G. Poulet

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.

A discharge flow mass-spectrometric study of the reaction between the NO3 radical and isoprene

The kinetics and mechanism of the reactionNO3+CH2=C(CH3)−CH=CH2→productswere studied in two laboratories at 298 K in the pressure range 0.7–3 torr using the discharge-flow mass-spectrometric method. The rate constant obtained under pseudo-first-order conditions with excess of either NO3 or isoprene was: k1=(7.8±0.6)×10−13 cm3 molecule−1 s−1. The product analysis indicated that the primary addition of NO3 occurred on both π-bonds of the isprene molecule.

Rate constant measurement for the reactions of OH and Cl with peroxyacetyl nitrate at 298 K

The gas phase reactions of peroxyacetyl nitrate (PAN) with OH and Cl have been studied using the discharge-flow EPR method. The rate constants are found to be k3=(7.5±1.4)×10-14 and k4=(3.7±1.7)×10-13 cm3 molecule-1 s-1 at 298 K, respectively. These results confirm that the OH+PAN reaction will be the dominant sink of PAN in the middle and upper troposphere, whereas the reaction Cl+PAN will be negligible in contrast with previous estimations.

Kinetics of the reactions of hydrogen iodide with hydroxyl and nitrate radicals

Abstract The reactions of HI with OH and NO 3 have been studied in discharge-flow reactors coupled to EPR and mass spectrometry for analysis. The rate constants for the reactions HI + OH → H 2 O + I (1) and HI + NO 3 → HNO 3 + I (2) were determined at 298 K: k 1 = (3.3 ± 0.2) × 10 −11 and K 2 = (2.5 ± 0.8) × 10 −15 cm 3 molecules −1 s −1 . Further measurements of k 2 over the temperature range 298–373 K yielded the expression k 2 = 1.3 × 10 −12 exp‘(− 1830 ± 300)/ T ’ cm 3 molecule −1 s −1 . The results for reaction (1) are compared with earlier literature data; those for reaction (2) are the first ones.

Kinetics and products of the reaction of hydroxyl radical with molecular bromine

Abstract The rate constant of the OH + Br 2 reaction has been measured at room temperature in two discharge-flow systems using either electron paramagnetic resonance or laser-induced fluorescence for OH analysis. The result is k = (4.2 ± 0.7) × 10 −11 cm 3 molecule −1 −1 . The measurement of the Br yield produced in this reaction indicates that OH reacts with Br 2 following the unique channel OH + Br 2 → Br + HOBr. This reaction could be a suitable source of HOBr for laboratory spectroscopic studies.

Kinetic Investigation of the OH + CH3Br Reaction between 248 and 390 K

The rate parameters for the reaction of the OH radical with CH3Br have been measured using the discharge flow-electron paramagnetic resonance method. The result isk1=(1.86±0.48)×10−12 exp[−(1230±150)/T] cm3 molecule−1 s−1. This value is compared to earlier data and is found to be in excellent agreement with the most recent results, which greatly increases the accuracy of the ozone depletion potential of CH3Br which can be derived from these kinetic data.

About the identification of some UV atmospheric absorptions: laboratory study of CIO

Abstract Concentrations of CIO radicals of about 5 × 10 16 radicals cm −12 were produced from the reaction: Cl + CLO 2 → 2ClO in a discharge flow reactor. UV absorption of the A 2 πue5f8X 2 π system of CIO was measured in the near UV where there is some possibility of stratospheric CIO being detected. Absorption cross sections were determined.

The Br+HO2 reaction revisited: Absolute determination of the rate constant at 298 K

Abstract The absolute determination of the rate constant for the reaction Br+HO 2 →HBr+O 2 has been done at 298 K using the discharge-flor EPR method. The value k 1 = (1.5±0.2) × 10 −12 cm 3 molecule −1 s −1 was obtained. Previous indirect measurements of k 1 from a discharge-flow, LIF/mass spectrometric study of the Br/H 2 CO/O 2 system have been reinterpreted, leading to values for k 1 ranging from 1.0 × 10 −12 to 2.2 × 10 −12 cm 3 molecule −1 s −1 at 298 K. These results are discussed and compared with other literature values.

Laser photolysis-resonance fluorescence investigation of the reactions of hydroxyl radicals with CCl3CHO and CF3CHO as a function of temperature

Absolute rate constants for the reactions of OH radicals with CCl3CHO and CF3CHO have been determined as a function of temperature using a pulsed laser photolysis-resonance fluorescence technique. The rate constant for reaction with CCl3CHO, k(CCl3CHO) equals 1.4 +/- 0.2 cm3 molecule-1 s-1, was found to be virtually independent of temperature over the range 233 - 313 K while the Arrhenius expression obtained for reaction with CF3CHO was k(CF3CHO) equals 3.5 +/- 1.0 X 10-12 exp [-(488 +/- 57)/T] cm3 molecule-1 s-1. Reactivity trends and atmospheric implications are discussed.