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The Master Chemical Mechanism (http://mcm.leeds.ac.uk/MCM) has been updated to version MCMv3.1 in order to incorporate recent kinetic and mechanistic improvements in the understanding of aromatic photo-oxidation. Carefully designed experiments have been carried out in the European Photoreactor (EUPHORE) to investigate key subsets of the toluene system. These results have been used to evaluate the toluene mechanism in MCMv3.1 (and by extension the other aromatic mechanisms in MCMv3.1) and where appropriate to refine the degradation schemes. MCMv3 and MCMv3,1 have also been evaluated using a high quality EUPHORE dataset on the photo-oxidation of benzene, toluene, p-xylene and 1,3,5-trimenthylbenzene. Significant deficiencies have been identified in the mechanisms, in particular: (1) an over-estimation of the ozone concentration, (2) an under estimation of the NO oxidation rate and (3) an under-estimation of OH. The use of MCMv3.1 improves the model-measurement agreement in some areas but significant discrepancies remain. Ideas for additional modifications to the mechanisms and for future experiments to further our knowledge of the details of aromatic photo-oxidation are discussed.

Large quantities of harmful VOCs are emitted into the troposphere from anthropogenic sources (Calvert , 2002). Aromatic hydrocarbons are an important class of VOCs present in the atmosphere, which contribute significantly to the chemistry of urban air (Atkinson, 2000; Atkinson and Arey, 2003). Estimations of the global emissions of aromatic hydrocarbon suggest that they comprise between 17-25% of the total anthropogenic NMVOC emissions (Calvert , 2002). The degradation of aromatic hydrocarbons is mainly initiated during the day by reaction with the hydroxyl radical (OH). Based on the presently accepted but inadequate mechanism for the atmospheric degradation of aromatic hydrocarbons, it has been calculated that this class of VOC could account for up to 30% of the photooxidant formation in urban areas (Derwent , 1996, 1998). Besides the photooxidant formation, this class of hydrocarbon is also assumed to make a significant contribution to secondary organic aerosol (SOA) formation in urban areas (Odum , 1996; Forstner , 1997; Hurley , 2001). This ranks aromatic hydrocarbon as one of the most important classes of hydrocarbons emitted into the urban atmosphere. Formulation of a realistic photo-oxidation mechanism for aromatic hydrocarbons represents one of the most important and challenging problems remaining to be solved in chemical models of tropospheric photooxidant formation.

Oxygenated volatile organic compounds form a major component of the trace gases found in the troposphere (Singh ., 2001). They are emitted directly into the atmosphere from biogenic sources, solvent and fuel additives use and are also formed in the tropospheric oxidation of all hydrocarbons. A number of oxygenated compounds are used as alternatives to aromatic and halocarbon solvents both in terms of toxicity problems and in the case of aromatic compounds as a means of reducing the levels of oxidant formation in the troposphere. It is apparent that oxygenated compounds will thus play an increasing role in determining the oxidising capacity of the troposphere both on a regional and global scale.

Chlorinated solvents are widely used in metal degreasing, dry cleaning, paint stripping, extraction of pharmaceuticals and foodstuffs, and electronic circuit board production (Sidebottom and Franklin, 1996). Emission data for trichloroethene (CHCl=CCl) and tetrachloroethene (CCl=CCl) show that emissions of these compounds are declining steadily, largely as a result of constant improvements in the efficiency with which they are being used and recycled. While 1,1,1-trichloroethane (CHCCl) was formerly widely used as a solvent, it is now strictly regulated as an ozone-depleting compound under the Montreal Agreement. Despite the decline in emissions of these chlorinated solvents their atmospheric fate and impact is still of concern.

The branching ratio for the reaction of OH radicals with CHCOOD was determined at 298 2±K and at atmospheric pressure using an indoor smog chamber (300 L) coupled to two different detection systems: (i) Gas Chromatograph (GC) with FTIR detection for determination of [CHCOOD], (ii) Cavity Ring Down Spectroscopy (CRDS) using a telecommunication diode laser as a source for the determination of [HDO] and [HO]. The reaction CHCOOD + OH occurs via D-atom abstraction with an efficiency of 36±20 %.

Alkyl halides (RX: X = Cl, I) are an important source of halogens in the atmosphere. The major tropospheric sinks of these compounds are photolysis (RBr, RI) and reaction with OH radicals. In the case of alkyl iodides (RI) relative kinetic studies of their OH reactions in photoreactors are complicated by fast reactions with the O(P) atoms generated by the photochemical OH radical sources. below shows a ln-ln plot of the kinetic data from an experiment performed in a large photoreactor to determine the OH rate coefficient for the reaction OH + CHCHCHI relative to OH + ethene using the photolysis of methyl nitrite (CHONO) as the OH radical source. A recent example of the implementation of the relative kinetic technique for the determination of OH radical rate coefficients in a photoreactor can be found in Olariu et al. (2000).

The reaction between CHOH and CFO has been studied in the temperature range between 20 and 122°C. A considerable decrease in the reaction rate with increase in temperature was noted, indicating a contribution of heterogeneous character. Different types of surfaces and coatings have been used. The possible involvement of aerosols for the reaction to occur in the atmosphere is discussed.

CFO radicals are present in the atmosphere as oxidized species of a series of CFC's, HFC's and HCFC's. They react with NO species. In particular, the reaction between CFO and NO which shows two reaction channels

Dimethyl sulphide (CHSCH, DMS) is the dominant natural sulphur compound emitted from the world’s oceans (Berresheim ., 1995, Urbanski and Wine, 1999), accounting for about one quarter of global sulphur gas emissions. Oceanic DMS, through its oxidation products, is proposed to play a key role in climate regulation, especially in the remote marine atmosphere (Charlson ., 1987).

Selected results of environmental chamber experiments carried out in the new large indoor environmental chamber at the University of California at Riverside (UCR) that are relevant to quantifying ozone impacts of volatile organic compounds (VOCs) are described. Issues and data needs for quantification of VOC reactivities towards ground-level ozone are described. The ability of the current SAPRC-99 chemical mechanism to simulate recent data from this chamber concerning the ozone formation from irradiations of an ambient reactive organic gas (ROG) - NO mixture and several new aromatics experiments are discussed. It was found that the mechanism consistently under predicts ozone formation in ambient surrogate - NO experiments at low ROG/NO ratios, and also under predicts the effects of adding CO to aromatic - NO irradiations. The two problems may be related and suggest problems with the formulation of current mechanisms for atmospheric reactions of aromatics.