Richard P. Wayne

The Nitrate Radical: Physics, Chemistry and the Atmosphere

Abstract This review surveys the present state of knowledge of the nitrate (NO 3 radical. Laboratory data on the physics and chemistry of the radical and atmospheric determination of the concentrations of the radical are both considered. One aim of the review is to highlight the relationship between the laboratory and the atmospheric studies. Although the emphasis of the review is on gas-phase processes, relevant studies conducted in condensed phases are mentioned because of their potential importance in the interpretation of cloud and aerosol chemistry. The spectroscopy, structure, and photochemistry of the radical are examined. Here, the object is to establich the spectroscopic basis for detection of the radical and measurement of its concentration in the laboratory and in the atmosphere. Infrared, visible, and paramagnetic resonance spectra are considered. An important quantity discussed is the absorption cross section in the visible region, which is required for quantitative measurements. Interpretation of the spectroscopic features requires an understanding of the geometrical and electronic structure of the radical in its ground and excited states; there is still some controversy about the groundstate geometry, but the most recent experimental evidence 9eg from laser induced fluorescence) and theoretical calculations suggest that the radical has D 3h symmetry. Photodissociation of the radical is important in the atmosphere, and the product channels, quantum yields, and dissociation dynamics are discussed. A short examination of the thermodynamics (heat and entropy of formation) of the radical is presented. The main exposition of laboratory studies of the chemistry of the nitrate radical is preceded by a consideration of the techniques used for kinetic and mechanistic studies. Methods for the generation and detection of the radical and the kinetic tools employed are all presented. The exact nature of the technique used in individual studies has some relevance to the way in which data must be analysed, and to the type of mechanistic information that can be extracted. Continuous and stopped flow, flash photolysis and pulse radiolysis, molecular modulation, and static reactor techniques can all provide absolute kinetic data, while relative rate measurements have been a further rich source of information. The treatment of the chemical reactions of the nitrate radical is formally divided into the interactions with non-radical inorganic (deemed to include NO and NO 2 ) and organic species, and with atoms and free radicals. In general, the reactions with open-shell species are much more rapid than those with closed-shell reactants. With the closed-shell partners, addition reactions are faster than abstraction reactions. An attempt is made to consider critically the published data on most reactions of importance, and to tabulate rate constants and temperature dependences where possible. However, it is not the objective of this review to provide recommendations for rate parameters. Evidence for the products of the reactions is sought, and for the branching ratios into the various channels where more than one exists. One theme of this part of the review is the elucidation of correlations of reactivity with structure and with the reactions of other radical species such as OH. The review turns next to a consideration of the role of NO 3 in the atmosphere, of its atmospheric sources and sinks, and of field measurements of concentrations of the radical. Long-path visible-absorption spectroscopy and matrix-isolation ESR have both been used successfully in field measurements in the troposphere as well as the stratosphere. Balloon-borne instruments and ground-based remote sensing have been used to obtain stratospheric concentrations. Two of the most important implications of the measurements are that the stratospheric profiles are consistent with accepted chemistry (and, in particular, do not require the postulation of an unidentified scavenging mechanism that had, at one stage, been proposed), and that the highly variable night-time tropospheric concentrations imply that NO 3 is a reactive tropospheric constituent. The inter-relation between laboratory studies and atmospheric observations, and the problems in extrapolating laboratory data to atmospheric conditions, are both explored. Initiation of night-time chemical transformations by NO 3 and the possible production of OH are considered. The available information is then brought together to see how far NO 3 is a sensitive indicator of the state of the atmosphere, and some speculations are presented about the involvement of NO 3 (or N 2 O 5 ) in damage to trees and plants. The final section of the review suggests some issues that remain unresolved concerning the NO 3 radical which is directly or indirectly relevant to a better knowledge of the part played by the radical in the atmosphere. Amongst the requirements noted are improved data for the heat of formation of the radical, its absorption cross section in the visible region (and, especially, the temperature dependence of the cross section), and the details of its photochemistry. There is also still a need for a definitive determination of the equilibrium constant and its temperature dependence for the association with NO 2 and the reverse dissociation of N 2 O 5 . A series of chemical reactions deserves further investigation, especially with regard to elucidation of product channels, and overall oxidation mechanisms also need to be defined better. Future atmospheric studies that are desirable include study of basic NO 3 chemistry in the field to understand the influence of humidity on the conversion (probably on surfaces) of N 2 O 5 to HNO 3 , and thus on NO 3 concentrations. In addition, a study of the chemistry of NO 3 in the presence of volatile organic compounds and at elevated concentrations of the oxides of nitrogen should help in the understanding of, for example, polluted marine coasts, forests, and urban areas.

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

Abstract 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 XONO2. 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.

Frontier molecular orbital correlations for predicting rate constants between alkenes and the tropospheric oxidants NO3, OH and O3

Two types of correlation relating the value of the energy of the highest occupied molecular orbital (HOMO) of an alkene to the logarithm of its rate constant for reaction with NO3, OH or O3 have been formulated. Both correlations have been shown to be consistent with frontier molecular orbital theory. The correlation can be used to predict the rate constants for the reaction of an alkene with NO3, OH or O3 by calculating the value of the HOMO energy of the alkene. The accuracy of these predictions is quoted as a 48, 40 and 97% minimum probability that the predicted rate constant for reaction of an alkene with NO3, O3 and OH, respectively, will be within a factor of two of the measured rate constant. This probability is increased to a minimum of 73, 80 and approaches 100% for the reactions of NO3, O3 and OH, respectively, with conjugated dienes.

Infra-red laser enhanced reactions: chemistry of vibrationally excited O3 with NO and O2(1Δ)

Abstract Vibrationally excited ozone, produced by absorption of CO2 laser radiation, was found to react significantly faster with NO and O2(1Δ) than thermal ozone. Using a modulation technique, absolute and relative rate constants at 300K for the following reactions were calculated assuming rapid equilibration between the three closely spaced vibrationally excited levels of O3, and that only the lowest level of these, the ν2 bending mode, is active in reaction. k1′ + k2′ = 2.7 × 10−13 cm3 molecule−1 s−1; (k1′ + k2′)/(k1 + k2) = 16.2 ± 4.0; k1′/k1 = 4.1 ± 2.0; k2′/k2 = 17.1 ± 4.3; k7′/k7 = 38 ± 20. These rate constants must be modified if a different combination of vibrationally excited levels is involved. The fraction of vibrational energy usable in chemical reaction was found to be about 15, 50 and ∼ 100% respectively for processes 1′, 2′ and 7′. Our measurements clearly differentiate between the participation of vibrational energy and thermal energy but do not distinguish differences between the individual vibrationally excited states. Details of the modulation technique, involving chemiluminescence detection of NO2 and resonance fluorescence detection of oxygen atoms, are described. Comparison of our results with a previous measurement of the summation reaction (1′ + 2′) shows excellent agreement.

The photochemistry of ozone

Ozone plays a major role in the atmosphere, important chemical transformations in the troposphere, stratosphere, and mesosphere all being initiated by the absorption of radiation by ozone. This article surveys laboratory data concerning the photochemistry of ozone, and indicates the relevance of the laboratory studies to interpretations of atmospheric chemistry.

Laboratory studies of some halogenated ethanes and ethers: Measurements of rates of reaction with OH and of infrared absorption cross-sections

Abstract We have measured, using a conventional discharge-flow resonance-fluorescence technique, the rates of reaction between the hydroxyl radical and a series of halogenated ethanes and ethers for the temperature range 230–423 K. Our measurements gave the following Arrhenius expressions (units are cm3 molecule−1 s−1): CF2HCH3 (HFC-152), 14.2 × 10−13 exp-(1050/T); CF2ClCH3 (HCFC-142b), 2.6 × 10−13 exp-(1230/T); CFCl2CH3 (HCFC-141b), 5.8 × 10−13 exp-(1100/T); CF3CFH2 (HFC-134a), 5.8 × 10−13 exp-(1350/T); CF3CF2H (HFC-125), 2.8 × 10−13 exp-(1350/T); CF3CCl2H (HCFC-123), 11.8 × 10−13 exp-(900/T); CF2HOCF2CFClH, (enflurane), 6.1 × 10−13 exp-(1080/T); CFH2OCH(CF3)2, (sevoflurane), 15.3 × 10−13 exp-(900/T). In two cases, we measured rate constants only at room temperature: CF3CClBrH (halothane), 6 × 10−14 and CF2HOCClHCF3 (isoflurane), 2.1 × 10−14. We also report the following values for the integrated absorption cross-sections of the compounds in the spectral region 800–1200 cm−1 in units of cm−2 atm−1: CF2HCH3, 1155; CF2ClCH3, 1422; CFCl2CH3, 1995; CF3CFH2, 2686; CF3CF2H, 1970, CF3CCl2H, 1411; CF3CClBrH, 1400; CF2HOCF2CFClH, 4800; CF2HOCClHCF3, 3900; CFH2OCH(CF3)2, 2550. We use our measurements to calculate ozone depletion potentials and greenhouse warming potentials relative to CFCl3 for each compound.

Infrared laser enhanced reactions: Spectral distribution of the NO2 chemiluminescence produced in the reaction of vibrationally excited O3 with NO

Vibrationally excited ozone, produced by CO2 laser radiation, was found to react significantly faster with NO than does thermal O3. The emission spectrum of the laser enhanced chemiluminescence from this reaction was measured from 520 to 810 nm. The lowest lying 12B2 state was identified as the primary source of NO*2 emission in the NO+O3 reaction. One quantum of vibrational excitation in the reactant O3 was found to introduce one quantum of vibrational energy in the product NO2 (12B2). The rate enhancement of the reaction channel producing NO2(12B2) as a result of vibrational excitation of O3 was 5.6±1.0. Thus, only about 50% of the available vibrational energy is used to enhance this reaction.

CF3 chemistry: Potential implications for stratospheric ozone

Previous evaluations of the impact of fluorine chemistry on stratospheric ozone have concluded that the role of fluorine compounds in catalytic ozone removal is negligible. However, recent investigations of the degradation pathways for compounds containing CF3 groups indicates that if the reaction of CF3O with O3 is sufficiently fast, there may be an ozone impact. Some recent measurements indicate that the reaction rate constant of CF3O+O3 is sufficiently low that the ozone impact is likely to be small. However, it is not possible a-priori to rule out significant ozone removal without additional kinetic data on other reactions. We present calculations to illustrate how different key reactions affect the calculated stratospheric concentrations of the CF3X species (CF3, CF3O, CF3O2, CF3OH, CF3OOH, CF3ONO2, CF3O2NO2, CF3OOCl) and their ability to remove stratospheric ozone. We utilize our results to suggest kinetic measurements that could substantially reduce the uncertainties in CF3 chemistry relevant to the determination of ozone depletion potential of CF3-bearing compounds.

A kinetic study of the reactions of NO3 with methyl vinyl ketone, methacrolein, acrolein, methyl acrylate and methyl methacrylate

Absolute and relative-rate techniques have been used to obtain rate coefficients for the reactions: NO3+CH3C(O)CHCH2→products (1), NO3+CH2C(CH3)CHO→products (2), NO3+CH2CHCHO→ products (3), and NO3+CH2CHC(O)OCH3→products (4). The reaction NO3+CH2C(CH3)C(O)OCH3→ products (5), has been investigated by a relative-rate method only. The rate coefficients obtained by the relative-rate method at T=296±2 K and P=760 Torr are k1=(4.7±1.7)×10-16 cm3 molecule-1 s-1, k2=(3.7±1.0)×10-15 cm3 molecule-1 s-1, k3=(1.1±0.4)×10-15 cm3 molecule-1 s-1, k4=(1.0±0.6)×10-16 cm3 molecule-1 s-1 and k5=(3.6±1.3)×10-15 cm3 molecule-1 s-1. The rate coefficients determined by the discharge-flow technique at low pressure (P=1–10 Torr) and at T=293–303 K are k1=(3.2±0.6)×10-16 cm3 molecule-1 s-1, k2=(9.6±2.0)×10-15 cm3 molecule-1 s-1, k3=(8.9±2.8)×10-15 cm3 molecule-1 s-1, k4=(1.9±0.4)×10-16 cm3 molecule-1 s-1. The discrepancy between the values obtained from the relative-rate technique and the absolute technique are discussed and explained in terms of interference in the absolute study caused by secondary chemistry and fast-reacting impurities. Product studies reveal that methyl glyoxal is a product of reactions (1) and (2) along with peroxymethacryloyl nitrate (MPAN) for reaction (2) in air. A diurnally varying boundary-layer model suggests that reaction (2) is an important loss process for methacrolein and that it can lead to the generation of OH at night.

A discharge–flow study of the kinetics of the reactions of IO with CH3O2 and CF3O2

We have determined the rate constants for the reactions IO + CH3O2 --> Products (1) and IO + CF3O2 --> Products (2) using a discharge-flow tube equipped with off-axis cavity-enhanced absorption spectroscopy (CEAS) for the detection of IO. NO2, produced from the titration of RO2 with NO, was also detected using the CEAS system. The rate constants obtained were k1 = (6.0 +/- 1.3) x 10(-11) cm3 molecule(-1) s(-1) and k2 = (3.7 +/- 0.9) x 10(-11) cm3 molecule(-1) s(-1) at T = 295 +/- 2 K and P = 2.5 +/- 0.3 Torr; this is the first determination of these rate constants. The possible products and the atmospheric implications of reaction (1) are discussed.