Howard Sidebottom

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.

Studies on the Atmospheric Degradation of Chlorpyrifos-Methyl

The gas-phase atmospheric degradation of chlorpyrifos-methyl (a widely used organophosphate insecticide in Southern European regions) has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. Photolysis under sunlight conditions and reaction with ozone were shown to be unimportant. The rate constant for reaction of chlorpyrifos-methyl with OH radicals was measured using a conventional relative rate method with cyclohexane and n-octane employed as reference compounds with k = (4.1 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) at 300 ± 5 K and atmospheric pressure. The available evidence indicates that tropospheric degradation of chlorpyrifos-methyl is mainly controlled by reaction with OH radicals and that the tropospheric lifetime is estimated to be around 3.5 h. Significant aerosol formation was observed following the reaction of chlorpyrifos-methyl with OH radicals, and the main carbon-containing products detected in the gas phase were chlorpyrifos-methyl oxone and 3,5,6-trichloro-2-pyridinol.

Rate constants for reactions of OH radicals with a series of asymmetrical ethers and tert‐Butyl alcohol

Absolute rate constants for the reactions of OH radicals with butyl ethyl ether (k1), methyl tert-butyl ether (k2), ethyl tert-butyl ether (k3) tert-amyl methyl ether (k4) and tert-butyl alcohol (k5) have been measured over the temperature range 230–372 K using a pulsed laser photolysis-laser induced fluorescence (PLP-LIF) technique. The temperature dependence of k1 − k5 when expressed in Arrhenius form gave: k1 = (6.59 ± 0.66) × 10 −12 exp|(362 ± 60)/T|, k2 = (5.03 ± 0.27) × 10−12 exp|&minus(133 ± 30)/T|, k3 = (4.40 ± 0.24) × 10−12 exp|(210 ± 37)/T|,k4 = (4.7 ± 0.7) × 10−12 exp|(82 ± 85)/T|, and k5 = (2.66 ± 0.48) × 10−12 exp| −(270 ± 130)/T|. However, the Arrhenius plots for k1–k5, were slightly curved and are best fitted by the three parameter fits which are given in the article. The room temperature values of k1, k2, k3, k4, and k5 are (2.08 ± 0.23) × 10−11, (3.13 ± 0.36) × 10−12, (8.80 ± 0.50) × 10−12, (6.28 ± 0.45) × 10−12, and (1.08 ± 0.10) × 10−12, respectively, in cm3 molecule−1 s−1.

An absolute- and relative-rate study of the gas-phase reaction of OH radicals and Cl atoms with n-alkyl nitrates

Abstract Rate constants for the reaction of OH radicals and Cl atoms with CH3ONO2, C2H5ONO2, n-C3H7ONO2, n-C4H9ONO2, and n-C5H11ONO2 have been determined at 298 ± 2 K and a total pressure of approximately 1 atm. The OH rate data were obtained using both the absolute-rate technique or pulse radiolysis combined with kinetic spectroscopy and a conventional photolytic relative-rate method. The Cl rate constants were measured using only the relative-rate method. Evidence is presented from the kinetic studies that reaction of OH radicals with alkyl nitrates may involve both addition and abstraction pathways. The data show that the —ONO2 group substantially decreases the rate constant for H-atom abstraction by OH radicals from groups bonded to the —ONO2 group and also decreases that for groups in the β position. Similar resuls were found for the reaction of Cl atoms with these compounds. The results are discussed in terms of reactivity trends.

A kinetic and mechanistic study of the gas-phase reactions of OH radicals and Cl atoms with some halogenated acetones and their atmospheric implications

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with a series of halogenated acetones of the type CX3COCH3 (X = H, Cl, F) have been determined using a photolytic relative-rate technique at T = 298 K and at 760 Torr total pressure. The reactions studied and the rate coefficients obtained are shown in the table. Reaction Reaction number Rate coefficient/cm3 molecule−1 s−1 OH + CH3COCH3 → products (1) (2.2 ± 0.5) × 10−13 OH + CH2ClCOCH3 → products (2) (4.2 ± 0.8) × 10−13 OH + CHCl2COCH3 → products (3) (3.8 ± 0.8) × 10−13 OH + CCl3COCH3 → products (4) (1.5 ± 0.3) × 10−14 OH + CH2FCOCH3 → products (5) (2.1 ± 0.4) × 10−13 OH + CF3COCH3 → products (6) (6.9 ± 1.3) × 10−15 Cl + CH3COCH3 → products (7) (2.2 ± 0.4) × 10−12 Cl + CH2ClCOCH3 → products (8) (2.0 ± 0.2) × 10−12 Cl + CHCl2COCH3 → products (9) (1.7 ± 0.3) × 10−13 Cl + CCl3COCH3 → products (10) (1.7 ± 0.3) × 10−14 Cl + CH2FCOCH3 → products (11) (8.2 ± 1.6) × 10−13 Cl + CF3COCH3 → products (12) (8.0 ± 1.6) × 10−15 The errors quoted reflect an estimate of the absolute uncertainty in the measured rate coefficients of ±20%. For reactions (7)–(12), Fourier transform infrared spectroscopy was used to identify products. Qualitative ultra-violet absorption spectra were also recorded for most of the halogenated species investigated in this study, and have been used together with the kinetic data to derive atmospheric lifetimes for these species.

Rate constants for the reaction of CF3O radicals with hydrocarbons at 298 K

Abstract Rate constant ratios of the reactions of CF 3 O radicals with a number of hydrocarbons have been determined at 298 ± 2 K and atmospheric pressure using a relative rate method. Using a previously determined value k (CF 3 O+C 2 H 6 ) = 1.2×10 −12 cm 3 molecule −1 s −1 these rate constant ratios provide estimates of the rate constants: k (CF 3 O+CH 4 ) = (1.2±0.1)×10 −14 , k (CF 3 O+ c -C 3 H 6 ) = (3.6±0.2)×10 −13 , k (CF 3 O+C 3 H 8 ) = (4.7±0.7)×10 −12 , k (CF 3 O+(CH 3 ) 3 CH) = (7.2±0.5)×10 −12 , k (CF 3 O+C 2 H 4 ) = (3.0±0.1)×10 −11 and k (CF 3 O+C 6 H 6 ) = (3.6±0.1)×10 −11 cm 3 molecule −1 s −1 . The importance of the reactions of CF 3 O radicals with hydrocarbons under atmospheric conditions is discussed.

Determination of Arrhenius parameters for thereactions of ozone with cycloalkenes

The kinetics of the gas-phase reactions of ozone with a series of cycloalkenes have been investigated using a conventional static system. Ozone loss was monitored in the presence of excess cycloalkene and rate data measured over the temperature range 240–331 K. Rate constants for the reactions of ozone with C 5 –C 8 cycloalkenes and several substituted cyclopentenes and cyclohexenes at 298 K were determined and Arrhenius parameters were also calculated from the experimental data. The rate parameters obtained in this study are compared with previous literature data and discussed in terms of structure–reactivity relationships.

A study of the IR and UV-Vis absorption cross-sections, photolysis and OH-initiated oxidation of CF3CHO and CF3CH2CHO

Infrared and ultraviolet-visible absorption cross-sections, effective quantum yields of photolysis and OH reaction rate coefficients for CF3CHO and CF3CH2CHO are reported. Relative rate measurements at 298(2) K and 1013(10) hPa, give k(OH + CF3CHO)/k(OH + CH3CH3) = 2.00(13), k(OH + CF3CH2CHO)/k(OH + CH3CH2OH) = 1.21(5) and k(OH + CF3CH2CHO)/k(OH + HC(O)OC2H5) = 3.51(9) (2σ). The effective quantum yield of photolysis was measured under pseudo-natural conditions in the European simulation chamber, Valencia, Spain (EUPHORE). Over the wavelength range 290–400 nm, the effective quantum yields of photolysis for CF3CHO and CF3CH2CHO are less than 2 × 10−2 and 4 × 10−2, respectively. The tropospheric lifetimes are estimated to be: τOH(CF3CHO) ∼ 26 days; τphotol(CF3CHO) > 27 days; τOH(CF3CH2CHO) ∼ 4 days; τphotol(CF3CH2CHO) > 15 days.

Studies of the gas phase reactions of linalool, 6-methyl-5-hepten-2-ol and 3-methyl-1-penten-3-ol with O3 and OH radicals.

The reactions of three unsaturated alcohols (linalool, 6-methyl-5-hepten-2-ol, and 3-methyl-1-penten-3-ol) with ozone and OH radicals have been studied using simulation chambers at T ∼ 296 K and P ∼ 760 Torr. The rate coefficient values (in cm(3) molecule(-1) s(-1)) determined for the three compounds are linalool, k(O3) = (4.1 ± 1.0) × 10(-16) and k(OH) = (1.7 ± 0.3) × 10(-10); 6-methyl-5-hepten-2-ol, k(O3) = (3.8 ± 1.2) × 10(-16) and k(OH) = (1.0 ± 0.3) × 10(-10); and 3-methyl-1-penten-3-ol, k(O3) = (5.2 ± 0.6) × 10(-18) and k(OH) = (6.2 ± 1.8) × 10(-11). From the kinetic data it is estimated that, for the reaction of O(3) with linalool, attack at the R-CH═C(CH(3))(2) group represents around (93 ± 52)% (k(6-methyl-5-hepten-2-ol)/k(linalool)) of the overall reaction, with reaction at the R-CH═CH(2) group accounting for about (1.3 ± 0.5)% (k(3-methyl-1-penten-3-ol)/k(linalool)). In a similar manner it has been calculated that for the reaction of OH radicals with linalool, attack of the OH radical at the R-CH═C(CH(3))(2) group represents around (59 ± 18)% (k(6-methyl-5-hepten-2-ol)/k(linalool)) of the total reaction, while addition of OH to the R-CH═CH(2) group is estimated to be around (36 ± 6)% (k(3-methyl-1-penten-3-ol)/k(linalool)). Analysis of the products from the reaction of O(3) with linalool confirmed that addition to the R-CH═C(CH(3))(2) group is the predominant reaction pathway. The presence of formaldehyde and hydroxyacetone in the reaction products together with compelling evidence for the generation of OH radicals in the system indicates that the hydroperoxide channel is important in the loss of the biradical [(CH(3))(2)COO]* formed in the reaction of O(3) with linalool. Studies on the reactions of O(3) with the unsaturated alcohols showed that the yields of secondary organic aerosols (SOAs) are higher in the absence of OH scavengers compared to the yields in their presence. However, even under low-NO(X) concentrations, the reactions of OH radicals with 3-methyl-1-penten-3-ol and 6-methyl-5-hepten-2-ol will make only a minor contribution to SOA formation under atmospheric conditions. Relatively high yields of SOAs were observed in the reactions of OH with linalool, although the initial concentrations of reactants were quite high. The importance of linalool in the formation of SOAs in the atmosphere requires further investigation. The impact following releases of these unsaturated alcohols into the atmosphere are discussed.

Rate constants for the gas-phase reactions of OH radicals with nitroethene, 3-nitropropene and 1-nitrocyclohexene at 298 K and 1 atm

Abstract Rate constants for the reactions of OH radicals with CH2=CHNO2, CH2=CH-CH2NO2 and c-C6H9NO2 have been determined at 298 ± 2 K and a total pressure of 1 atm. Rate data for the reaction of OH radicals with CH2=CH2, CH2=CH-CH3 and c-C6H10 were also obtained for comparison purposes. The OH rate data for all the compounds except c-C6H9NO2 were obtained using the absolute technique of pulse radiolysis combined with kinetic UV spectroscopy. Water-argon mixtures were irradiated and the OH kinetics recorded by following the transient light absorption at 309 nm. The rate constant for the OH + c-C6H9NO2 reaction was derived using a conventional photolytic relative-rate method. The results are discussed in terms of structure-reactivity relationships.