ClO Reaction Research Articles

A test of the partitioning between ClO and ClONO2 using simultaneous UARS measurements of ClO, NO2, and ClONO2

The photochemical theory of the partitioning hetween the chlorine species ClONO2 and ClO in the lower and mid stratosphere is tested for the first time using simultaneous measurements of ClO, NO2, and ClONO2. The theory suggests that over most of the day the production of ClONO2 through the three‐body reaction of ClO and NO2 is balanced by the loss of ClONO2 through photolysis. Our analysis tests this theory by utilizing simultaneous measurements of these species from instruments aboard the Upper Atmosphere Research Satellite (UARS), as well as calculations of the photolysis rate of ClONO2 and laboratory measurements of the rate constant for the reaction between ClO and NO2. Between 32 and 10 hPa (∼24 and 32 km), averaged instantaneous production and loss rates of ClONO2 agree within ∼10%. At 46 hPa (∼21 km), the agreement between averaged instantaneous production and loss of ClONO2 is poorer than at higher altitudes, but still within uncertainties. This analysis provides no evidence for any problems in our understanding of the partitioning between the chlorine species ClONO2 and ClO over the range of pressures (46 to 10 hPa) and latitudes (60°S to 60°N) considered here.

Simultaneous measurements of stratospheric HOx, NOx, and Clx: Comparison with a photochemical model

We report simultaneous measurements of the stratospheric concentration profiles of OH, HO2, H2O2, H2O, O3, HNO3, NO2, N2O, HCl, HOCl, and ClO during a midlatitude balloon flight in 1989. Measurements were made over almost an entire diurnal cycle by the Smithsonian Astrophysical Observatory Far‐Infrared Spectrometer (FIRS‐2) and the Jet Propulsion Laboratory Balloon Microwave Limb Sounder (BMLS). We analyze these measurements using a photochemical model constrained by observations of long‐lived gases. Measured HOx species (OH and HO2) and H2O2 show fair agreement with theory throughout the diurnal cycle. Measurements of HNO3 are higher than theory near the concentration peak, while the levels of NO2 are consistent with the model at most altitudes. Measurements of ClO and HOCl are less than predicted concentrations, suggesting a source of HCl in addition to the reaction of Cl with CH4. Possibilities for such a source include a minor HCl + O2 product channel for the reaction of ClO with OH and a minor HCl + O3 channel for the reaction of ClO with HO2.

Validation of CLAES ClONO2 measurements

The cryogenic limb array etalon spectrometer (CLAES) aboard the Upper Atmosphere Research Satellite has made extensive measurements of thermal infrared radiation from the Earth's limb from which vertical concentration profiles of several stratospheric gases and multiwavelength aerosol absorption coefficients have been retrieved for the period from January 9, 1992, to May 5, 1993. This work examines stratospheric ClONO2 concentrations from the current calibration and retrieval software which are designated version 7 data. These data provide the first near‐global view of this stratospheric species. This work evaluates data quality through (1) an analysis of estimated uncertainties and biases in the remote sensing process, (2) comparison with calculations using a two‐dimensional chemical model, (3) comparison with correlative data, and (4) an examination of various known limitations. The precision of CLAES ClONO2 volume mixing ratio retrievals are within 15% in the range (10 < P < 50 mbar). The upper limit on estimated systematic error is 28% in the range (10 < P < 100 mbar) based on studies of error sources in midlatitude retrievals. The global distribution of ClONO2 computed with the Lawrence Livermore National Laboratory two‐dimensional stratospheric chemistry model and the CLAES measurements agree qualitatively. However, above the profile peak the calculated concentration frequently exceeds the measurement. CLAES and ATMOS measurements show relatively good midlatitude agreement, suggesting that the major source of discrepancy is in the model. A possible explanation in terms of a missing reaction ClO + OH → HCl + O2 is suggested. Also, the ClONO2 diurnal cycle constructed from more than 30 days of CLAES data agrees well with the model. The CLAES ClONO2 data diifer from correlative data acquired on flights of the shuttle‐based ATMOS and balloon‐borne instruments by less than 25% on the average in the 10 < P < 50 mbar range. At altitudes above 10 mbar the CLAES measurement is biased low with respect to correlative measurements. This discrepancy at high altitudes is consistent with the analysis showing a large increase of systematic errors above 10 mbar. Heavy tropical volcanic aerosol from the Mount Pinatubo eruption in June 1991 apparently interfered with ClONO2 retrievals in the period before July 1992, causing anomalous peaks in the 20 < P < 30 mbar region accompanied by very small concentrations below the peak (P > 30 mbar). A similar effect associated with thick polar stratospheric clouds is identified. Overall, this validation study indicates that the majority of these data are of good quality and should be very useful in quantitative and qualitative chemical studies of the stratosphere.

Factors Affecting Competitive Ion−Molecule Reactions: ClO- + C2H5Cl and C2D5Cl via E2 and SN2 Channels

Dual-level generalized transition state theory and statistical calculations based on correlated electronic structure calculations with augmented correlated basis sets are used to predict rate constants and deuterium kinetic isotope effects for the competing SN2 and E2 reactions of ClO- with C2H5Cl. The E2 reaction is favored by zero point effects despite its higher barrier. Furthermore, the entropic contribution of low-frequency transition state modes promotes the E2 reaction by about an order of magnitude at 300 K, despite nearly equal combined electronic/zero point propensities for the two reactions. The kinetic isotope effect for the E2 reaction is predicted to be 8.3 at 100 K and 3.1 at room temperature. The kinetic isotope effect of the SN2 reaction is predicted to be surprisingly low at low temperatures (0.06 at 100 K) and 0.60 at room temperature. The activation energy for both reactions is predicted to be negative below about 900 K and positive above 1000 K. Variational effects on the location of ...

Stratospheric trace constituents simulated by a three‐dimensional general circulation model: Comparison with UARS data

Constituent distributions are presented from the NASA Langley three‐dimensional general circulation model, incorporating a comprehensive chemistry scheme. A 7‐year, gas phase model simulation was performed to investigate long‐term model stability. In addition, a 1‐year simulation was made using parameterized polar heterogeneous processes and reactions occurring on sulfate aerosols. The results of these simulations are compared with species climatologies and with satellite data sets in order to characterize and evaluate model performance and identify aspects of the chemical scheme requiring improvement. The agreement between the modeled seasonal variation of total ozone and the measurement climatologies is satisfactory but with some differences with respect to the depth and persistence of the southern springtime ozone depletion. Comparisons of the model simulation with observations made from UARS were performed. There is good accord between the microwave limb sounder observations of ozone and the model. Areas of agreement and disagreement are revealed between the model and the cryogenic array etalon spectrometer measurements of HNO3 and ClONO2, suggesting the need for a more detailed representation of sulfate aerosol processes in the model. The comparison between the modeled and the measured partitioning of odd chlorine species is improved in the upper stratosphere by the inclusion of an additional pathway to HCl from the reaction of ClO + OH.

Efficiency of formation of CH3O in the reaction of CH3O2 with ClO

A discharge‐flow apparatus was used to determine the branching ratio α for the channel of the reaction of ClO with CH3O2 that leads to the formation of CH3O. The experiments were performed at 2 Torr pressure and at room temperature. ClO was the excess reactant and was measured mass spectrometrically; CH3O was detected by laser‐induced fluorescence. The value of α was shown to depend on the fraction β of the reaction of CH3O2 with Cl atoms that leads to the formation of CH3O. For β = 0, 0.5 and 1, the derived values of α are 0.28±0.07, 0.32±0.08 and 0.42±0.09, respectively. Comparison with other published results suggests the possibility of three significant product channels for the reaction of CH3O2 with ClO.

Negative chlorine ion chemistry in the upper stratosphere and its application to an artificially created dense electron cloud

Abstract. This paper discusses new potential reactions of chlorine-bearing anions (negative ions) in the upper stratosphere. These reactions are then applied to the negative-ion chemistry following the injection of an electron cloud of very high density, of the order of 106-107 e- cm-3, in the 40-45-km region. The idea is to evaluate the recently proposed scheme to mitigate ozone depletion by converting the reactive chlorine atoms at these altitudes into Cl- ions which are unreactive towards ozone, i.e., electron scavenging of Cl. We find that the previously neglected photodetachment from Cl- is fast. For an overhead sun, this process may have a rate coefficient of 0.08 s-1 when multiple scattering is included. The rate could be even higher, depending on the ground albedo. Switching reaction between Cl-·H2O and HCl might lead to the formation of Cl-·HCl anion. Possible reactions of Cl-·H2O and Cl-·HCl with O atoms could produce ClO- and Cl-2. The production of ClO- in this manner is significant because Cl- having a high photodetachment rate constant would be regenerated in the very likely reactions of ClO- with O. When these possibilities are considered, then it is found that the chlorine anions may not be the major ions inside the electron cloud due to the rapid photodetachment from Cl-. Furthermore, in such a cloud, there may be the hazard that the Cl--Cl-·H2O-ClO--Cl- cycle amounts to catalytic destruction of two O atoms. Thus, the scheme could be risky if practised in the altitude region where atomic oxygen is an important constituent. Similar conclusions apply even if the ClO- species forms ClO-3 by three-body association with O2, instead of reacting with O. It must be emphasized that the present study is speculative at this time, because none of the relevant reactions have been investigated in the laboratory as yet. Nevertheless, it is very safe to say that the scheme of ozone preservation by electron scavenging of the upper stratospheric Cl is much less certain than implied in the studies reported by its original proponents, because those studies neglected the photodetachment from Cl- and made the highly unlikely assumption that the Cl-·H2O anion neither photodissociates nor reacts any further. The situation at the lower altitudes could be even more complex due to the formation of large cluster ions and the ion-induced aerosol formation. The lower atmospheric situation, therefore, requires much more study. The uncertainties in the scavenging scheme due to the electrostatic repulsion in the cloud should also be addressed. Despite the uncertainties about its environmental engineering usefulness, the emerging technology for artificial creation of plasmas, with any desired density and charge in the stratosphere, could have significant pure scientific values in the studies of stratospheric ion chemistry and ion-induced aerosol formation. Such studies have perennially suffered from the extremely low densities of the naturally occurring plasma.

Stratospheric OClO measurements as a poor quantitative indicator of chlorine activation

In the past measurements of OClO have been used to determine the degree of chlorine activation in the Arctic and Antarctic lower stratospheres. In this study we have carried out box model calculations to investigate the behaviour of OClO concentrations at 50hPa for cold denoxified conditions, typical of the polar winter. The dependence of the OClO abundance on ClOx (ClO+2Cl2O2) and BrOx (BrO+BrCl), as well as the sensitivity to nitrogen species, latitude and temperature is investigated. The net production of OClO from the reaction ClO + BrO → OClO + Br, the major source of OClO, is found to be first order in BrO for BrOx values from 0 to 22 pptv, whereas it becomes pseudo zero order in ClO for ClOx greater than ∼400 pptv. This behaviour is due to the rate at which BrOx is tied up as BrCl at sunset. The insensitivity of the OClO concentration to high levels of ClO makes OClO a poor indicator for distinguishing between medium and high levels of ClO. OClO is only a qualitatively good indicator of chlorine activation but is a good quantitative indicator of BrO.

Observation of the CFCl2CH2O Radical in the Flash Photolysis of CFCl2CH3 in the Presence of O2. Kinetics of the Reactions of Cl and ClO with CFCl2CH2O2 and CH2CFClO2

Observation of the CFCl2CH2O Radical in the Flash Photolysis of CFCl2CH3 in the Presence of O2. Kinetics of the Reactions of Cl and ClO with CFCl2CH2O2 and CH2CFClO2

Halogen oxides: Radicals, sources and reservoirs in the laboratory and in the atmosphere

The central topic of this review concerns the species XO, where X is F, Cl, Br or I. These molecules are thus the radicals FO, ClO, BrO and IO, but attention is also given to some of their precursors in the laboratory and the atmosphere, as well as to their reservoirs, sinks, and other related species of potential atmospheric importance. Laboratory data on the physics and chemistry of the species and atmospheric determinations of their concentrations are both considered. One aim of the review is to highlight the relationship between the laboratory investigations and the atmospheric studies. The emphasis of the review is on gas-phase processes. After a brief introductory section, the review continues with an examination of laboratory techniques for the study of the halogen-oxide species. This section fast looks at the general properties of the oxides and sources of them for laboratory experiments, then discusses the detection and measurement of the monoxide radicals in the laboratory, and ends with a description of the kinetic tools that have been harnessed in the various studies. The spectroscopy, structure, photochemistry and thermochemistry, of the halogen oxides are discussed in Section III. Both experimental and theoretical aspects are presented. The objectives of the work described are on the one hand to establish the basis for the detection of the radical and the measurement of its concentration in the laboratory and in the atmosphere, and on the other to provide the framework for interpreting pathways, mechanisms and efficiencies of photochemical and thermal reactions. Sections IV, V and VI of the review address the main issues of observed chemistry and its kinetics. Section IV gathers together available kinetic and mechanistic information on gas-phase reactions of FO, ClO, BrO and IO radicals, and the available data are summarized in appropriate tables. Section V reports on the corresponding data available for the gas-phase reactions of certain species containing the XO grouping, which include most of the so-called atmospheric reservoirs of XO radicals. There are three sub-sections, which deal in turn with oxide species, HOX, and XONO 2. Heterogeneous processes are introduced in Section VI. Heterogeneous chemistry in the atmosphere is that which occurs on or in ambient condensed phases that are in contact with the gas phase, such as aerosols, clouds, surface waters, and so on. It is becoming increasingly clear that such processes are of importance not only in the stratosphere, but also in the troposphere. Section VII of the review is concerned directly with the atmosphere. The sources and sinks of the compounds, the reaction pathways, temporary and permanent reservoirs, observational evidence, the involvement of the species in atmospheric chemistry, and modelling studies are considered for the troposphere and the stratosphere in turn. The section concludes with a more detailed exposition of the role of modelling of the halogen compounds in the stratosphere. The review concludes with an examination of issues in regard to the halogen oxide species that are unresolved, uncertain, or in need of further research. Further data are required, for example, on the spectroscopy and photochemistry of reservoir compounds, on potential organic sources of atmospheric iodine, and even on the channels for photolysis of compounds such as OClO. Within the field of reaction kinetics, there is a need for further study of the kinetics of dimer formation, and of certain other reactions of the radicals themselves (especially of IO) and some of their reservoirs. A substantial number of problems in heterogeneous chemistry of the species remain to be solved. Not only are some key physical measurements missing, but most of what has been achieved in both chemistry and physics is limited to chlorine-containing species, so that the work needs to be extended to the other halogens. There is also a need for a search for novel reactions occurring on conventional surfaces and for all types of reaction occurring on surfaces that exist within the atmosphere but which have not yet been the subject of laboratory study. So far as the atmosphere itself is concerned, there are important issues to be resolved. They include (i) the involvement of halogen species in episodic tropospherec ozone depletion in the Arctic (and a further question about whether or not such depletion is more widespread); (ii) the role of an active halogen chemistry in the oxidation of VOC; (iii) the significance and detail of stratospheric iodine and iodine-catalysed ozone removal; and (iv) the quantitative description of heterogeneous stratospheric chemistry.

Kinetics and Equilibrium Constant of the Reversible Reaction ClO + ClO + M <=> Cl2O2 + M at 295 K.

Kinetics and Equilibrium Constant of the Reversible Reaction ClO + ClO + M <=> Cl2O2 + M at 295 K.

Ground‐based FTIR measurements of CLONO2 Vertical column amounts in the Arctic

During the EASOE campaign measurements have been performed in March at Ny‐Ålesund (79°N, 11°E) by high resolution FTIR spectroscopy to derive the vertical column amounts of ClONO2. For the first part of this period the polar vortex was found to be situated above Ny‐Ålesund.The measurements give ClONO2 zenith column amounts of about 3.9×1015 mol cm−2 outside the polar vortex, much higher than midlatitude values for an undisturbed atmosphere. Inside the vortex strong variabilities with burdens between 2.8 and 6.8×1015 mol cm−2 were observed. These high values might be explained by the reaction of ClO with NO2 and the relatively warm stratosphere ensuring that the ClONO2 so formed remains in the gas phase.

Reaction of ClO with NO3: OClO formation and night‐time O3 loss

The atmospheric importance of the reaction of ClO with NO3 has been tested in a one‐dimensional photochemical model. This reaction has two product channels which are of atmospheric importance. The channel which forms Cl and NO2 can destroy ozone in a catalytic cycle. This cycle constitutes the main ozone loss mechanism during the night near 40 km. The other channel forms OClO and is a significant source of OClO in the upper stratosphere during the night. In the polar lower stratosphere this reaction does not significantly enhance OClO.

Atmospheric chemistry of the reaction ClO + O2 ↔ ClO · O2: Where it stands, what needs to be done, and why?

Possible existence and chemistry of ClO · O2 was originally proposed to explain the Norrish‐Neville effect that O2 suppresses chlorine photosensitized loss of ozone. It was also thought that ClO · O2 might have some atmospheric chemistry significance. Recently, doubts have been cast on this proposal, because certain laboratory data seem to imply that the equilibrium constant of the title reaction is so small that ClO · O2 may be too unstable to matter. However, those data create only a superficial illusion to that effect, because on a closer analysis they do not disprove a moderately stable and chemically significant ClO · O2. Furthermore, our state‐of‐the‐science accurate computational chemistry calculations also suggest that ClO · O2 may be a weakly bound ClOOO radical with a reactive 2A ground electronic state. There is therefore a need to design and perform definitive experimental tests of the existence and chemistry of the ClO · O2 species, which we discuss and which have the potential to mediate the chlorine‐catalyzed stratospheric ozone depletion.

Influence of the heterogeneous reaction HCl + HOCl on an ozone hole model with hydrocarbon additions

Injection of ethane or propane has been suggested as a means for reducing ozone loss within the Antarctic vortex because alkanes can convert active chlorine radicals into hydrochloric acid. In kinetic models of vortex chemistry including as heterogeneous processes only the hydrolysis and HCl reactions of ClONO2 and N2O5, parts per billion by volume levels of the light alkanes counteract ozone depletion by sequestering chlorine atoms. Introduction of the surface reaction of HCl with HOCl causes ethane to deepen baseline ozone holes and generally works to impede any mitigation by hydrocarbons. The increased depletion occurs because HCl + HOCl can be driven by HOx radicals released during organic oxidation. Following initial hydrogen abstraction by chlorine, alkane breakdown leads to a net hydrochloric acid activation as the remaining hydrogen atoms enter the photochemical system. Lowering the rate constant for reactions of organic peroxy radicals with ClO to 10−13 cm3 molecule−1 s−1 does not alter results, and the major conclusions are insensitive to the timing of the ethane additions. Ignoring the organic peroxy radical plus ClO reactions entirely restores remediation capabilities by allowing HOx removal independent of HCl. Remediation also returns if early evaporation of polar stratospheric clouds leaves hydrogen atoms trapped in aldehyde intermediates, but real ozone losses are small in such cases.

Kinetics and mechanism of the chlorine oxide ClO + ClO reaction: pressure and temperature dependences of the bimolecular and termolecular channels and thermal decomposition of chlorine peroxide

The kinetics and mechanism of the CIO + CIO reaction and the thermal decomposition of CIOOCI were studied using the flash photolysis/long path ultraviolet absorption technique. Pressure and temperature dependences were determined for the rate coefficients for the bimolecular and termolecular reaction channels, and for the thermal decompositon of CIOOCI.

Observation of chlorine oxide (ClO3) radical in aqueous chlorate solution by pulse radiolysis

A pulse radiolysis study on concentrated aqueous chlorate solution was carried out. In the radiolysis of the chlorate system, ClO 2 and ClO 3 radicals are produced as a result of direct action of radiation on chlorate ion with G values of 1.0 and 1.5, respectively, whereas the slow formation process of ClO 3 radicals is not observed. The absorption peak of ClO 3 radicals is around 330±10 nm and the absorption coefficient is evaluated as 4100 M -1 cm -1 . The rate constants of some reactions of ClO 3 radical with additives were determined

Determination of product branching ratio of the ClO self‐reaction at 298 K

The Cl2‐sensitized continuous photolysis of O3 mixtures in O2 was investigated at 298 K using time resolved single wavelength and diode array spectroscopy to monitor the decay of O3 and the formation of OClO. Φ−O3 was found to be 4.1±0.4. This value, combined with the determination of the initial rates of OClO formation, allowed the evaluation of the following branching ratios into the three bimolecular disproportionation channels of ClO reactions; k3a/k3 = 0.40 ± 0.08, k3b/k3 = 0.42 ± 0.08 and k3c/k3 = 0.18 ± 0.02 where ClO + ClO → Cl2 + O2 (3a), ClO + ClO → Cl + Cl + O2 (3b) and ClO + ClO → Cl + OClO (3c).

Chlorine catalyzed destruction of ozone: Implications for ozone variability in the upper stratosphere

The annual mean and the annual amplitude of ozone have been derived from ozone measurements from the SBUV and SBUV/2 spectrometers on board the Nimbus‐7 and NOAA‐11 satellites. These values differ significantly from values calculated using a two‐dimensional model of stratospheric photochemistry and dynamics with standard chemistry. We have found that the differences between the calculated and data‐derived values are considerably improved by changing the partitioning in the Cly family to create a larger reservoir of HCl and reducing ClO. This is accomplished by including a channel for the products HCl+O2 from the reaction ClO+OH in addition to the products Cl+HO2. This partitioning also improves the agreement between the calculated and measured values of ClO/HCl ratio.

Chemiluminescence method for the determination of gaseous chlorine species in ambient air

The chemiluminescence reaction of ClO − with H 2O 2 in alkaline solution was applied to the determination of gaseous chlorine species such as Cl 2 and HCl in ambient air. The detection limit for Cl 2 was 1 ppm (v/v) and the relative standard deviation of five measurements of 2.5 ppm Cl 2 was 3.8%. Hydrocarbons, ozone, nitrogen oxides and sulphur dioxide did not interfere in the determination of Cl 2 and HCl. The results obtained were in good agreement with those obtained by ion chromatography.