Theoretical study of chlorine nitrates: implications for stratospheric chlorine chemistry

Reported here is a theoretical study of possible stratospheric chlorine reservoir species including isomers of chemical formula ClNO(4) and ClNO(5), in addition to the well-known ClONO(2) reservoir species. Density functional theory (DFT) in conjunction with large one-particle basis sets has been used to determine equilibrium structures, dipole moments, rotational constants, harmonic vibrational frequencies, and infrared intensities. The B3LYP functional was used since it has previously been shown to perform well for similar compounds. The equilibrium geometry and vibrational spectra of ClONO(2) are shown to be in good agreement with the experimental data and also with high-level coupled-cluster calculations reported previously. Three stable isomers have been identified for each ClNO(4) and ClNO(5). The vibrational spectrum of O(2)ClONO(2) has been compared with the available experimental data and found to be in good agreement. The relative energetics of the ClNO(4) and ClNO(5) isomers have been determined using large atomic natural orbital (ANO) basis sets in conjunction with the singles and doubles coupled-cluster method that includes a perturbational correction for triple excitations, denoted CCSD(T). Accurate heats of formation have been evaluated by computing energies for isodesmic reactions involving the ClNO(4) and ClNO(5) isomers. The stability of these molecules with respect to thermal dissociation is examined. The present study suggests that isomers of ClNO(4) and ClNO(5) may have no atmospheric chemical relevance because the atmospheric concentrations of the necessary reactants are insufficient, but it is also found that under laboratory conditions the formation of O(2)ClONO(2) cannot be ignored.

The 3nu1 + nu2 Combination Band of HOCl: Assignments, Perturbations, and Line Intensities

The high-resolution spectra (0.012 cm-1) of the 3nu1 + nu2 combination band of hypochlorous acid HO35(37)Cl in the near infrared ( approximately 11 478 cm-1) have been measured using a titanium:sapphire intracavity laser absorption (ICLA) spectrometer. Line assignments, absolute intensities, and the total band intensity for both isotopomers are reported. In the course of the band analysis two Ka branches (Ka = 2,3) were found to be perturbed via low-order Fermi-type (anharmonic) resonances by a dark perturber which has been identified to be the 2nu1 + 2nu2 + 3nu3 state. The data are compared with intensity predictions from simple empirical models and discussed with regard to detection limits for this molecule in the near infrared spectral region of the atmosphere. Copyright 1997 Academic Press. Copyright 1997Academic Press

The "Ozone Deficit" Problem: O2(X, v ge 26) + O(3P) from 226-nm Ozone Photodissociation

Highly vibrationally excited O(2)(X(3)sigmag(-), v >/= 26) has been observed from the photodissociation of ozone (O(3)), and the quantum yield for this reaction has been determined for excitation at 226 nanometers. This observation may help to address the "ozone deficit" problem, or why the previously predicted stratospheric O(3) concentration is less than that observed. Recent kinetic studies have suggested that O(2)(X(3)sigmag(-), v >/= 26) can react rapidly with O(2) to form O(3) + O and have led to speculation that, if produced in the photodissociation of O(3), this species might be involved in resolving the discrepancy. The sequence O(3) + hv --> O(2)(X(3)sigmag(-), v >/= 26) + O; O(2)(X(3)sigmag(-), v >/= 26) + O(2) --> O(3) + O (where hv is a photon) would be an autocatalytic mechanism for production of odd oxygen. A two-dimensional atmospheric model has been used to evaluate the importance of this new mechanism. The new mechanism can completely account for the tropical O(3) deficit at an altitude of 43 kilometers, but it does not completely account for the deficit at higher altitudes. The mechanism also provides for isotopic fractionation and may contribute to an explanation for the anomalously high concentration of heavy O(3) in the stratosphere.

Enhancement of atmospheric radiation by an aerosol layer

The presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer. Such haze layers result from the injection of aerosol-forming material into the stratosphere by volcanic eruptions. Simple heuristic arguments show that the increase in flux below the haze layer, relative to a clear sky case, is a consequence of "photon trapping." We explore the magnitude of these flux perturbations, as a function of aerosol properties and illumination conditions, with a new radiative transfer model that can accurately compute fluxes in an inhomogenous atmosphere with nonconservative scatterers having arbitrary phase function. One calculated consequence of the El Chichon volcanic eruption is an increase in the midday surface actinic flux at 20 degrees N latitude, summer, by as much as 45% at 2900 angstroms. This increase in flux in the UV-B wavelength range was caused entirely by aerosol scattering, without any reduction in the overhead ozone column.

Isotopic exchange between carbon dioxide and ozone via O(1D) in the stratosphere

We propose a novel mechanism for isotopic exchange between CO2 and O3 via O(1D) + CO2 --> CO3* followed by CO3* --> CO2 + O(3P). A one-dimensional model calculation shows that this mechanism can account for the enrichment in 18O in the stratospheric CO2 observed by Gamo et al. [1989], using the heavy O3 profile observed by Mauersberger [1981]. The implications of this mechanism for other stratospheric species and as a source of isotopically heavy CO2 in the troposphere are briefly discussed.

Instrumental Requirements for Global Atmospheric Chemistry

The field of atmospheric chemistry is data-limited, primarily because of the challenge of measuring the key chemical constituents in the global environment. Several recent advances, however, in rugged, portable, remotesensing, ground-based instrumentation and accurate, fast-response airborne instrumentation have provided powerful tools for the understanding of stratospheric ozone, particularly in polar regions. Current discoveries of the role of heterogeneous chemical processes point to the need for better techniques for characterization of stratospheric aerosols. In the troposphere, advances in in situ, sensitive methods for detecting reactive nitrogen compounds have demonstrated the role that these compounds have in controlling global oxidation processes, but better measurements of the reservoir species by which the long-ranged transport of pollutant-reactive nitrogen compounds is thought to occur are urgently needed. The role of hydrocarbons, particularly those of natural origin, in ozone formation in rural areas has focused attention on the requirement for better speciation of these ubiquitous compounds. Lastly, rigorous instrument intercomparison experiments have provided unbiased estimates of measurement capabilities.

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.

Rate of formation of the ClO dimer in the polar stratosphere: implications for ozone loss

The gas-phase recombination of chlorine monoxide (ClO) has been investigated under the conditions of pressure and temperature that prevail in the Antarctic stratosphere during the period of maximum ozone (O3) disappearance. Measured rate constants are less than one-half as great as the previously accepted values. One-dimensional model calculations based on the new rate data indicate that currently accepted chemical mechanisms can quantitatively account for the observed O3 losses in late spring (17 September to 7 October). A qualitative assessment indicates that the existing mechanisms can only account for at most one-half of the measured O3 depletion in the early spring (28 August to 17 September), indicating that there may be additional catalytic cycles, besides those currently recognized, that destroy O3.

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.

A New Laboratory Source of Ozone and Its Potential Atmospheric Implications

Although 248-nanometer radiation falls 0.12 electron volt short of the energy needed to dissociate O(2) large densities of ozone (O(3)) can be produced from unfocused 248-nanometer KrF excimer laser irradiation of pure O(2). The process is initiated in some undefined manner, possibly through weak two-photon O(2) dissociation, which results in a small amount of O(3) being generated. As soon as any O(3) is present, it strongly absorbs the 248-nanometer radiation and dissociates to vibrationally excited ground state O(2) (among other products), with a quantum yield of 0.1 to 0.15. During the laser pulse, a portion of these molecules absorb a photon and dissociate, which results in the production of three oxygen atoms for one O(3) molecule destroyed. Recombination then converts these atoms to O(3), and thus O(3) production in the system is autocatalytic. A deficiency exists in current models of O(3) photochemistry in the upper stratosphere and mesosphere, in that more O(3) iS found than can be explained. A detailed analysis of the system as it applies to the upper atmosphere is not yet possible, but with reasonable assumptions about O(2) vibrational distributions resulting from O(3) photodissociation and about relaxation rates of vibrationally excited O(2) a case can be made for the importance of incuding this mechanism in the models.