDEs (345−350 kJ mol-1). Thus, the peroxy radicals are oxidizing species comparable in strength to thiyl radicals and peptide backbone αC-centered radicals. Each peroxy radical can abstract a hydrogen atom from the backbone αC site of the Met residue to yield the corresponding αC-centered radical/hydroperoxide in a weakly exothermic process with modest barriers in the range of 64−92 kJ mol-1.

“Fenton-like” reactions of methylhydroperoxide and ethylhydroperoxide with Fe2+ in liquid aerosols under tropospheric conditions

Laboratory experiments were performed to study the “Fenton-like” reduction of methylhydroperoxide and ethylhydroperoxide by Fe2+ ions in aqueous solutions, under tropospheric conditions. The rate constants obtained at 6°C and pH=2 (16±5 and 24±9 M−1 s−1, respectively) are of the same order as the rate constant of H2O2 reduction by Fe2+. The major reaction products identified were the corresponding aldehydes (formaldehyde and acetaldehyde). The other identified organic products (the corresponding acids and hydroxy hydroperoxides) accounted for <10% of the total organic reaction products. Aldehydes and organic acids total production represented >87% of the consumed hydroperoxides. Between the two possible mechanisms (i.e. Fe2++ROOH→Fe3++OH+RO (M1) and Fe2++ROOH→Fe3++OH+RO− (M2)), the experiments showed that M2 is negligible. The alkoxy (RO) radicals formed in (M1) further rearrange in water solution, and react with oxygen to form peroxy radicals which decompose, leading to the formation of aldehydes. This decomposition is in competition with (i) the self-reaction forming the corresponding acids and H2O2, and (ii) the reaction with HO2 radicals forming the corresponding hydroxy hydroperoxides. The atmospheric implications of these reactions are discussed.

The effect of humic acids on nitrobenzene oxidation by ozonation and O 3/UV processes

Three types of commercially available humic acids from different sources were used to simulate natural organic matter in water for the investigation of nitrobenzene oxidation by ozonation and O 3/UV. Despite the structural differences among the Fluka, Aldrich and Suwanee River humic acids as reflected by the UV absorptivity, their effects on nitrobenzene removal rate was observed to be similar for the two processes. Removal rate of nitrobenzene was hindered by the addition of humic acids in ozonation as well as in O 3/UV processes. However, the hindrance by the humic acids was more pronounced in O 3/UV as compared to the ozonation process. The effect of humic acid in O 3/UV was primarily a UV light screening. Addition of humic acids above a certain concentration did not cause any further retardation on nitrobenzene removal rate by ozonation and O 3/UV. Accumulation of hydrogen peroxide as well as probable formation of peroxy radicals in the solutions might induce chain promoting reactions to produce hydroxyl radical during the nitrobenzene oxidation. For waters containing high levels of humic acid, ozonation alone might be as effective as O 3/UV process for the removal of nitrobenzene.

Nighttime formation of peroxy and hydroxyl radicals during the BERLIOZ campaign: Observations and modeling studies

Traditionally, tropospheric radical chemistry is discussed in terms of the daytime photochemically produced hydroxyl radical (OH). Radicals, however, are also important during nighttime: this is especially true for ozone and the nitrate radical (NO3), which both act as key initiators of the degradation of alkenes such as biogenic monoterpenes. These reactions lead to the formation of peroxy radicals (HO2 and RO2) and hydroxyl radicals at night. We present recent observations of nighttime concentrations of NO3, RO2, HO2, and OH by differential optical absorption spectroscopy (DOAS), matrix isolation electron spin resonance (MIESR), laser‐induced fluorescence (LIF), and a chemical amplifier (CA) in the framework of the Berliner Ozonexperiment (BERLIOZ) campaign at Pabstthum, Germany, together with modeling studies of nocturnal radical chemistry. Modeled RO2 mixing ratios reached 40 ppt while the measured ROx level went up to 22 ppt at the same time. Modeled and measured HO2 mixing ratios were up to 6 and 4 ppt, respectively. In the case of OH, a nocturnal concentration of (1.85 ± 0.82) × 105 cm−3 was measured during one night. At this time, the model yielded an OH level of (4.1 ± 0.7) × 105 cm−3. This overestimation by the model could point to a missing nocturnal sink of OH. Nitrate radical reactions with terpenes were found responsible for producing 77% of the RO2 radicals, 53% of the HO2, and 36% of the OH radicals during night. Nighttime ozonolysis formed 12% of the RO2, 47% of the HO2, and 64% of the OH radicals. Another 11% of the RO2 radicals were formed by OH–volatile organic compound (VOC) reactions. A positive linear correlation of RO2 and NO3 was observed and could be reproduced in model calculations originating from the loss of both radicals by reaction with NO and the NO3‐initiated RO2 production. The contribution of nighttime OH to the atmosphere's oxidation capacity (oxidation rate of VOCs, CO, and CH4) was found negligible (&lt;0.5%).

Friction‐reducing and antioxidant capabilities of engine oil additive systems under oxidative conditions. II. Understanding ligand exchange in a molybdenum dialkyldithiocarbamate/zinc dialkyldithiophosphate additive system in various base oils

AbstractThe ligand‐exchange reactions between molybdenum dialkyldithiocarbamate, Mo(dtc)2, and zinc dialkyldithiophosphate, Zn(dtp)2, have been investigated during inhibited oxidation in a model hydrocarbon (hexadecane) and in a series of Group I–IV base oils at 160°C. These investigations revealed that the ligand exchange leads to formation of single‐exchange products, Mo(dtc)(dtp) and Zn(dtp)(dtc), and double‐exchange products, Mo(dtp)2 and Zn(dtc)2, and that the extent of the exchange is significantly affected by oxidation and inhibition reactions involving the original additives, the ligand‐exchange products, the base oil, and base oil‐derived oxidation products.It is concluded that there are two reaction sequences that control the product distribution and additive consumption during oxidation in different base oils. The first sequence involves the formation of peroxy radicals and hydroperoxides, and the decomposition of hydroperoxides by Zn(dtp)2 and Zn(dtp)(dtc). One of the most important factors affecting this sequence is the base oil oxidisability. The second sequence involves inhibition of oxidation by peroxy radical‐trapping antioxidants naturally present in or added to the oil, or formed during the oxidation. Zn(dtc)2 and Zn(dtp)(dtc) appear to be very effective in this respect and are, therefore, preferentially consumed. This preferential consumption shifts the ligand‐exchange equilibrium towards the formation of Mo(dtc)(dtp) and Mo(dtp)2.The combined effect of the two sequences depends on base oil properties. The first sequence predominates in base oils exhibiting high oxidisability (e.g., in Group I oils) and the second in paraffinic oils, such as Group III and IV oils, having low oxidisability and no sulphur or aromatics. the retention of friction‐reducing capability with the Mo(dtc)2/Zn(dtp)2 additive system during oxidation appears to be tied to the first sequence, which leads to consumption of Zn (dtp)2, since the friction‐reducing capability ceases when Zn (dtp)2 is consumed and hydroperoxides can accumulate in the system. Consequently, the best retention of friction‐reducing capability is achieved in bases oils with low oxidisability.

Overview of the FIELDVOC'94 experiment in a eucalyptus forest of Portugal

The aim of the EU FIELDVOC'94 project (field study on the tropospheric degradation mechanisms of biogenic VOCs, isoprene and dimethylsulfide) was to understand the contribution of the isoprene chemistry to the budget of ozone and the formation of peroxy radicals (RO2) under various conditions, particularly high and low nitrogen oxides concentrations (NOx). An additional objective was to evaluate the involvement of nitrate radicals (NO3) in the production of RO2 during night-time. An experiment was undertaken in June–July 1994, at a forest site in central Portugal. During this campaign most of the compounds involved in the ozone/isoprene chemistry were measured, i.e., nitrogen oxides, carbon monoxide, degradation and secondary products (aldehydes, ketones, organic acids and peroxides), as well as free radicals (RO2 and NO3). Several intensive measurement periods were carried out under different conditions of high and low photochemical activity. Isoprene was the most abundant hydrocarbon measured at the site, with mixing ratios as high as 10–12 ppbv during periods of high photochemical activity and elevated temperatures. Terpenes were also produced in the ppbv level with maximum concentrations observed at night. Ozone exhibited a diurnal variability, with maximum levels in the afternoon of 80–100 ppbv attributed to the interaction of polluted air masses and the isoprene chemistry. NO3 radicals never exceeded a maximum of 6 pptv. RO2 radicals displayed a diurnal maximum of 200–250 pptv and in some case reached night-time levels of 10–20 pptv. Radicals and ozone budgets deduced from photostationary models or chemical box models showed that hydroxyl radical (OH) initiated isoprene chemistry contributed to about 45% of the observed NO3 levels during daytime and that ozone initiated chemistry accounted for approximately half of the RO2 observed. The chemistry of isoprene strongly enhanced the ozone net production and reduced by a factor 2–3 the OH concentration.

Formation of Peroxy Radicals from OH−Toluene Adducts and O2

The gas-phase reaction of OH radicals with toluene and toluene-d8 and reactions of the resulting OH adducts with O2 were studied in N2/O2 mixtures at atmospheric pressure and room temperature. OH production was performed by pulsed 248 nm photolysis of H2O2. Cw UV-laser long-path absorption at 308 nm was used for time-resolved detection of OH and OH−toluene adducts. The reaction OH + toluene was studied in N2 and O2 and similar rate constants of (5.70 ± 0.19) × 10-12 cm3 s-1 (N2) and (5.60 ± 0.14) × 10-12 cm3 s-1 (O2) were obtained at 100 kPa. For toluene-d8 the corresponding rate constants are (5.34 ± 0.34) × 10-12 cm3 s-1 (N2) and (5.47 ± 0.14) × 10-12 cm3 s-1 (O2). Absorption cross-sections of (1.1 ± 0.2) × 10-17 cm2 were determined for both the OH−toluene and OH−toluene-d8 adduct at 308 nm. Rate constants of (4.7 ± 1.4) × 10-11 cm3 s-1 and (1.8 ± 0.5) × 10-10 cm3 s-1 were determined for the OH−toluene adduct self-reaction and the OH−toluene adduct + HO2 reaction, respectively. Upper limits ≤8 × 10-15 cm3 s-1 were estimated for any reactions of the OH−toluene adduct with H2O2 or toluene. Adduct kinetics in the presence of O2 is consistent with reversible formation of peroxy radicals with an estimated rate constant of (3 ± 2) × 10-15 cm3 s-1 and an equilibrium constant of (3.25 ± 0.33) × 10-19 cm3 (toluene-d8: (3.1 ± 1.2) × 10-19 cm3). The effective adduct loss from the equilibrium can be explained by (i) an additional, irreversible reaction of the adduct with O2 with a rate constant of (6.0 ± 0.5) × 10-16 cm3 s-1 (toluene-d8: (4.7 ± 1.2) × 10-16 cm3 s-1), or (ii) a unimolecular reaction of the peroxy radical, with a rate constant of (1.85 ± 0.15) × 103 s-1 (toluene-d8: (1.5 ± 0.4) × 103 s-1). Consequences for the atmospheric degradation of toluene will be discussed.

Production of peroxy radicals at night via reactions of ozone and the nitrate radical in the marine boundary layer

In this paper, a substantial set of simultaneous measurements of the sum of peroxy radicals, [HO2 + RO2], NO3, hydrocarbons (HCs), and ozone, taken at Mace Head on the Atlantic coast of Ireland in spring 1997, is presented. Conditions encountered during the experiment ranged from semipolluted air masses advected from Britain and continental Europe to clean air masses off the North and mid‐Atlantic, where mixing ratios of pollution tracers approached Northern Hemispheric background mixing ratios. Average mixing ratios of peroxy radicals varied from 2.5 to 5.5 parts per trillion by volume (pptv) at night depending on wind sector, and were observed to decay only very slowly from late afternoon to early morning (0.1–0.5 pptv h−1). Measurements of OH and HO2 on two nights using the Fluorescence Assay by Gas Expansion (FAGE) technique give an upper limit for [OH] of 2.5×105 molecules cm−3 and an average upper limit [HO2]/[HO2 + RO2] ratio of 0.27. A modeling study of the 1/e lifetimes of the peroxy radicals, assuming no radical production at night, yielded mean lifetimes of between ∼8–23 min for HO2 and 3–18 min for CH3O2. Given these lifetimes, it may be concluded that the peroxy‐radical mixing ratios observed could not be maintained without substantial production at night. No significant correlation is observed between measured [HO2 + RO2] and [NO3] under any conditions. Calculation of the reaction rates for ozone and NO3 with hydrocarbons (HCs) shows that the ozone‐initiated oxidation routes of HCs outweighed those of NO3 in the NE, SE and NW wind sectors. In the SW sector, however, the two mechanisms operated at similar rates on average, and oxidation by NO3 was the dominant route in the westerly sector. The oxidation of alkenes at night by ozone was greater by a factor of 4 than that by NO3 over the whole data set. The HC degradation rates from the three “westerly” sectors, where tracer mixing ratios were relatively low, may be representative of the nighttime oxidative capacity of the marine boundary layer throughout the background Northern Hemisphere. Further calculations using literature values for OH yields and inferred RO2 yields from the ozone‐alkene reactions show that peroxy radicals derived from the ozone reactions were likely to make up the major part of the peroxy‐radical signal at night (mean value 66%). However, the NO3 source was of similar magnitude in the middle of the night, when [NO3] was generally at its maximum. The estimated total rates of formation of peroxy radicals are much higher under semipolluted conditions (mean 8.0×104 molecules cm−3 s−1 in the SE wind sector) than under cleaner conditions (mean 2.4×104 molecules cm−3 s−1 in the westerly wind sector). A model study using a campaign‐tailored box model (CTBM) for semipolluted conditions shows that the major primary sources of OH, HO2, and CH3O2 (the most abundant organic peroxy radical) were the Criegee biradical intermediates formed in the reactions of ozone with alkenes.

Biogenic volatile organic compounds at Azusa and elevated sites during the 1997 Southern California Ozone Study

Biogenic volatile organic compounds (BVOC) were measured at Azusa and at either Pine Mountain or Mount Baldy, elevated sites 11 km north and 25 km northeast of Azusa, respectively, during four intensive sampling periods of the 1997 Southern California Ozone Study. During the sampling periods there was a consistent pattern of isolation of the mountain sites from valley air masses at night, followed by transport of valley air with elevated levels of O3 and NOx to the mountain sites as the mixing height increased throughout the day. Isoprene was the dominant BVOC at the mountain sites with afternoon concentrations reaching 2 ppbv, and its decrease to a low mixing ratio after sunset was attributed to reaction with NO3 radicals. At Azusa the BVOC mixing ratios were highest in the morning with the concentrations of monoterpenes and of methacrolein (MACR) and methyl vinyl ketone (MVK), isoprene photooxidation products, generally exceeding the maximum isoprene measured at Azusa. The high daytime ratio of (MVK + MACR)/isoprene suggested that nighttime drainage flows into Azusa, from elevated sites where the isoprene was depleted by chemical reaction, may have been responsible for much of the isoprene and its photoxidation products. The data also indicated local isoprene sources at Azusa and a possible contribution of MVK and MACR from vehicle emissions. Instances of high mixing ratios of limonene at Azusa suggested an intermittent anthropogenic source. During this study, particularly in early morning, BVOC are calculated to make a significant contribution to peroxy radical formation at Azusa.