Absolute Rate Method Research Articles

Reaction of F Atoms with Carbonyl Sulfide: Experimental and Theoretical Study.

The gas phase reaction of fluorine atoms with carbonyl sulfide, F + OCS → SF + CO (1), has been investigated over a wide temperature range, T = 220-960 K, in a discharge-flow system combined with mass spectrometry for the analysis of the reactive mixture. The reaction rate coefficient was determined as a function of temperature using both absolute (under pseudo-first-order conditions using an excess of F atoms) and relative rate methods: k1 = (8.95 ± 0.52) × 10-11 exp((102 ± 20)/T) cm3 molecule-1 s-1 (with an estimated total uncertainty of 15% independent of temperature). The SF radical was observed as the main primary reaction product. The experimental results are supported by theoretical calculations, which evidence that the reaction is exothermic with a weak negative temperature dependence.

Kinetic and Mechanistic Study of the Reactions of NO3 Radicals with Unsaturated Aldehydes: 2-Butenal, 2-Methyl-2-butenal, and 3-Methyl-2-butenal.

The kinetics and mechanisms of the gas-phase reactions of NO3 radical with two branched unsaturated aldehydes, 2-methyl-2-butenal (also called 2-methyl-crotonaldehyde) and 3-methyl-2-butenal (or 3-methyl-crotonaldehyde), have been investigated by experimental and theoretical approaches. Kinetic data were also provided, for comparison, for 2-butenal (or crotonaldehyde). Experiments were performed in a simulation chamber at 295 ± 3 K and atmospheric pressure. Rate constants were determined using both absolute and relative rate methods. Experimental results were found to be in good agreement leading to the following rate constants (in cm3 molecule-1 s-1): k(2-butenal + NO3) = (4.6 ± 1.3) × 10-15; k(2-methyl-2-butenal + NO3) = (14.0 ± 2.8) × 10-15; and k(3-methyl-2-butenal + NO3) = (19.1 ± 4.1) × 10-15. Theoretical calculations were also performed using the DFT-BH&HLYP/6-311++G(d,p) method and lead to rate constants in agreement with experiments and allow us to explore mechanisms for abstraction and addition pathways. Impact on atmospheric chemistry is discussed.

Reaction Rate Coefficient of OH Radicals with d 9-Butanol as a Function of Temperature.

d9-Butanol or 1-butan-d9-ol (D9B) is often used as an OH radical tracer in atmospheric chemistry studies to determine OH exposure, a useful universal metric that describes the extent of OH radical oxidation chemistry. Despite its frequent application, there is only one study that reports the rate coefficient of D9B with OH radicals, k1(295 K), which limits its usefulness as an OH tracer for studying processes at temperatures lower or higher than room temperature. In this study, two complementary experimental techniques were used to measure the rate coefficient of D9B with OH radicals, k1(T), at temperatures between 240 and 750 K and at pressures within 2–760 Torr. A thermally regulated atmospheric simulation chamber was used to determine k1(T) in the temperature range of 263–353 K and at atmospheric pressure using the relative rate method. A low-pressure (2–10 Torr) discharge flow tube reactor coupled with a mass spectrometer was used to measure k1(T) at temperatures within 240–750 K, using both the absolute and relative rate methods. The agreement between the two experimental aproaches followed in this study was very good, within 6%, in the overlapping temperature range, and k1(295 ± 3 K) was 3.42 ± 0.26 × 10–12 cm3 molecule–1 s–1, where the quoted error is the overall uncertainty of the measurements. The temperature dependence of the rate coefficient is well described by the modified Arrhenius expression, k1 = (1.57 ± 0.88) × 10–14 × (T/293)4.60±0.4 × exp(1606 ± 164/T) cm3 molecule–1 s–1 in the range of 240–750 K, where the quoted error represents the 2σ standard deviation of the fit. The results of the current study enable an accurate estimation of OH exposure in atmospheric simulation experiments and expand the applicability of D9B as an OH radical tracer at temperatures other than room temperature.

Rate constants for reactions of OH radicals with (Z)-CF3CCl=CHCl, CHF2CF=CF2, (E)-CF3CH=CHF, (Z)-CF3CH=CHF, CH3CF=CH2, and CH2FCH=CH2

The rate constants for the reaction of OH radicals with six halogenated propenes—(Z)-CF3CCl = CHCl (k1), CHF2CF=CF2 (k2), (E)-CF3CH=CHF (k3), (Z)-CF3CH=CHF (k4), CH3CF=CH2 (k5), and CH2FCH=CH2 (k6)—were measured over 250–430 K using the absolute rate method. The Arrhenius expressions for the rate constants were determined as k1 = (9.30 ± 0.54) × 10−13 exp [−(180 ± 20)/T], k2 = (1.43 ± 0.04) × 10−12 exp [(490 ± 10)/T], k3 = (4.86 ± 0.13) × 10−13 (T/298)0.727 exp [(110 ± 10)/T], k4 = (4.55 ± 0.12) × 10−13 (T/298)0.367 exp [(350 ± 10)/T], k5 = (3.22 ± 0.20) × 10−12 exp [(490 ± 20)/T], and k6 = (2.87 ± 0.19) × 10−12 exp [(440 ± 20)/T] cm3 molecule−1 s−1. Infrared absorption spectra of the halogenated propenes were measured at room temperature. The atmospheric lifetimes, global warming potentials of the halogenated propenes, and ozone depletion potential of (Z)-CF3CCl = CHCl were estimated. The correlation between the reactivity and structure of 44 halogenated alkenes, including the six halogenated propenes, was investigated.

Impact of Scene Content on High Resolution Video Quality.

This paper deals with the impact of content on the perceived video quality evaluated using the subjective Absolute Category Rating (ACR) method. The assessment was conducted on eight types of video sequences with diverse content obtained from the SJTU dataset. The sequences were encoded at 5 different constant bitrates in two widely video compression standards H.264/AVC and H.265/HEVC at Full HD and Ultra HD resolutions, which means 160 annotated video sequences were created. The length of Group of Pictures (GOP) was set to half the framerate value, as is typical for video intended for transmission over a noisy communication channel. The evaluation was performed in two laboratories: one situated at the University of Zilina, and the second at the VSB—Technical University in Ostrava. The results acquired in both laboratories reached/showed a high correlation. Notwithstanding the fact that the sequences with low Spatial Information (SI) and Temporal Information (TI) values reached better Mean Opinion Score (MOS) score than the sequences with higher SI and TI values, these two parameters are not sufficient for scene description, and this domain should be the subject of further research. The evaluation results led us to the conclusion that it is unnecessary to use the H.265/HEVC codec for compression of Full HD sequences and the compression efficiency of the H.265 codec by the Ultra HD resolution reaches the compression efficiency of both codecs by the Full HD resolution. This paper also includes the recommendations for minimum bitrate thresholds at which the video sequences at both resolutions retain good and fair subjectively perceived quality.

The gas-phase reaction kinetics of different structure of unsaturated alcohols and ketones with O3

Unsaturated alcohols and ketones are a category of oxygenated biogenic volatile organic compounds, which reacted with tropospheric ozone and then made a significant contribution to the formation of secondary organic aerosols (SOA). However, the chemical pathways of their reaction with O3 are poorly understood. Hence, the reactions of O3 with three unsaturated alcohols and ketones were conducted in 100 L FEP Teflon smog chamber and the rate constants were obtained by using the absolute rate method. The rate constant of 4-methyl-3-penten-2-one, 2-methyl-4-penten-2-ol, 4-penten-2-ol reacting with O3 at 298 K and 760 Torr was (7.68 ± 0.80) × 10−18, (4.27 ± 0.44) × 10−18 and (5.40 ± 0.57) × 10−18 cm3molecule−1s−1, respectively. Combining with theoretical calculation, the factors leading to the different rate constants of a series of similar structure compounds were analyzed, including the steric effect of methyl in specific position and the electron withdrawing effect of hydroxyl group in the gamma position for alcohols and carbonyl group in the beta position for ketones. Moreover, the major products of three unsaturated organics reacted with ozone were detected, and the reaction mechanisms were deduced by further supporting of theoretical calculation. The atmospheric lifetime of the target compounds and the environmental significant were explored.

Atmospheric chemistry of ketones: Reaction of OH radicals with 2-methyl-3-pentanone, 3-methyl-2-pentanone and 4-methyl-2-pentanone.

This work reports new kinetic and mechanistic information on the atmospheric chemistry of ketones. Both absolute and relative rate methods were used to determine the rate constants for OH reactions with 2-methyl-3-pentanone (2M3P), 3-methyl-2-pentanone (3M2P) and 4-methyl-2-pentanone (4M2P), three widely used compounds in the industry. This work constitutes the first temperature dependence study of the reactions of OH with 2M3P and 3M2P. The following rate constants values are recommended at 298 K (in 10-12 cm3 molecule-1 s-1): kOH+2M3P = 3.49 ± 0.5; kOH+3M2P = 6.02 ± 0.14 and kOH+4M2P = 11.02 ± 0.42. The following Arrhenius expressions (in units of cm3 molecule-1 s-1) adequately describe the measured rate constants for OH reactions with 2M3P and 3M2P in the temperature range 263-373 K: k2M3P = (2.33 ± 0.06) × 10-12 exp((127.4 ± 18.6)/T) and k3M2P = (1.05 ± 0.14) × 10-12 exp((537 ± 41)/T). Products studies from the reactions of OH with the investigated ketones were conducted in a 7.3 m3 simulation chamber using PTR-ToF-MS, UHPLC-MS and GC-MS. A series of short chain carbonyl compounds including formaldehyde, acetone, acetaldehyde, 2-butanone and 2-methypropanal were observed as products. Combining the yields of carbonyls measured with those estimated from the SAR method, we propose various mechanistic degradation schemes of the investigated ketones initiated by reaction with OH radicals.

Quantitative Analysis Methods of Chloride Deposition on Silver for Atmospheric Corrosion Monitoring in South Korea

The chloride deposition rate is one of critical factors affecting the atmospheric corrosion of metals, which can be monitored by exposing Ag specimens and analyzing their surfaces using various ways. Although a coulometric reduction method can provide an absolute chloride deposition rate, new and alternative methods can be pursued, i.e., methods using typical surface analysis tools (scanning electron microscopy-energy dispersive x-ray spectroscopy [SEM-EDS], x-ray diffraction [XRD], and x-ray photoelectron spectroscopy [XPS]). As the surface analysis tools can only provide relative values, a correlation between the coulometric reduction and surface analyses should be developed. To develop the correlation between them, Ag specimens were prepared by exposing them to the atmosphere at 12 locations in South Korea for one year. Surface analysis of retrieved Ag specimens yielded correlations for the chloride deposition rates between the coulometric reduction method and typical surface analysis tools. Each correlation showed a fairly linear relationship, especially ones obtained from SEM-EDS and XPS. The developed correlations can be used when estimating the chloride deposition rate by using surface analysis tools as an alternative to the coulometric reduction method.

Kinetic studies of the gas phase reaction of 1,2-propylene oxide with the OH radical over a temperature range of 261–335 K

Abstract The rate coefficient for the gas phase reaction of the OH radical with 1,2-propylene oxide (PPO) has been measured at 298 K using a relative rate method, employing Gas Chromatography (GC) technique, using ethane and 1,2-dichloroethane as reference compounds. Temperature dependence of the above reaction is also studied using an absolute rate method, employing Laser Photolysis-Laser Induced Fluorescence (LP-LIF) technique, over a range of 261–335 K. The measured bimolecular rate coefficients were fitted into the Arrhenius plot, which yielded a straight line roughly parallel to the x-axis and implies no or very weak temperature dependence. Within the experimental error limits, rate coefficients obtained at various temperatures, are found to vary within a narrow range from (4.65 ± 0.24) × 10−13 to (4.80 ± 0.24) × 10−13 cm3 molecule−1 s−1. Room temperature rate coefficient of the reaction of PPO with the OH radical was found to be (4.65 ± 0.24) × 10−13 cm3 molecule−1 s−1, using LP-LIF technique. These results have been discussed with available literature to unveil the probable mechanism. An average tropospheric lifetime of PPO has been estimated to be about two weeks for degradation by the OH radical. In addition, atmospheric parameters such as Radiative Efficiency (RE) and Global Warming Potential (GWP) for PPO are determined. Stable products formed are characterized using GC-MS and FT-IR techniques. Theoretical calculations are performed to understand the energetics and mechanism of H atom abstraction by OH, and also to complement the experimental observations.

Kinetic and mechanism studies of the ozonolysis of three unsaturated ketones

Reaction rate constants and products of 1-octen-3-one, 3-octen-2-one and 4-hexen-3-one with ozone were studied in a 100-L fluorinated ethylene propylene (FEP) Teflon film bag using absolute rate method at 298 ± 1 K and atmospheric pressure. The rate constants were (1.09 ± 0.12) × 10−17, (3.48 ± 0.36) × 10−17 and (5.70 ± 0.60) × 10−17 cm3/(molecule⋅sec), respectively. According to the obtained rate constants, the effects of carbonyl were discussed. The carbonyl group in β position has a net withdrawing effect with respect to an olefinic bond, then causing the decline of rate constants. The quantum chemical calculation was used to explain the results of rate constants. The products of ozonolysis were mainly aldehydes, which have significant influence on the formation of SOA, and hence play an important role in the atmosphere. In this work, we detected the main products of reaction and proposed the reaction mechanism by combining the results of quantum chemical calculations. Atmospheric lifetime for three unsaturated ketones reacted with ozone was 36.4, 11.4 and 6.9 hr for 1-octen-3-one, 3-octen-2-one and 4-hexen-3-one, respectively.

Temperature-Dependent Kinetic Study of the Reaction of Hydroxyl Radicals with Hydroxyacetone

The kinetics of the reaction of OH radicals with hydroxyacetone has been investigated as a function of temperature at (2.0 - 2.1) Torr total pressure of helium and over an extended temperature range, T = 250 - 830 K, and as a function of pressure at T = 301 K in the pressure range 1.0-10.4 Torr. The rate constant of the reaction OH + CH3C(O)CH2OH  products (1) was measured using both absolute (from the kinetics of OH consumption in excess of hydroxyacetone) and relative rate method, k1 = 4.7×10-22×T3.25exp(1410/T) cm3molecule-1s-1 at T = 250-830 K. The present data combined with selected previous temperature dependent studies of reaction (1) yield k1 = 4.4×10-20×T2.63exp(1110/T) cm3molecule-1s-1, which is recommended from the present work at T = 230-830K (with conservative uncertainty of 20% at all temperatures). k1 was found to be pressure independent in the pressure range from 1.0 to 10.4 Torr of He at T = 301 K. The present results are compared with previous experimental and theoretical data.

Study of Subjective Data Integrity for Image Quality Data Sets with Consumer Camera Content.

We need data sets of images and subjective scores to develop robust no reference (or blind) visual quality metrics for consumer applications. These applications have many uncontrolled variables because the camera creates the original media and the impairment simultaneously. We do not fully understand how this impacts the integrity of our subjective data. We put forward two new data sets of images from consumer cameras. The first data set, CCRIQ2, uses a strict experiment design, more suitable for camera performance evaluation. The second data set, VIME1, uses a loose experiment design that resembles the behavior of consumer photographers. We gather subjective scores through a subjective experiment with 24 participants using the Absolute Category Rating method. We make these two new data sets available royalty-free on the Consumer Digital Video Library. We also present their integrity analysis (proposing one new approach) and explore the possibility of combining CCRIQ2 with its legacy counterpart. We conclude that the loose experiment design yields unreliable data, despite adhering to international recommendations. This suggests that the classical subjective study design may not be suitable for studies using consumer content. Finally, we show that Hoßfeld–Schatz–Egger failed to detect important differences between the two data sets.

Temperature-Dependent Rate Constant for the Reaction of Hydroxyl Radical with 3-Hydroxy-3-methyl-2-butanone.

Reactions of hydroxyketones with OH radicals are of importance in atmospheric chemistry and represent a theoretical interest because they proceed through two reaction pathways, formation of a hydrogen-bonded prereactive complex and direct H-atom abstraction. In this work, the kinetics of the reaction of OH radicals with 3-hydroxy-3-methyl-2-butanone (3H3M2B) has been investigated at 2 Torr total pressure of helium over a wide temperature range, T = 278-830 K, using a discharge flow reactor combined with an electron impact ionization quadrupole mass spectrometer. The rate constant of the reaction OH + 3H3M2B → products (1) was determined using both a relative rate method and absolute measurements under pseudo-first-order conditions, monitoring the kinetics of OH consumption in excess of 3H3M2B, k1= 5.44 × 10-41T9.7exp (2820/T) and 1.23 × 10-11 exp (-970/T) cm3 molecule-1 s-1 at T = 278-400 and 400-830 K, respectively (with a total uncertainty of 20% at all temperatures). The rate constant of the reaction OH + Br2 → HOBr + Br (2) was measured as a part of this study using both absolute and relative rate methods: k2 = 2.16 × 10-11 exp (207/T) cm3 molecule-1 s-1 at T = 220-950 K (with conservative 10% uncertainty). The kinetic data from the present study are discussed in comparison with previous measurements and theoretical calculations.

Atmospheric Chemistry of 2-Methoxypropene and 2-Ethoxypropene: Kinetics and Mechanism Study of Reactions with Ozone

Rate coefficients at ambient temperature and atmospheric pressure for the reaction of ozone with 2-methoxypropene (2-MPE) and 2-ethoxypropene (2-EPE) were determined in an evacuable 100 L Teflon reaction chamber using absolute and relative rate methods. The product experiments were carried out using a 50 L Teflon reaction chamber in conjunction with FTIR as the detection technique. The rate coefficients (k in units of cm3 molecule−1 s−1) obtained are 1.18 ± 0.13 × 10−17 and 1.89 ± 0.23 × 10−17 for reactions with 2-MPE and 2-EPE, respectively. The effects of the alkoxy group on the gas-phase reactivity of alkyl vinyl ethers toward ozone are compared and discussed. The major ozonolysis products are methyl acetate, formaldehyde and CO2 for 2-MPE, and ethyl acetate, formaldehyde and CO2 for 2-EPE. Possible mechanisms for the two vinyl ethers are proposed based on the observed reaction products. Additionally, atmospheric lifetimes of 32 h and 21 h for 2-MPE and 2-EPE were estimated based on the measured rate constants and the ambient tropospheric concentration of ozone, respectively. The obtained values of the lifetimes indicate that the reaction with ozone is an important loss process for these vinyl ethers in the atmosphere, especially in polluted areas.

Kinetics of atmospheric reactions of 4-chloro-1-butene.

Chloroalkenes are among the important anthropogenic organic compounds emitted in the atmosphere as a result of their wide use in synthetic processes in industry. Despite their well-known adverse effects on human health and air quality, the chemistry of these chloroalkenes remains poorly explored. In this work, reactions of 4-chloro-1-butene (CBE), a representative example of chloroalkenes, with O3, OH, NO3, and Cl are investigated in a 100-L Teflon reaction chamber equipped with gas chromatography-flame ionization detector (GC-FID). The absolute rate method was used for the reaction with O3 while the relative rate method was used for reactions with OH, NO3, and Cl. The following rate constants were obtained at room temperature (298 ± 2)K and atmospheric pressure: (3.96 ± 0.43) × 10-18, (2.63 ± 0.96) × 10-11, (4.48 ± 1.23) × 10-15, and (2.35 ± 0.90) × 10-10cm3 molecule-1s-1, for reactions with O3, OH, NO3, and Cl, respectively. Atmospheric lifetimes of CBE calculated from rate constants of the different reactions obtained in this work showed that reaction with OH is the main loss process for CBE, while in coastal areas and in the marine boundary layer, the CBE loss by Cl reaction becomes important. Estimation of the value of the photochemical ozone creation potential (POCP) indicated that CBE has a large ozone formation potential. The present work underlines the need for further studies on the atmospheric chemistry of chlorinated VOCs.

Kinetic study of the gas-phase reaction of O3 with three unsaturated alcohols

Rate constants for the reactions of ozone with 1-octen-3-ol, 1-nonen-3-ol and 1-nonen-4-ol have been determined at 298±1K and atmospheric pressure for the first time. The experiments were performed in a 100-L FEP Teflon film bag using absolute rate method; the rate constants were (1.91±0.19)×10−17, (1.89±0.20)×10−17, and (0.83±0.08)×10−17cm3/(molecule·sec) for 1-octen-3-ol, 1-nonen-3-ol, and 1-nonen-4-ol, respectively. The rate constants have been compared with those of unsaturated alcohols structural homologs, and used to estimate the reaction reactivity. The electronegativity of carbon–carbon double bond was calculated by atomic charges analysis. The calculated results show that the electronic effect of the lone pair electrons of hydroxyl oxygen is the main cause of the difference in rate coefficient. According to the obtained rate constants, the atmospheric lifetimes of studied unsaturated alcohols were also estimated, which indicates that the reaction with ozone is an important loss pathway in the atmosphere, especially in polluted areas.

Gas-Phase Oxidation of Allyl Acetate by O3, OH, Cl, and NO3: Reaction Kinetics and Mechanism.

Allyl acetate (AA) is widely used as monomer and intermediate in industrial chemicals synthesis. To evaluate the atmospheric outcome of AA, kinetics and mechanism of its gas-phase reaction with main atmospheric oxidants (O3, OH, Cl, and NO3) have been investigated in a Teflon reactor at 298 ± 3 K. Both absolute and relative rate methods were used to determine the rate constants for AA reactions with the four atmospheric oxidants. The obtained rate constants (in units of cm3 molecule-1 s-1) are (1.8 ± 0.3) × 10-18, (3.1 ± 0.7) × 10-11, (2.5 ± 0.5) × 10-10, and (1.1 ± 0.4) × 10-14, for reactions with O3, OH, Cl, and NO3, respectively. While results for reactions with O3, OH and Cl are in good agreement with previous studies, the kinetics for the reaction with NO3 is reported for the first time in this study. On the basis of determined rate constants, the tropospheric lifetimes of AA are τO3 = 9 days, τOH = 5 h, τCl = 5 days, τNO3 = 2 days. On the basis of the products study, reaction mechanisms for these oxidations have been proposed and the reaction products were detected using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) and Fourier transform infrared spectroscopy (FTIR). Results show that the main products formed in these reactions are carbonyl compounds. In particular, 2-oxoethyl acetate was detected in all four AA oxidation reactions. Compared to previous studies, several new products were determined for reactions with OH and Cl. These results form a set of comprehensive kinetic data for AA reactions with main atmospheric oxidants and provide a better understanding of the degradation and atmospheric outcome of unsaturated acetate esters in the troposphere, during both daytime and nighttime.

Gas-phase kinetic and mechanism study of the reactions of O3, OH, Cl and NO3 with unsaturated acetates

Environmental contextAcetate esters, emitted into the atmosphere from both natural and anthropogenic sources, can participate in photochemical processes leading to the formation of secondary organic aerosols. We report a study on the kinetics and mechanisms of the reaction of two acetate esters with O3, OH, NO3, and Cl radicals. The results help our understanding of the chemical behaviour of this important class of compounds in the atmosphere. AbstractThe photodegradation reactions of isopropenyl acetate (IPA) and 2-methyl-2-propenyl acetate (MPA) initiated by O3, OH, Cl and NO3 radicals have been investigated in a 100 L Teflon reactor at 293 ± 3 K, by using gas chromatography with flame-ionisation detection as well as thermal desorption–gas chromatography–mass spectrometry to monitor the reactants and the products. The rate constants for the reactions of IPA and MPA with the four atmospheric oxidants were determined by using either absolute or relative rate methods. The following rate constants (in units of cm3 molecule−1 s−1) were obtained: k(O3 + IPA) = (0.37 ± 0.06) × 10−18, k(OH + IPA) = (6.44 ± 0.74) × 10−11, k(Cl+ IPA) = (4.33 ± 0.52) × 10−10, k(NO3 + IPA) = (1.62 ± 0.22) × 10−14, k(O3 + MPA) = (2.76 ± 0.40) × 10−18, k(OH + MPA) = (7.41 ± 0.92) × 10−11, k(Cl + MPA) = (3.33 ± 0.39) × 10−10, k(NO3 + MPA) = (1.34 ± 0.23) × 10−14. With the exception of the kinetic study of the reactions of O3 and OH with IPA and the mechanistic study of the reaction of IPA with O3, the current research reports the first kinetic and mechanistic investigation for these reactions at atmospheric pressure. Acetic anhydride and 1-acetoxyacetone are the main products of the reactions of IPA and MPA, respectively. On the basis of the products and estimated tropospheric lifetimes of the two esters, reaction mechanisms are proposed and the atmospheric implications are discussed.

Kinetics and Products of the Reactions of Fluorine Atoms with ClNO and Br2 from 295 to 950 K.

The kinetics and products of the reactions of F atoms with Br2 and ClNO have been studied in a flow reactor coupled with an electron impact ionization mass spectrometer at nearly 2 Torr total pressure of helium and over a wide temperature range, T = 295-950 K. The rate constant of the reaction F + ClNO → products (1) was determined under pseudo-first order conditions, monitoring the kinetics of F atom consumption in excess of ClNO. The measured temperature independent rate constant, k1 = (1.29 ± 0.13) × 10-10 cm3 molecule-1 s-1 (T = 299-950 K), was found to be in excellent agreement with the only previous low temperature study which allowed to recommend the value of k1 in an extended temperature range, 228-950 K. FCl and Cl atoms were observed as the reactions products (corresponding to two reaction pathways: Cl-atom abstraction and replacement with fluorine atom, respectively) with the independent of temperature, in the range 295-948 K, yields of 0.68 ± 0.10 and 0.32 ± 0.05, respectively. Rate constant of the reaction F + Br2 (2), k2 = (1.28 ± 0.20) × 10-10 cm3 molecule-1 s-1, determined using both absolute and relative rate methods, was found to be independent of temperature at T = 299-940 K.

Kinetics of the Reactions of NO3 Radical with Methacrylate Esters.

Two different experimental methods (relative rate and absolute rate methods) were used to measure the rate coefficients for the reactions of NO3 radical with six methacrylate esters: methyl methacrylate (MMA, k1), ethyl methacrylate (EMA, k2), propyl methacrylate (PMA, k3), isopropyl methacrylate (IPMA, k4), butyl methacrylate (BMA, k5), and isobutyl methacrylate (IBMA, k6). In the relative rate method, the loss of the esters relative to that of a reference compound was followed in a 7300 L Teflon-walled chamber at 298 ± 2 K and 1000 ± 5 hpa. In the absolute method, the temporal profiles of NO3 and N2O5 were followed by using a dual channel cavity ring-down spectrometer in the presence of an excess of ester in the 7300 L chamber. The rate coefficients from these two methods (weighted averages) in the units of 10-15 cm3 molecule-1 s-1 at 298 K are k1 = 2.98 ± 0.35, k2 = 4.67 ± 0.49, k3 = 5.23 ± 0.60, k4 = 7.91 ± 1.00, k5 = 5.91 ± 0.58, and k6 = 6.24 ± 0.66. The quoted uncertainties are at the 2σ level and include estimated systematic errors. Unweighted averages are also reported. In addition, the rate coefficient k7 for the reaction of NO3 radical with deuterated methyl methacrylate (MMA-d8) was measured by using the relative rate method to be essentially the same as k1. The trends in the measured rate coefficient with the length and nature of the alkyl group, along with the equivalence of k1 and k7, strongly suggest that the reaction of NO3 with the methacrylate esters proceeds via addition to the double bond on the methacrylate group. The present results are compared with those from previous studies. Using the measured values of the rate coefficients, along with those for reactions of these esters with OH, O3, and chlorine atoms, we calculated the atmospheric lifetimes of methacrylate esters. We suggest that NO3 radicals do contribute to the atmospheric loss of these unsaturated esters, but to a lesser extent than their reactions with OH and O3.