Studies of ClO and BrO reactions important in the polar stratosphere: Kinetics and mechanism of the ClO+BrO and ClO+ClO reactions

The reactions, BrO + ClO yields Br + ClOO (1a) yields Br + OClO (1b) yields BrCl + O2 (1c) and ClO + ClO yields Cl + CiOO (2a) yields Cl + OClO (2b) yields Cl2 + O2 (2c) yields (ClO)2 (2d) have assumed new importance in explaining the unusual springtime depletion of ozone observed in the Antarctic stratosphere. The mechanisms of these reactions involve the formation of metastable intermediates which subsequently decompose through several energetically allowed products providing the motivation to study these reactions using both the discharge flow-mass spectrometric and flash photolysis - ultraviolet absorption techniques. These methods have also been used to explore aspects of the kinetics and spectroscopy of the ClO dimer

Kinetics of the HO_2 + BrO reaction over the temperature range 233–348 K

The reaction BrO + HO_2 → products is the rate-limiting step in a key catalytic ozone destruction cycle in the lower stratosphere. In this study a discharge-flow reactor coupled with molecular beam mass spectrometry has been used to study the BrO + HO_2 reaction over the temperature range 233-348 K. Rate constants were measured under pseudo-first-order conditions in separate experiments with first HO_2 and then BrO in excess in an effort to identify possible complications in the reaction conditions. At 298 K, the rate constant was determined to be (1.73 ± 0.61) x 10^(-11) cm^3 molecule^(-1) s^(-1) with HO_2 in excess and (2.05 ± 0.64) x 10^(-11) cm^3 molecule^(-1) s^(-1) with BrO in excess. The combined results of the temperature-dependent experiments gave the following fit to the Arrhenius expression : k = (3.13 ± 0.33)]10^(-12) exp(536 ± 206/T) where the quoted uncertainties represent two standard deviations. The reaction mechanism is discussed in light of recent ab initio results on the thermochemistry of isomers of possible reaction intermediates

Kinetics and product studies of the BrO + ClO Reaction: Implications for Antarctic chemistry

The reaction of ClO with BrO has been investigated by two independent techniques, discharge flow‐mass spectrometry and flash photolysis‐UV spectrometry, over the temperature range 220‐400 K and the pressure range 1‐760 torr. Rate constants have been determined for three product channels; a) Br + ClOO, b) Br + OClO, and c) BrCl + O_2. The rate constants for the overall reaction and each reaction branch were found to be inversely dependent on temperature and independent of pressure. The results for the temperature dependence of the overall rate constant from the discharge flow and flash photolysis studies are in excellent agreement, and collectively disagree substantially with the only previous temperature dependence study. Also, in contrast to previous studies, the channel forming BrCl is found to be significant (≃ 8%). These kinetic measurements have an important impact on the modeling of Antarctic chemistry; for temperatures found in the Antarctic stratosphere the rate coefficients for the channels yielding ClOO and OClO are a factor of 2‐3 larger than previously estimated. In addition, the BrCl channel, which has an impact on the nighttime partitioning of BrO_X and the diurnal variability of OClO, has been omitted from previous atmospheric models

A kinetics study of the homogeneous and heterogeneous components of the HCl + ClONO_2 reaction

The kinetics of the reaction HCl + ClONO_2 → Cl_2 + HNO_3 were investigated at 298 K using a flow reactor with FTIR analysis to assess the importance of this reaction for stratospheric chemistry. The observed reaction was characteristic of a heterogeneous process; an upper limit of 5 × 10^(−18) cm³ molecule^(−1) s^(−1) was obtained for the homogeneous gas phase rate constant. From calculations of the first order wall rate constant, estimates were made of the reaction rate on stratospheric aerosols. Because both HCl and ClONO_2 need to be adsorbed on the particle surface, the reaction will be of negligible importance under most stratospheric conditions

Production of NO2 from Photolysis of Peroxyacetyl Nitrate

Peroxyacetyl nitrate (PAN) vapor was photolyzed at 248 nm, and the NO2 photoproduct was detected by laser-induced fluorescence. The quantum yield for the production of NO2 from PAN photolysis was determined by comparison to HNO3 photolysis data taken under identical experimental conditions. The average of data collected over a range of total pressures, precursor concentrations, and buffer gases was 0.83 +/- 0.09 for the NO2 quantum yield, where the statistical uncertainty is 2 standard deviations

Atmospheric Effects of Aviation: First Report of the Subsonic Assessment Project

This document is the first report from the Office of Aeronautics Advanced Subsonic Technology (AST) Program's Subsonic Assessment (SASS) Project. This effort, initiated in late 1993, has as its objective the assessment of the atmospheric effects of the current and predicted future aviation fleet. The two areas of impact are ozone (stratospheric and tropospheric) and radiative forcing. These are driven, respectively, by possible perturbations from aircraft emissions of NOX and soot and/or sulfur-containing particles. The report presents the major questions to which project assessments will be directed (Introduction) and the status of six programmatic elements: Emissions Scenarios, Exhaust Characterization, Near-Field Interactions, Kinetics and Laboratory Studies, Global Modeling, and Atmospheric Observations (field studies)

Measurements of quantum yields of bromine atoms in the photolysis of bromoform from 266 to 324 nm

The quantum yield for the formation of bromine atoms in the photolysis of bromoform, CHBr_3, has been measured between 266 and 324 nm. For 303 to 306 nm the quantum yields are unity within the experimental uncertainty of the measurements. At longer wavelengths, where the bromoform cross sections decrease rapidly, an apparent trend to slightly lower quantum yields is probably the result of systematic and random errors or incorrect CHBr_3 absorption cross sections. Support for a unit quantum yield for all wavelengths longer than 300 nm comes from the recent theoretical calculations of Peterson and Francisco. At 266 nm the bromine atom quantum yield is 0.76 (±0.03), indicating that at least one additional dissociation channel becomes important at shorter wavelengths. For modeling of the troposphere, it is recommended that a quantum yield of unity be used for wavelengths of 300 nm and longer

A kinetics study of the homogeneous and heterogeneous components of the HCl + ClONO_2 reaction

The kinetics of the reaction HCl + ClONO_2 → Cl_2 + HNO_3 were investigated at 298 K using a flow reactor with FTIR analysis to assess the importance of this reaction for stratospheric chemistry. The observed reaction was characteristic of a heterogeneous process; an upper limit of 5 × 10^(−18) cm³ molecule^(−1) s^(−1) was obtained for the homogeneous gas phase rate constant. From calculations of the first order wall rate constant, estimates were made of the reaction rate on stratospheric aerosols. Because both HCl and ClONO_2 need to be adsorbed on the particle surface, the reaction will be of negligible importance under most stratospheric conditions

Interaction of peroxynitric acid with solid H_2O ice

The uptake of peroxynitric acid (PNA), HO_2NO_2 or HNO_4, on solid H_2O ice at 193 K (−80°C) was studied using a fast flow‐mass spectrometric technique. An uptake coefficient of 0.15 ± 0.10 was measured, where the quoted uncertainty denotes 2 standard deviations. The uptake process did not result in the production of gas phase products. The composition of the condensed phase was investigated using programmed heating (3 K min^(−1)) of the substrate coupled with mass spectrometric detection of desorbed species. Significant quantities of HNO_4 and HNO_3 desorbed from the substrates at temperatures above 225 K and 246 K, respectively. The desorbed HNO_3, which was less than 9% of the desorbed HNO_4 and remained unchanged upon incubation of the substrate, was likely due to impurities in the HNO_4 samples rather than reaction of HNO_4 on the substrate. The onset temperatures for HNO_4 desorption increased with increasing H_2O to HNO_4 ratios, indicating that HNO_4, like HNO_3, tends to be hydrated in the presence of water. These observations suggest possible mechanisms for removal of HNO_4 or repartitioning of total odd nitrogen species in the Earth's upper troposphere and stratosphere

Measurements of quantum yields of bromine atoms in the photolysis of bromoform from 266 to 324 nm

The quantum yield for the formation of bromine atoms in the photolysis of bromoform, CHBr_3, has been measured between 266 and 324 nm. For 303 to 306 nm the quantum yields are unity within the experimental uncertainty of the measurements. At longer wavelengths, where the bromoform cross sections decrease rapidly, an apparent trend to slightly lower quantum yields is probably the result of systematic and random errors or incorrect CHBr_3 absorption cross sections. Support for a unit quantum yield for all wavelengths longer than 300 nm comes from the recent theoretical calculations of Peterson and Francisco. At 266 nm the bromine atom quantum yield is 0.76 (±0.03), indicating that at least one additional dissociation channel becomes important at shorter wavelengths. For modeling of the troposphere, it is recommended that a quantum yield of unity be used for wavelengths of 300 nm and longer