Carlos E. Canosa-Mas

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.

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.

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.

Molecular iodine reduction in seawater, an improved rate equation considering organic compounds

Abstract The earlier modelling of the reduction of micro-molar amounts of molecular iodine added to seawater by means of linked first-order reactions considering only iodine species is shown to be inappropriate. Reasons are given for believing that the characteristic of a rapid initial phase followed by a much slower one results from speciation of the reducing organic compounds, not the iodine. An alternative approach using two competing reductions, one fast the other much slower, is explored. Both reactions are first order with respect to concentrations of iodine and organics, with the rapidly reduced material being consumed fully. The similarity between iodines reaction with reducing species in seawater and that of chlorine and ozone is considered.

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.

Is the reaction between CH3C(O)O2 and NO3 important in the night-time troposphere?

A discharge-flow system equipped with a laser-induced fluorescence (LIF) cell to detect NO2 and a multi-pass absorption cell to detect NO3 has been used to study the reaction CH3C(O)O2+ NO3→ CH3C(O)O + NO2+ O2(1) at T= 403–443 K and P= 2–2.4 Torr. The rate constant was found to be independent of temperature with a value of k1=(4 ± 1)× 10–12 cm3 molecule–1 s–1. The likely mechanism for the reaction is discussed. The atmospheric implications of reaction (1) are investigated using a range of models and several case studies are presented, comparing model results with actual field measurements. It is concluded that reaction (1) participates in a cycle which can generate OH at night. This reaction cycle (see text) can operate throughout the continental boundary layer, but may even occur in remote regions.

Kinetics of the reactions of the nitrate radical with a series of halogenobutenes. A study of the effect of substituents on the rate of addition of NO3 to alkenes

Rate coefficients for the reaction of NO3 with a series of halogenobutenes have been measured using the discharge-flow technique coupled to optical-absorption detection of NO3. The following room-temperature rate coefficients and Arrhenius parameters (cm3 molecule–1 s–1) were measured: 1-chlorobut-1-ene, 1.2 × 10–14; 2-chlorobut-1-ene, 7.0 × 10–14; 2-chlorobut-2-ene, 11.0 × 10–14; 1-chloromethylpropene, 9.0 × 10–14; 3-bromobut-1-ene, 0.4 × 10–14; 4-bromobut-1-ene, 0.5 × 10–14; 2-bromobut-2-ene, 13.4 × 10–14; 3-chlorobut-1-ene, 2.4 × 10–12 exp(–1992/T); 1-chlorobut-2-ene, 6.0 × 10–13 exp(–981/T); 3-chloromethylpropene, 1.7 × 10–12 exp(–1277/T). Trends in the reactivities of the compounds towards NO3 are discussed in terms of the relative energies of the interacting orbitals, and the data are used to calculate group reactivity factors. These factors can be used to estimate rate constants which have not, as yet, been measured.

Investigation into the pressure dependence between 1 and 10 Torr of the reactions of NO2 with CH3 and CH3O

The kinetics and pressure dependence of the reactions of NO2 with CH3 and CH3O have been investigated in the gas phase at 298 K, at pressures from 1 to 10 Torr. A low-pressure discharge-flow laser-induced fluorescence (LIF) technique was used. In a consecutive process, CH3 reacted with NO2 to form CH3O, CH3+ NO2→ CH3O + NO (1), which further reacted with NO2 to form products, CH3O + NO2→ products (2). Reaction (1) displayed no discernible pressure dependence over the pressure range 1–7 Torr, and k1 was calculated to be (2.3 ± 0.3)× 10–11 cm3 molecule–1 s–1. Reaction (2) displayed a strong pressure dependence and an RRKM analysis yielded the following limiting low- and high-pressure rate constants in He, k0= 5.9 × 10–29 cm6 molecule–2 s–1 and k∞= 2.1 × 10–11 cm3 molecule–1 s–1. It is unrealistic to quote errors for this type of analysis. Parametrisation in the standard Troe form with Fc= 0.6 yielded k0=(5.3 ± 0.2)× 10–29 cm6 molecule–2 s–1 and k∞=(1.4 ± 0.1)× 10–11 cm3 molecule–1 s–1. Atmospheric implications and possible reaction mechanisms are discussed.

Correlations between rate parameters and calculated molecular properties in the reactions of the nitrate radical with alkenes

A study of the reactivity of the No3 radical towards a series of alkenes and halogenoalkenes has been carried out. For those compounds not containing a vinylic chlorine atom, the activation energy, the logarithm of the pre-exponential A factor and the logarithm of the room-temperature rate coefficient (k) correlate strongly with the calculated ionization potential (Ei) of the organic reactant. In the case of alkenes containing vinylic chlorine atoms, the correlations of k and activation barriers with Ei break down, although the correlation between log A and Ei still holds. On the basis of the extent of interaction between the chlorine atom lone-pair orbitals and the carbon–carbon π bond, corrections to the Ei s can be made and the activation barriers and log ks then correlate well with these corrected Ei s. Using these correlations, Arrhenius parameters can be estimated and used to calculate ks, which are compared with measured values. Excellent agreement is observed, providing support for this method of estimating Arrhenius parameters. The observations are rationalized in terms of frontier orbital theory.