Garry D. Hayman

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.

Photochemical ozone creation potentials for oxygenated volatile organic compounds: sensitivity to variations in kinetic and mechanistic parameters

The sensitivity of Photochemical Ozone Creation Potentials (POCP) to a series of systematic variations in the rates and products of reactions of radical intermediates and oxygenated products is investigated for the C4 alcohols, 1-butanol (n-butanol) and 2-methyl-1-propanol (i-butanol), using the recently developed Master Chemical Mechanism (MCM) as the base case. The POCP values are determined from the calculated formation of ozone in the boundary layer over a period of approximately five days along an idealised straight line trajectory, using a photochemical trajectory model and methodology described in detail previously. The results allow the relative impacts on calculated ozone formation of various classes of chemical reaction within the degradation chemistry to be assessed. The calculated POCP is found to be very insensitive to many of the changes investigated. However, it is found to be sensitive to variations in the rate coefficient for the initiating reaction with OH (kOH), although the sensitivity decreases with increasing kOH. The POCP appears to vary approximately linearly with kOH at low values (i.e. kOH less than ca. 4×10-13 cm3 molecule-1 s-1), whereas at high reactivities (i.e. kOH greater than ca. 4×10-11 cm3 molecule-1 s-1), the calculated POCP value is comparatively insensitive to the precise value of kOH. The POCP is also very sensitive to mechanistic changes which influence the yields of unreactive oxygenated products (i.e. those with OH reactivities below ca. 10-12 cm3 molecule-1 s-1), for example acetone. The propensity of the organic compound to produce organic nitrates (which act as comparatively unreactive reservoirs for free radicals and NOx) also appears to have a notable influence on the calculated POCP. Recently reported information relevant to the degradation of oxygenated VOCs is then used to update the chemical schemes for the 17 alcohols and glycols, 10 ethers and glycol ethers, and 8 esters included in the MCM, and new schemes are incorporated for dimethoxy methane (CH3OCH2OCH3) and dimethyl carbonate (CH3OC(O)OCH3), which are proposed fuel additives. New or updated POCP values are calculated for all 37 oxygenated VOCs and, where applicable, these are compared with the previous POCP values and reported Maximum Incremental Reactivity (MIR) values.

Methods for comparing gridded inventories of atmospheric emissions—application for Milan province, Italy and the Greater Athens Area, Greece

A set of methods has been compiled to compare gridded air emission inventories that have been derived, on the same spatial grid, using widely differing techniques. Top-down and bottom-up inventories for Milan, Italy and for the Greater Athens area (GAA), Greece were used to test and apply these methods. The applicability of each method to certain source sectors was assessed by conducting sensitivity analyses. Whilst some of the methods (such as regression calculations or the Moran coefficient) appeared very sensitive to variations of point source emissions, others (e.g. the construction of difference maps) proved more appropriate for characterizing line source differences. Area sources could best be dealt with by using the newly developed acceptability criterion. The development of these tools allows a reproducible comparison of sets of emission inventories and consequently supports developments towards improvement.

Kinetics of the reaction of IO radicals with HO2 radicals at 298 K

The molecular-modulation—UV/visible-absorption-spectroscopy technique has been used to investigate the simultaneous time-resolved behaviour of IO and HO2 produced by the photolysis of O3 in the presence of CH3OH, I2, O2 and Ar. This has enabled a direct measurement of the rate coefficient for the reaction IO + HO2 → HOI + O2 to be made at 760 Torr and 289 K. The value obtained was k5= (6.4 ± 0.7) × 10−11 cm3 molecule−1 s−1.

Laboratory Studies of Peroxy Radical Reactions of Importance for Tropospheric Chemistry

Studies of the UV absorption spectra as well as the kinetics and products of reactions of the following organic peroxy radicals have been carried out: CH3O2, CH2=CHCH2O2, HOCH2CH2O2, CH3CH(OH)CH(O2)CH3, (CH3)2C(OH)CH2O2, (CH3)2CH(OH)CH(O2)(CH3)2, CH3OCH2O2, CH3C(O)CH2O2 and C2H5C(O)O2. These radicals were chosen to be representative of those formed from the degradation of some classes of volatile organic compound (VOC) likely to be important in tropospheric chemistry (i.e. alkanes, alkenes, conjugated dienes, alcohols, ethers, aldehydes and ketones). Kinetic studies of the self reactions and reactions with HO2 were carried out using the molecular modulation technique and the laser flash photolysis technique, coupled with UV absorption spectroscopy. Product studies were carried out by long pathlength Fourier transform infra-red spectroscopy and UV-visible diode array spectroscopy. This work has also allowed investigation of a variety of laboratory sources of peroxy radicals.