Product study of the reactions of γ-caprolactone and γ-heptalactone initiated by OH radicals at 298 K and atmospheric pressure: Formation of acyl peroxynitrates (APN).

A product study was performed for the reaction of γ-caprolactone (GCL) and γ-heptalactone (GHL) initiated by OH radicals at (298 ± 2) K and atmospheric pressure, in presence of NOx. The identification and quantification of the products were performed in a glass reactor coupled with in situ FT-IR spectroscopy. The following products were identified and quantified with the corresponding formation yields (in %) for the OH + GCL reaction: peroxy propionyl nitrate (PPN) (52 ± 3), peroxy acetyl nitrate (PAN) (25 ± 1), and succinic anhydride (48 ± 2). For the GHL + OH reaction, the products detected with their corresponding formation yields (in %) were the following: peroxy n-butyryl nitrate (PnBN) (56 ± 2), peroxy propionyl nitrate (PPN) (30 ± 1) and succinic anhydride and (35 ± 1). Upon these results, an oxidation mechanism is postulated for the title reactions. The positions with the highest H-abstraction probabilities for both lactones are analyzed. Specifically, the increased reactivity of the C5 site, as indicated by structure reactivity estimations (SAR), is suggested by the identified products. For both GCL and GHL degradation appears to follow degradation paths including ring preservation and opening. The atmospheric implications of the APN formation as a photochemical pollutant and as NOx reservoirs of species is assessed.

Organic acid formation in the gas-phase ozonolysis of α,ß-unsaturated ketones.

Organic acids are key species in determining the radiative properties of the atmosphere due to their contribution to particle formation. Reported discrepancies between field measurements and modelling suggest significant missing sources. Herein, we present a mechanistic investigation on the gas-phase ozonolysis of ethyl vinyl ketone (EVK, 1-penten-3-one), which we chose as a model compound for α,ß-unsaturated ketones. Experiments were performed in a 1080 L quartz-glass reaction chamber (QUAREC) at 990 ± 15 mbar and 298 ± 2 K (r. h. ≪ 0.1%) and analysed via long-path FTIR spectrometry and PTR-ToF-MS. The experiments were performed in the presence of an excess of CO to suppress the chemistry of OH radicals. For a comprehensive picture, in selected experiments, SO2 was also added to the reaction system to scavenge the stabilized Criegee intermediates (sCIs) and to investigate their formation yield. Combining the results of both set-ups allowed us to quantify 2-oxobutanal, for which we report vapour-phase FTIR spectra. In addition, we introduce the first-ever infrared spectra of perpropionic acid, which was also positively identified in the EVK + O3 system. A detailed analysis of the experimental findings allowed us to link the identified reaction products (acetaldehyde, ethyl hydroperoxide, and perpropionic acid) to known bimolecular reactions of RO2 radicals. Thereby, it is shown that the EVK + O3 reaction yields formic acid, HC(O)OH, and propionic acid, C2H5C(O)OH, and their formation is not covered by mechanisms reported in the literature. Three different pathways accounting for their formation from chemically activated CIs are proposed and possible implications for the ozonolysis of α,ß-unsaturated ketones in the atmosphere are discussed.

Kinetics, product distribution and atmospheric implications of the gas-phase oxidation of allyl sulfides by OH radicals.

Relative rate coefficients of the OH radical -initiated oxidation of allyl methyl sulfide (AMS, H2CCHCH2SCH3) and allyl ethyl sulfide (AES, H2CCHCH2SCH2CH3) have been measured at atmospheric pressure of synthetic air and 298 K: kAMS= (4.98 ± 1.42) and kAES= (6.88 ± 1.49) × 10-11 cm3 molecule-1 s-1 by means of in situ FTIR spectroscopy. In addition, the molar yields of the main reaction products of AMS with OH radicals formed in the absence and presence of nitric oxides (NOX) were determined to be the following: sulfur dioxide (95 ± 12) % and (51 ± 12) % for acrolein (50 ± 9) % and (41 ± 9) %. In the reaction of AES with OH radicals, the following molar yields were obtained: for sulfur dioxide (88 ± 13) % and (56 ± 12) % for acrolein (36 ± 9) % and (41 ± 9) %. The present results suggest that the abstraction at C3 plays an important role in the oxidation mechanism as the addition to the double bond. This work represents the first study of the OH radical interaction with AMS and AES carried out under atmospheric conditions. The atmospheric implications were discussed in terms of the atmospheric residence times of the sulfur-containing compounds studied and the products formed in the presence and absence of NOx. SO2 formation seems to be the main fate of the gas-phase allyl sulfides oxidation with significant acidifying potentials and short-chain aldehydes production like formaldehyde and acetaldehyde.

Diurnal photodegradation of fluorinated diketones (FDKs) by OH radicals using different atmospheric simulation chambers: Role of keto-enol tautomerization on reactivity.

Rate coefficients for the gas-phase reactions of OH radicals with a series of fluorinated diketones have been determined for the first time at (298 ± 3) K and atmospheric pressure using the relative method and FTIR spectroscopy and GC-FID to monitor both reactants and references. The following values, in 10-11 cm3 molecule-1 s-1, were obtained for 1,1,1-trifluoro-2,4-pentanedione (TFP), 1,1,1-trifluoro-2,4-hexanedione (TFH) and 1,1,1-trifluoro-5-methyl-2,4-hexanedione (TFMH), respectively: k1(TFP + OH) = (1.3 ± 0.4), k2(TFH + OH) = (2.2 ± 0.8), k3(TFMH + OH) = (3.3 ± 1.0). The results are discussed with respect to the keto-enolic tautomerization specific for ß-diketones. Based on the present results, the tropospheric lifetimes of TFP, TFH and TFMH upon degradation by OH radicals were calculated as 21, 13 and 8 h, respectively indicating that transport might play a role in the atmospheric fate of the studied compounds. Photochemical ozone creation potentials were estimated for TFP, TFH and TFMH to be: 23, 29 and 34, respectively.

Temperature dependencies of the degradation of NO, NO2 and HONO on a photocatalytic dispersion paint.

The photocatalytic decomposition of nitrogen oxides (NOx) has attracted significant interest as a potential measure of reducing NOx levels in the urban atmosphere. Since photocatalytic activity is highly variable depending on atmospheric conditions, the uptake of NO, NO2 and HONO was studied on a commercial photocatalytic dispersion paint in a flow photoreactor as a function of the relative humidity and temperature. Since the relative humidity is a function of the surface's temperature, here both dependencies were carefully decoupled for the first time. In addition, for the first time the temperature dependence of the whole NOx reaction system including the important intermediate HONO was investigated. While for NO and NO2 strong negative humidity dependencies were observed, the photocatalytic uptake of HONO increased with humidity. For constant relative humidity no temperature dependence of the photocatalytic oxidation of NO was observed, whereas the photocatalytic NO2 uptake decreased with increasing temperature, which is explained by a temperature dependent adsorption equilibrium of the surface active NO2. HONO uptake showed a positive temperature dependence confirming the proposed photocatalysis of nitrite in a layer of adsorbed water on the surface of the photocatalyst. The missing/negative temperature dependencies of the photocatalysis of NO/NO2 are overcompensated by their strong negative relative humidity dependencies, leading to increasing uptake for both pollutants when photocatalytic surfaces are heated by solar irradiation in the atmosphere.

Experimental and theoretical study of the reactivity of a series of epoxides with chlorine atoms at 298 K.

Evaluating the reactivity of epoxides in the gas phase is very important due to their wide distribution in the atmosphere, potential health implications and atmospheric impact. The kinetic rate constants for the oxidation of epoxides have been very little studied until now. From the experimental data obtained in this work has been observed that there is an increase in reactivity towards chlorine atoms as a CH2 group is added to the hydrocarbon chain. The Structure Activity Relationship (SAR) method usually provides a good approximation of the rate constant for a wide series of compounds especially for those without complex structure and multiple organic functions. However, a good determination of the factors included in SAR estimations depends largely on the database of these compounds, which in the case of epoxides is very limited. The SAR estimation method also does not take into account other possible factors that could affect reactivity, such as the geometry of the molecule. The aim of this work is to further evaluate the reactivity of epoxides with chlorine atoms using experimental determinations, theoretical calculations and SAR estimations. For this, rate coefficients have been measured at 298 ± 2 K and 1000 ± 4 mbar pressure of synthetic air in a 1080 l Quartz Reactor (QUAREC) and a 480 l Duran glass reactor for the reaction of chlorine atoms with cyclohexene oxide (CHO), 1,2-epoxyhexane (12EHX), 1,2-epoxybutane (12EB), trans-2,3-epoxybutane (tEB) and cis-2,3-epoxybutane (cEB). Theoretical calculations for the reactions studied are in good agreement with our experimental findings and provide insights about the position of the H atom abstraction and reactivity trends for a series of epoxides. The importance of taking into consideration the geometrical distribution and the ring influence to improve SAR calculations is discussed.

OH-initiated degradation of methyl 2-chloroacetoacetate and ethyl 2-chloroacetoacetate: Kinetics, products and mechanisms at 298 K and atmospheric pressure.

Rate coefficients for the gas-phase reactions of OH radicals with CH3C(O)CHClC(O)OCH3 (k1) and CH3C(O)CHClC(O)OCH2CH3 (k2) were measured using the relative technique with different reference compounds. The experiments were performed at (298 ± 2) K and 750 Torr of nitrogen or synthetic air by in situ FTIR spectroscopy and GC-FID chromatography. The following rate coefficients (in units of cm3molecule-1 s-1) were obtained: k1FTIR= (2.70 ± 0.51) × 10-11; k1GC-FID= (2.30 ± 0.71) × 10-11 and k2FTIR= (3.37 ± 0.62) × 10-11; k2GC-FID= (3.26 ± 0.85) × 10-11. This work reports the first kinetic study for the reactions of OH radicals with the mentioned chloroacetoacetates. Additionally, product studies are reported in similar conditions of the kinetic experiments. Acetic acid, acetaldehyde, formyl chloride, and methyl 2-chloro-2-oxoacetate were positively identified and quantified as degradation products. According to the identified products, atmospheric chemical mechanisms were proposed. The environmental implications of these reactions were assessed by the tropospheric lifetimes calculations of the title chloroesters. Significant average ozone production of 4.16 ppm for CH3C(O)CHClC(O)OCH3 and 5.98 ppm for CH3C(O)CHClC(O)OCH2CH3, respectively were calculated.

Gas-phase reactivity of acyclic α,ß-unsaturated carbonyls towards ozone.

We evaluated different approaches to discuss the reactivity of α,ß-unsaturated carbonyls comparative to alkene analogues. It was found that the reactivity factors xr, defined as the relative ratio between the rate coefficient of the carbonyl and a core structure, allow a semi-quantitative estimation of substituent effects in α,ß-unsaturated acids, aldehydes and esters when the carbonyl containing substituent is replaced by a hydrogen atom. By contrast, it can be shown that the reactivity of the corresponding ketones differs from the other carbonyls. A linear correlation is presented between the xr- values and the number of carbon atoms of the alkyl group of the unsaturated esters, which can be used to predict ozonolysis rate coefficients. For this systematic analysis the following rate coefficients (in 10-18 cm3 molecule-1 s-1) have been determined at 298 ± 2 K and 990 ± 15 mbar and under dry conditions using the relative rate method: k(O3 + methyl methacrylate) = 7.0 ± 0.9, k(O3 + methyl crotonate) = 5.5 ± 1.4, k(O3 + methyl 3-methyl-3-butenoate) = 1.3 ± 0.3, k(O3 + methyl tiglate) = 65 ± 11, k(O3 + 3-penten-2-one) = 31 ± 7, k(O3 + 3-methyl-3-penten-2-one) = 80 ± 19, k(O3 + 4-methyl-3-penten-2-one) = 8.4 ± 0.8.

FTIR product study of the Cl-initiated oxidation products of CFC replacements: (E/Z)-1,2,3,3,3-pentafluoropropene and hexafluoroisobutylene.

A product study of the reactions of (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF[double bond, length as m-dash]CHF) and hexafluoroisobutylene ((CF3)2C[double bond, length as m-dash]CH2) initiated by Cl atoms were developed at 298 ± 2 K and atmospheric pressure. The experiments were carried out in a 1080 L quartz-glass environmental chamber coupled via in situ FTIR spectroscopy to monitor the reactants and products. The main products observed and their yields were as follows: CF3C(O)F (106 ± 9)% with HC(O)F (100 ± 8)% as a co-product for (E/Z)-CF3CF[double bond, length as m-dash]CHF, and CF3C(O)CF3 (94 ± 5)% with HC(O)Cl (90 ± 7)% as a co-product for (CF3)2C[double bond, length as m-dash]CH2. Atmospheric implications of the end-product degradation are assessed in terms of their impact on ecosystems to help environmental policymakers consider HFOs as acceptable replacements.

Determination of the emission indices for NO, NO2 , HONO, HCHO, CO, and particles emitted from candles.

In the present study, emission indices for NO, NO2 , HONO, HCHO, CO, particle mass, and particle numbers including particle size distributions for three different offering candles were determined. The candles investigated showed similar emission characteristics with emission indices (g/kg) in good agreement with former candle emission studies. An average HONO/NOx emission ratio of 6.6 ± 1.1% was obtained, which is much higher compared to most other combustion sources, indicating that candles may be a significant indoor source of this important trace gas. The particle size distributions indicate that the majority of the emitted particles are in the size range 7 - 15 nm. Three modes were observed during burning the candles with very different emission profiles: a "normal burning" mode characterized by low particle number emission rates and small particles; an initial "sooting" behavior after ignition, and a final "smoldering" phase upon candle extinction with higher particle number emission rates and larger particles. The particle emission upon extinction is dependent on the extinction method. The NOx emission indices were applied in a simple box model to calculate typical indoor NOx concentration levels from candle emissions, which were in excellent agreement with direct measurements in a typical indoor environment.

Product distribution and mechanism of the OH- initiated tropospheric degradation of three CFC replacement candidates: CH3CF[double bond, length as m-dash]CH2, (CF3)2C[double bond, length as m-dash]CH2 and (E/Z)-CF3CF[double bond, length as m-dash]CHF.

The OH radical initiated photodegradation of 2-fluoropropene (CH3CF[double bond, length as m-dash]CH2), 3,3,3-trifluoro-2-(tri-fluoromethyl)propene ((CF3)2C[double bond, length as m-dash]CH2) and (E/Z)-1,2,3,3,3-pentafluoropropene ((E/Z)-CF3CF[double bond, length as m-dash]CHF) has been investigated for the first time using a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air coupled with in situ FTIR spectroscopy to monitor reactants and products. The major products observed in the OH reaction were CH3C(O)F (98 ± 5)% together with HC(O)H (89 ± 7)% as a co-product, CF3C(O)F (103 ± 8)% together with HC(O)F (96 ± 7)% as a co-product and CF3C(O)CF3 (91 ± 8)% together with HC(O)H (98 ± 12)% as a co-product from the C1-C2 bond cleavage channel of the intermediate hydroxyalkoxy radical, formed by addition of OH to the terminal carbon of the double bond which is designated C1 of 2-fluoropropene, (E/Z)-1,2,3,3,3-pentafluoropropene and 3,3,3-trifluoro-2-(tri-fluoromethyl)propene, respectively. The present results are compared with previous studies for the reaction of OH with the separate isomers (E) and (Z) of 1,2,3,3,3-pentafluoropropene. In addition, atmospheric implications of the reactions studied are discussed.

Gas-phase degradation of 2-butanethiol initiated by OH radicals and Cl atoms: kinetics, product yields and mechanism at 298 K and atmospheric pressure.

Relative rate coefficients and product distribution of the reaction of 2-butanethiol (2butSH) with OH radicals and Cl atoms were obtained at atmospheric pressure and 298 K. The experiments were performed in a 480 L borosilicate glass photoreactor in synthetic air coupled to a long path "in situ" FTIR spectrometer. The rate coefficients obtained by averaging the values from different experiments were: k OH = (2.58 ± 0.21) × 10-11 cm3 per molecule per s and k Cl = (2.49 ± 0.19) × 10-10 cm3 per molecule per s. The kinetic values were compared with related alkyl thiols and homologous alkyl alcohols, where it was found that thiols react faster with both oxidants, OH radicals and Cl atoms. SO2 and 2-butanone were the major products identified for the reactions of 2-butanethiol with OH radicals and Cl atoms. The product yield of the reaction of 2-butanethiol and OH radicals were (81 ± 2)%, and (42 ± 1)% for SO2 and 2-butanone, respectively. For the reactions of 2-butanethiol with Cl atom, yields of SO2 and 2-butanone were (59 ± 2)% and (39 ± 2)%, respectively. A degradation mechanism was proposed for the pathways that leads to formation of identified products. The product distribution observed indicated that the H-atom of the S-H group abstraction channel is the main pathway for the reaction of OH radicals and Cl atoms with 2-butanethiol.

Atmospheric sink of ß-ocimene and camphene initiated by Cl atoms: kinetics and products at NO x free-air.

Rate coefficients for the gas-phase reactions of Cl atoms with ß-ocimene and camphene were determined to be (in units of 10-10 cm3 per molecule per s) 5.5 ± 0.7 and 3.3 ± 0.4, respectively. The experiments were performed by the relative technique in an environmental chamber with FTIR detection of the reactants at 298 K and 760 torr. Product identification experiments were carried out by gas chromatography with mass spectrometry detection (GC-MS) using the solid-phase microextraction (SPME) method employing on-fiber carbonyl compound derivatization with o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine hydrochloride. An analysis of the available rates of addition of Cl atoms and OH radicals to the double bond of alkenes and cyclic and acyclic terpenes with a conjugated double bond at 298 K is presented. The atmospheric persistence of these compounds was calculated taking into account the measured rate coefficients. In addition, tropospheric chemical mechanisms for the title reactions are postulated.

Mechanism and Product Distribution of the O3-Initiated Degradation of (E)-2-Heptenal, (E)-2-Octenal, and (E)-2-Nonenal.

The O3-molecule initiated degradation of three 2-alkenals (E)-2-heptenal, (E)-2-octenal, and (E)-2-nonenal has been investigated in a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air using in situ FTIR spectroscopy to monitor the reactants and products. The experiments were performed in the absence of an OH scavenger. The molar yields of the primary products formed were glyoxal (49 ± 4) % and pentanal (34 ± 3) % from the reaction of (E)-2-heptenal with O3, glyoxal (41 ± 3) % and hexanal (39 ± 3) % from the reaction of (E)-2-octenal with O3, and glyoxal (45 ± 3) % and heptanal (46 ± 3) % from the reaction of (E)-2-nonenal with O3. The residual bands in the infrared product spectra for each of the studied reactions are attributed to 2-oxoaldehyde compounds. Based on the observed products, a general mechanism for the ozonolysis reaction of long chain unsaturated aldehydes is proposed, and the results are compared with the available literature data.

Rate Coefficients for the Reactions of OH Radicals with a Series of Alkyl-Substituted Amines.

Rate coefficients have been determined at (298 ± 2) K and atmospheric pressure for the reaction of OH radicals with two primary monoalkyl-, one secondary dialkyl-, and five tertiary trialkyl-substituted amines in a large volume photoreactor by the relative kinetic technique. The following rate coefficients (in cm3 molecule-1 s-1 units) have been obtained: (1) 1,2-dimethylpropylamine, CH3C(CH3)HC(CH3)H)NH2, (5.08 ± 1.02) × 10-11; (2) tert-amylamine (tert-pentylamine), CH3CH2C(CH3)2NH2, (0.86 ± 0.17) × 10-11; (3) diethylamine, (CH3CH2)2NH, (7.36 ± 1.47) × 10-11; (4) N,N-dimethylethylamine, (CH3)2NCH2CH3, (7.37 ± 1.47) × 10-11; (5) N,N-dimethylpropylamine, (CH3)2NCH2CH2CH3, (10.97 ± 1.78) × 10-11; (6) N,N-dimethylisopropylamine, (CH3)2NCH(CH3)2, (9.79 ± 1.75) × 10-11; (7) N,N-diethylmethylamine, (CH3CH2)2NCH3, (8.92 ± 1.54) × 10-11; (8) triethylamine, (CH3CH2)3N, (10.86 ± 1.88) × 10-11. The quoted errors are the 2σ deviations from least-squares linear analysis of the kinetic data plus a contribution to take into account uncertainties associated with the reference compounds. With the exception of diethylamine, this study represents the first determinations of the rate coefficients for reaction of the compounds with OH radicals. The results are compared with rate coefficients available in the literature for smaller alkyl-substituted amines and also the values predicted for the compounds investigated by using a structure activity relationship (SAR). With the exception of tert-amylamine, the SAR-predicted OH rate coefficients are in good agreement with the experimental values.

Atmospheric Sink of (E)-3-Hexen-1-ol, (Z)-3-Hepten-1-ol, and (Z)-3-Octen-1-ol: Rate Coefficients and Mechanisms of the OH-Radical Initiated Degradation.

A kinetic study of the gas-phase reactions of OH radicals with three unsaturated biogenic alcohols, (E)-3-hexen-1-ol, (Z)-3-hepten-1-ol, and (Z)-3-octen-1-ol, has been performed. The rate coefficients obtained are (in units of 10(-10) cm(3) molecule(-1) s(-1)) k1 (OH + (E)-CH2(OH)CH2CH═CHCH2CH3) = (1.14 ± 0.14), k2 (OH + (Z)-CH2(OH)CH2CH═CHCH2CH2CH3) = (1.28 ± 0.23), and k3 (OH + (Z)-CH2(OH)CH2CH═CHCH2CH2CH2CH3) = (1.49 ± 0.35). In addition, a product study on the reactions of OH with (E)-3-hexen-1-ol and (Z)-3-hepten-1-ol is reported. All the experiments were performed at (298 ± 2) K and 1 atm of NOx-free air in a 1080 L photoreactor with in situ FTIR detection of organics. This work constitutes the first kinetic study of the reactions of OH radicals with (Z)-3-hepten-1-ol and (Z)-3-octen-1-ol as well as the first determination of the fate of the hydroxy alkoxy radicals formed in the title reactions. An analysis of the available rates of addition of OH and Cl to the double bond of different unsaturated alcohols at 298 K has shown that they can be related by the expression log kOH = (0.29 ± 0.04) log kCl - 10.8. The atmospheric lifetimes of the alcohols studies were estimated to be around 1 h for reaction with OH radicals. The products formed in the title reactions are mainly carbonylic compounds that can contribute to the formation of ozone and PANs-type compounds in the troposphere.

Rate Coefficients for the Gas-Phase Reactions of Hydroxyl Radicals with a Series of Methoxylated Aromatic Compounds.

Rate coefficients for the reactions of hydroxyl radicals (OH) with a series of oxygenated aromatics (two methoxybenzene and five methoxyphenol isomers) have been obtained using the relative kinetic method in 1080 and 480 L photoreactors at the University of Wuppertal, Germany. The experiments were realized at 295 ± 2 K and 1 bar total pressure of synthetic air using in situ Fourier transform infrared spectroscopy for the chemical analysis. The following rate coefficients (in units of cm(3) molecule(-1) s(-1)) were determined: methoxybenzene (anisole), (2.08 ± 0.21) × 10(-11); 1-methoxy-2-methylbenzene, (4.56 ± 0.50) × 10(-11); 2-methoxyphenol (guaiacol), (5.40 ± 0.72) × 10(-11); 3-methoxyphenol, (6.93 ± 0.67) × 10(-11); 4-methoxyphenol, (5.66 ± 0.55) × 10(-11); 2-methoxy-4-methylphenol, (7.51 ± 0.68) × 10(-11); 2,3-dimethoxyphenol, (7.49 ± 0.81) × 10(-11); and 2,6-dimethoxyphenol (syringol), (8.10 ± 0.98) × 10(-11). The rate coefficients for the reactions of OH with 2,3-dimethoxyphenol and 1-methoxy-2-methylbenzene are first time measurements. The rate coefficients determined in this work are compared with previous determinations reported in the literature and also with the values estimated using a structure-activity relationship method. A comparison is performed between the OH rate coefficients obtained for methoxylated aromatics with those of other substituted aromatics in order to understand the influence of the type, number, and position of the different substituents on the reactivity of aromatics toward OH. In addition, a comparison is made between the OH and Cl rate coefficients for the compounds. The principal atmospheric sink of these methoxylated aromatic compounds during daytime is their reaction with OH radicals. The corresponding lifetimes for reaction with OH radicals and Cl atoms are 2-8 and 11-50 h, respectively.

Kinetic study of the gas-phase reactions of chlorine atoms with 2-chlorophenol, 2-nitrophenol, and four methyl-2-nitrophenol isomers.

Anthropogenic activities are the main source of nitrophenols and chlorophenols in the atmosphere. Nitro and chlorophenols have a high potential to form ozone and secondary organic aerosol, thus investigations on the major photo oxidation pathways of these compounds are important to assess their contribution to urban air pollution and human health. Presented here are rate coefficients determined at atmospheric pressure and (298 ± 2) K using a relative kinetic method for the reactions of chlorine atoms with 2-chlorophenol (2ClP), 2-nitrophenol (2NP) and four methyl-2-nitrophenol (2-nitrocresol, nM2NP (n = 3,4,5,6)) isomers. The following rate coefficients (in units of cm(3) molecule(-1) s(-1)) have been obtained: (5.9 ± 1.5) × 10(-12) for 2ClP, (6.8 ± 2.3) × 10(-12) for 2NP, and (14.0 ± 4.9) × 10(-11), (4.3 ± 1.5) × 10(-11), (1.94 ± 0.67) × 10(-11) and (2.68 ± 0.75) × 10(-11) for the four methyl-2-nitrophenol isomers 3M2NP, 4M2NP, 5M2NP, and 6M2NP, respectively. This study represents the first kinetic investigation for the reaction of chlorine atoms with all the nitrophenols. In addition, to assist in the interpretation of the results, rate coefficients for the reactions of Cl atoms with the cresol ortho, meta, and para isomers have been determined for the first time. The rate coefficient for the reaction with 2ClP is in good agreement with previous data and the relative reactivity of 2NP, 4M2NP, 5M2NP, and 6M2NP can be rationalized based on known substituent effects. The rate coefficient for 3M2NP is anomalously large; the observation of significant NO2 production in only this reaction suggests that an ipso substitution mechanism is the cause of the enhanced reactivity.

Rate coefficients for the gas-phase reaction of chlorine atoms with a series of methoxylated aromatic compounds.

The reaction of a series of oxygenated aromatics (two methoxybenzene and six methoxyphenol isomers) with chlorine atoms has been studied in two simulation chambers with volumes of 1080 and 480 L at the University of Wuppertal. Experiments were performed at 295 ± 2 K and a total pressure of synthetic air of 1 bar using the relative kinetic method with in situ Fourier transform infrared spectroscopy for chemical analysis. The following rate coefficients (in units of cubic centimeter per molecule per second) were determined: (1.07 ± 0.24) × 10(-10) for methoxybenzene, (1.20 ± 0.24) × 10(-10) for 1-methoxy-2-methylbenzene, (2.97 ± 0.66) × 10(-10) for 2-methoxyphenol (guaiacol), (2.99 ± 0.62) × 10(-10) for 3-methoxyphenol, (2.86 ± 0.58) × 10(-10) for 4-methoxyphenol, (3.35 ± 0.68) × 10(-10) for 2-methoxy-4-methylphenol, (4.73 ± 1.06) × 10(-10) for 2,3-dimethoxyphenol, and (2.71 ± 0.61) × 10(-10) for 2,6-dimethoxyphenol (syringol). To the best of our knowledge, this work represents the first determination of the rate coefficients for the gas-phase reaction of the chlorine atoms with the methoxy-aromatic compounds investigated. The reactivity of the methoxylated aromatics toward Cl is compared with that of other substituted aromatic compounds, and the differences in the rate coefficients are interpreted in terms of the type, number, and position of the different substituents on the aromatic ring. The atmospheric implications of the studied reactions are also discussed.

Products and mechanism of the reactions of OH radicals and Cl atoms with methyl methacrylate (CH2═C(CH3)C(O)OCH3) in the presence of NOx.

The OH radical and Cl atom initiated photodegradation of methyl methacrylate has been investigated in a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air using in situ FTIR spectroscopy to monitor the reactants and products. The major products observed in the OH reaction were methyl pyruvate (92 ± 16%) together with formaldehyde (87 ± 12%) as a coproduct from the C1-C2 bond cleavage channel of the intermediate 1,2-hydroxyalkoxy radical, formed by the addition of OH to the terminal carbon of the double bond which is designated C1. For the Cl atom reaction, the products identified were chloroacetone (41 ± 6%) together with its coproduct formaldehyde (35 ± 5%) and methyl pyruvate (24 ± 4%) together with its coproduct formylchloride (25 ± 4%). The results show that the fate of the intermediate 1,2-chloroalkoxy radical involves not only cleavage of the C1-C2 bond but also quite substantial cleavage of the C2-C3 bond. The present results are compared with previous studies of acrylates, showing different branching ratios for the OH and Cl addition reactions in the presence of NOx. Atmospheric implications are discussed.