Gas-Phase Reaction between CF2O and CF3C(O)OH: Characterization of CF3C(O)OC(O)F

The thermal decomposition of trifluoroacetic acid and carbonyl fluoride (CF₂O) has been extensively studied because of their importance in the oxidation of hydrochlorofluorocarbons in the atmosphere. We hitherto present the study of the thermal reaction between these two molecules. The reaction mechanism was studied using Fourier transform infrared spectroscopy in the temperature range of 513-573 K. The reaction proceeds homogeneously in the gas phase through the formation of a reaction intermediate, here characterized as CF₃C(O)OC(O)F (detected for the first time in this work), the major final products being CF₃C(O)F, HF, and CO₂. We demonstrate that the reaction is first-order with respect to each reagent, second-order global and the mechanism consists of two steps, the first being the rate-determining one. The E_a = 110.1 ± 6.1 kJ mol^-1 and A = (1.2 ± 0.2) × 10^-12 cm³ molec^-1 s^-1 values were obtained from the experimental data. The low activation energy is explained by the hydrogen-bond interactions between the -OH group of the acid and the F atom of the CF₂O. First-principles calculations at the G4MP2 level of theory were carried out to understand the dynamics of the decomposition. Thermodynamic activation values found for this reaction are as follows: Δ H^⧧ = 105.6 ± 6.4 kJ mol^-1, Δ S^⧧ = -88.6 ± 9.7 J mol^-1 K^-1, and Δ G^⧧ = 153.7 ± 13.5 kJ mol^-1. The comparison between theory and experimental results showed excellent similarities, thus strengthening the proposed mechanism.

Diode laser absorption measurements of CH(3)Cl and CH(4) near 1.65 microm

Two distributed-feedback (InGaAsP) diode lasers were used to record high-resolution absorption spectra of the parallel and the perpendicular components of the 2nu(4) band of methyl chloride (CH(3)Cl) and the 2nu(3) band of methane (CH(4)) near 1.65 mum. The room-temperature absorption measurements, which were conducted in a multipass cell with a variable path length (878-1020 cm), were used to determine the mole fractions of the constituent gases and thus demonstrate species-specific, nonintrusive concentration measurements of species with overlapping spectra.

Infrared measurements of HF and HCl total column abundances above Kitt Peak, 1977-1990: seasonal cycles, long-term increases, and comparisons with model calculations

Series of high-resolution (approximately 0.01 cm-1) solar absorption spectra recorded with the McMath Fourier transform spectrometer on Kitt Peak (altitude 2.09 km, 31.9 degrees N, 111.6 degrees W) have been analyzed to deduce total column amounts of HF on 93 different days and HCl on 35 different days between May 1977 and June 1990. The results are based on the analysis of the HF and H35Cl (1-0) vibration-rotation band R(1) lines which are located at 4038.9625 and 2925.8970 cm-1, respectively. All of the data were analyzed using a multilayer, nonlinear least squares spectral fitting procedure and a consistent set of spectroscopic line parameters. The results indicate a rapid increase in total HF and a more gradual increase in total HCl with both trends superimposed on short-term variability. In addition, the total columns of both gases undergo a seasonal cycle with an early spring maximum and an early fall minimum, with peak-to-peak amplitudes equal to 25% for HF and 13% for HCl. In the case of HF, the changes over the 13 years of measurement are sufficiently large to determine that a better fit is obtained assuming a linear rather than an exponential increase with time. For HCl, linear and exponential models fit the data equally well. Referenced to calendar year 1981.0 and assuming a sinusoidal seasonal cycle superimposed on a linear total column increase with time, HF and HCl increase rates of (10.9 +/- 1.1)% yr-1 and (5.1 +/- 0.7)% yr-1 and total columns of (3.17 +/- 0.11) x 10(14) and (1.92 +/- 0.06) x 10(15) molecules cm-2 (2 sigma) are derived, respectively; the corresponding best fit mean exponential increase rates are equal to (7.6 +/- 0.6)% yr-1 and (4.2 +/- 0.5)% yr-1 (2 sigma). Over the 13-year observing period, the HF and HCl total columns increased by factors of 3.2 and 1.8, respectively. Based on HF and HCl total columns deduced from measurements on the same day, the HF/HCl total columns ratio increased from 0.14 in May 1977 to 0.23 in June 1990. Short-term temporal variations in the HF and HCl total columns are highly correlated; these fluctuations are believed to be caused by dynamical variability in the lower stratosphere. The results of this investigation are compared with previously reported measurements and with time-dependent, two-dimensional model calculations of HF and HCl total columns based on emission histories and photo-oxidation rates for the source molecules.

ATMOS data processing and science analysis methods

The ATMOS (atmospheric trace molecule spectroscopy) instrument, a high speed Fourier transform spectrometer operating in the middle IR (2.2-16 microm), recorded more than 1500 solar spectra at approximately 0.0105-cm(-1) resolution during its first mission onboard the shuttle Challenger in the spring of 1985. These spectra were acquired during high sun conditions for studies of the solar atmosphere and during low sun conditions for studies of the earth's upper atmosphere. This paper describes the steps by which the telemetry data were converted into spectra suitable for analysis, the analysis software and methods developed for the atmospheric and solar studies, and the ATMOS data analysis facility.

El Chichon volcanic aerosols: impact of radiative, thermal, and chemical perturbations

We examine the consequences of the eruption of the El Chichon volcano on the Earth's stratospheric chemistry. Formed after the eruption, the volcanic aerosol cloud, with a peak particle density at 27 km, was very efficient at altering the radiation field. The results of a one-dimensional radiative transfer model show that the total radiation increased by 8% within the aerosol layer longward of 3000 angstroms. At certain altitudes and wavelengths below 3000 angstroms, the total radiation decreased by 15%. Consequently, there are changes in the photolysis rates obtained with a one-dimensional photochemical model: for example, O2 photodissociation rate constants decrease by 10%, while O3 photodissociation rate constants increase by a comparable amount. A combination of this radiation change and the effect of a temperature variation of a few degrees causes the abundance of O3 to decrease by 7% at 24 km, in good agreement with the Solar Backscattered Ultraviolet experiment (SBUV) measurements of a 5-10% decrease. The combined radiative and thermal perturbations on the concentrations of O, O(1 D), OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO3, HO2NO2, Cl, ClO, ClO2, HOCl, ClNO3, and HCl are computed and presented in detail. However, these changes as calculated are insufficient to explain the observations of significant decreases in NO and NO2 and increases in HCl. A heterogeneous reaction catalyzed by aerosol surfaces which transforms ClNO3 into HCl provides a pathway for sequestering NOx, and at the same time reduces ClNO3 in favor of HCL. The inclusion of this reaction in the model leads to a satisfactory single-step explanation of the otherwise puzzling observations of NO, NO2, and HCl. The observed lack of change in HNO3 cannot be explained by this hypothesis. The effects of a number of heterogeneous reactions, some believed to be important for the Antarctic stratosphere, have been assessed with our model. We also examine the hypothesis of direct injection of gases from the volcano into the stratosphere. Only an unrealistically large injection (60% column increase above 12 km) results in an HCl increase in agreement with observations. An equally large water injection decreases HCl, and decreases the NO and NO2 by as much as 20%, but still does not simulate the observed NOx decrease.

Nonlocal thermodynamic equilibrium effects in stratospheric HF by collisional energy transfer from electronically excited O(2) and implications for infrared remote sensing

A possible nonlocal thermodynamic equilibrium (non-LTE) effect involving stratospheric HF arising from the direct photochemical excitation of vibrationally excited HF by collisional energy transfer from electronically excited O(2) [O(2)((1)Delta), O(2)((1)Sigma)] is presented. Although this non-LTE effect is smaller than one associated with the direct solar excitation of both HF(nu= 1) and HF(nu= 2), calculations show that inclusion of the mechanism proposed here into retrieval algorithms is necessary if correct daytime upper stratosphere HF profiles are to be inferred in future infrared thermal emission measurements. Accurate determinations of the state-to-state rate constants of the reactions involved in these excitation processes and of the quenching rates of vibrationally excited HF are needed if IR thermal emission techniques are to be accurately employed in the measurement of stratospheric HF.

Changing Composition of the Global Stratosphere

The current understanding of stratospheric chemistry is reviewed with particular attention to the influence of human activity. Models are in good agreement with measurements for a variety of species in the mid-latitude stratosphere, with the possible exception of ozone (O(3)) at high altitude. Rates calculated for loss of O(3) exceed rates for production by about 40 percent at 40 kilometers, indicating a possible but as yet unidentified source of high-altitude O(3). The rapid loss of O(3) beginning in the mid-1970s at low altitudes over Antarctica in the spring is due primarily to catalytic cycles involving halogen radicals. Reactions on surfaces of polar stratospheric clouds play an important role in regulating the abundance of these radicals. Similar effects could occur in northern polar regions and in cold regions of the tropics. It is argued that the Antarctic phenomenon is likely to persist: prompt drastic reduction in the emission of industrial halocarbons is required if the damage to stratospheric O(3) is to be reversed.