Effects of the oXidation of Aromatic Compounds in the Troposphere

Contract number: EVK2-CT-1999-00053

Project Coordinator:

Prof. Mike Pilling, School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK

Contents

[Consortium members and web links]  [Introduction]  [Innovation]  
[The EUPHORE chamber] [Project work plan]  [Workpackage description]  
[Collaboratory for Aromatic Oxidation Mechanisms] [Publications]   
[For further information...]   [Useful links]  [References]

Consortium members and web links

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Introduction

It has been estimated that aromatic species contribute about 10% to the total global anthropogenic non-methane hydrocarbon (NMHC) emissions, the major source being car exhaust from petrol-vehicles, with a significant contribution from solvent usage (Houweling et al., 1998).  For instance, for UK emissions of toluene, which is the second highest emitted VOC species after butane, petrol usage accounts for 51% and solvent usage 41% (PORG, 1997).  For larger species such as p-xylene, the percentage of emissions from petrol usage rises to 78%, whilst solvent usage accounts for only 15% (PORG, 1997).  In regions where there are relatively strong traffic emissions, the percentage contribution of aromatics to total NMHC emissions can be substantially greater than 10%. In the UK, for example, aromatic emissions make up about 21% of the total emissions of NMHC (PORG, 1997). The contribution of aromatics to the formation of regional ozone is even greater, because of their high reactivity (high ozone creation potentials). Again taking the UK as a European example, Derwent et al. (1996) have estimated that the contribution of aromatics to ozone production in the UK is 30%.

In addition to their high atmospheric photochemical reactivity and their consequent major influence on the formation of tropospheric ozone and on the oxidising capacity of the atmosphere, there is now strong evidence that the photochemistry of aromatic compounds can lead to the formation of secondary organic aerosols (SOA), which are known to be harmful to human and ecosystem health (Forstner et al., 1997; Liang et al., 1997).  In fact, studies have shown that the atmospheric organic aerosol formation potential of whole gasoline vapour can be accounted for solely in terms of the aromatic fraction of the fuel (Odum et al., 1997).  As emissions of aromatics are concentrated in urban areas, where many people live and work, the formation of secondary organic aerosols becomes a more acute problem.

The assessment of the impact of European aromatic emissions on regional ozone formation, on secondary organic aerosol and on the oxidising capacity of the atmosphere must be based on a proper understanding of the chemical mechanisms for their atmospheric oxidation.

The mechanisms describing the oxidation of aromatic species need urgent attention.  Despite much recent research, the rate of formation of ozone cannot be accounted for by current mechanisms.  Calculations of the impact of aromatics on ozone formation have led to estimates that are clearly too low (Dr. R.G. Derwent, UK Meteorlogical Office, Bracknell, UK, personal communication). A quantitative understanding of the tropospheric oxidation of aromatic compounds is required, to enable the development of policy on emissions of these species.  A better mechanistic understanding would assist the control of photochemical smog and transboundary air pollution, and allow the assessment of the impact of aromatic species on climate change caused by the formation of ozone and aerosols.  Even if Europe follows the USA in reducing the aromatic content of fuel, there will still be significant emissions from solvent usage to contend with.  It is also likely that the expanding car industry in Eastern Europe will result in large aromatic emissions there in the future.

Ozone formation in much of northern Europe is limited by emissions of VOCs rather than of nitrogen oxides, and policy must, therefore, recognise that VOC controls in general and control of aromatics in particular, must form a central plank of strategies for the reduction of ozone exceedances. Secondary particulates form a substantial (around 30%) component of the overall atmospheric particulate loading in Europe. In northern and central Europe, aromatics make a significant contribution to the formation of SOA and hence to the particulate loading. Particulates represent a substantial threat to human health, especially in urban areas, and their control and reduction are important national and EU goals.

The investigation of the problems described requires the multidisciplinary approach. We have assembled a team of European scientists encompassing all of the skills needed to address the issues defined.  The expertise and planned interactions within this team make it unique in a world-wide context and ensure the strength required to solve the problems to be undertaken. The timing of such an initiative is helped by  (i) the recent development of new techniques for studying important short-lived intermediates in aromatic oxidation directly in the laboratory, (ii) advances made in mechanistic investigations through the combination of synthetic organic chemistry and modern analytical techniques enabling the synthesis of key intermediates for direct photochemical study (iii) the successful developments at the European Photochemical Reactor (EUPHORE) for the study of oxidation under atmospheric conditions, with comprehensive instrumentation and (iv) the proven use of a master chemical mechanism (MCM) in incorporating our understanding of detailed chemical mechanisms in evaluated models that can be used for policy applications (Derwent et al., 1998).

These considerations have led to the design of project EXACT. The overall aims of the proposed programme are:

1.   To investigate the kinetics and mechanism of the oxidation of benzene, toluene, xylenes and 1,3,5-trimethylbenzene using laser flash photolysis/absorption spectroscopy, coupled with ab initio calculations (Bordeaux and Hannover) , and a combination of synthetic organic chemistry (Newcastle) and steady-state photochemistry (Cork, Wuppertal). These data will then be incorporated in the master chemical mechanism, together with estimates of uncertainty (Leeds). The deliverables of this section of the programme will be:

2.   To evaluate the mechanisms by performing experiments at EUPHORE designed to test key aspects of the mechanism, especially in relation to the formation of ozone (Leeds, Cork, Wuppertal, CEAM). The deliverables will be:

3.   To investigate formation of aerosols (i) in the oxidation of the aromatics and aromatic mixtures and (ii) by their partially oxidised products (e.g. muconaldehydes) (Wuppertal. Newcastle). Deliverables will be:

4.  To carry out modelling of the effects of aromatics on regional scale ozone formation, in the European context, using a photochemical trajectory model. Assessments will also be made of secondary aerosol formation and of the formation of longer lived species, e.g. ketones, which can be exported to the free troposphere where they can affect global chemical processes. A careful analysis will also be made of model uncertainties and their impact on model outputs. (Leeds). Deliverables will be:

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Innovation

The oxidation of aromatic compounds proceeds via abstraction from any alkyl substituent and, primarily, by addition of OH to the ring. The adduct can react with O₂ to form the following observed products (Kwok et al., 1997): (i) an oxepin + HO₂; (ii) a peroxy radical which subsequently reacts with NO; (iii) a phenol + HO₂; (iv) a bicyclic peroxy alkyl radical; (v) an epoxide alkoxy radical.

Earlier mechanisms were based largely on channel (ii) and led to high photochemical ozone creation potentials (POCPs), in agreement with smog chamber studies, although the predicted values were higher than expected from experiment. The current implementation of the MCM identifies channel (i) as the most important, based on available data (Klotz et al., 1995) when the aromatic schemes were constructed (1997). This route leads to “prompt” formation of HO₂ in agreement with experiment, but generates ozone at a much lower initial rate than is observed experimentally (Klotz et al., 1999). Calculation of POCP values using the MCM with the channel (i) products gave unrealistically low values, which are incompatible with smog chamber measurements (Jenkin et al., 1999). Thus the oxepin mechanism may not be as significant as had been thought and it has even been suggested that there is no reaction via this route (Klotz et al., 1999; Ghigo and Tonachini, 1998).

Recently, Bjergbakke et al. (1996) detected the benzene-OH adduct via its absorption spectrum between 250-350 nm. Subsequent laser flash photolysis experiments showed that the reaction of the adduct with O₂ leads to rapid equilibration, but with an unexpectedly small equilibrium constant, K_c = 2.7 x 10^-19 cm³ (Bohn and Zetzsch, 1999). This conclusion is confirmed by theoretical calculations (Lay et al., 1996) and by measurements in the liquid phase (Pan and von Sonntag, 1990). Bohn and Zetzsch (1999) also detected the formation of the peroxy radical from the decreased absorption of the OH-adduct in rapid equilibrium with the peroxy radical in the presence of O₂ and from an apparently decreased O₂-reactivity of the adduct at high O₂. These results show that concentrations of the OH adduct and the peroxy radical are comparable under atmospheric conditions, in contrast to other peroxy radical formation reactions that are essentially irreversible at these temperatures. The equilibrated pair react with an atmospheric lifetime of 2.5 ms, much shorter than that of a typical peroxy radical, which precludes its reaction with NO to form NO₂ and hence O₃, under atmospheric conditions. The reaction of the pair, accounting for the 2.5 ms lifetime, is either a second channel of the adduct + O₂ or a unimolecular reaction of the peroxy radical. The reactions of the adduct with NO_x and O₂ have also been observed less directly by Knipsel et al. (1990).

There have been several investigations of the mechanism of the oxidation of aromatics, with and without added NO, using product analysis (Atkinson, 1994; Atkinson and Aschmann, 1994; Klotz et al., 1998; Knipsel et al., 1990; Kwok et al., 1997; Yu et al., 1997; Yu and Jeffries, 1997). These studies have led to the identification of channels (i)-(v) through the identification of products including substituted benzaldehydes from the minor abstraction route, phenols from channel (iii) and a range of dialdehydes and diketones, which result from ring cleavage. Mass balance is generally poor and there has been only limited quantification of the kinetics of the intermediates (e.g. Kwok et al., 1997; Klotz et al., 1998).

A recent collaboration between two of the partners defined efficient synthetic methods for obtaining several key compounds for smog chamber studies.  Especially notable were the development of a new method for preparing muconaldehydes and the synthesis of novel precursors of the 6-hydroxycyclohexa-2,4-dienyl radical.  These successes will be built upon in the present project by the extension of the synthetic methodology to related compounds (e.g. those corresponding to toluene).

Recent measurements suggest that oxidation of aromatic hydrocarbons is one of the principal contributors to the formation of aerosol in urban areas. Smog chamber studies give conflicting results for the yields, however, with the difference being quite probably due to the different conditions employed, i.e. natural sunlight versus fluorescent lamps. Factors controlling the aerosol yield such as temperature and solar intensity have not yet been systematically investigated. Further, the identity of many of the compounds responsible for the aerosol formation, as well as the reaction mechanisms that lead to the observed products, have not been identified (Seinfeld and Pandis, 1998).

The master chemical mechanism (MCM) was constructed for policy development from funding by the UK Department of the Environment. It has been extensively used to estimate photochemical ozone creation potentials for Europe (Jenkin et al., 1999). It is based on the best available kinetic and mechanistic data, much of which has derived from EU programmes. The MCM represents the best way currently available of using detailed chemical kinetic and mechanistic information to assess the atmospheric impact of individual species. It is available for use on the web and represents a general community research tool.

EXACT will contribute to an improved, quantitative understanding of the atmospheric chemistry of aromatic compounds, and the application of that understanding to a quantification of the effects of aromatic emissions in the atmosphere, in the following ways:

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The EUPHORE chamber in Valencia

The chamber experiments in EXACT will take place in the European Photoreactor (EUPHORE) chamber in Valencia, Spain. EUPHORE contains two chambers, each one consisting of a half-spherical Teflon bag with a volume of about 204 m³. The chambers are made from a fluorine-ethene-propene (FEP) foil with a thickness of 0.127 mm, which has a transmission >80% in the wavelength range between 280 and 640 nm. The chambers are protected from the elements when not in use by two half-sphere shaped protective houses.

Each chamber floor consists of 32 symmetrically arranged aluminium panels (2.50 x 1.25 m) covered with FEP foil (0.25 mm). Located on each chamber floor are 10 openings with a diameter of 0.6 m and 20 openings (diameter of 0.2 m), each closed with aluminium flanges. The chamber framework structure itself is supported on a circular wall and two heavy crossed girders. To compensate for heating of the chamber by solar radiation the chamber floor panels are cooled by a refrigerating system with a cooling capacity of 0.75 kW/m². The inlet and outlet ports and other accessories, like mixing fans, analytical systems, mechanical excess pressure valves, are located on the floor to leave the chamber surface is free for light entry. Integrated on the flanges are ports for the input of reactants and the sampling lines for the different analytical instruments.

Each chamber can be filled with air from a separate air purification system. The air is dried in absorption dryers (Zander, Type HEA 1400) with an air throughput of ca. 500 m³/h. With the help of a special charcoal adsorber NO_X is eliminated and oil vapour and non-methane hydrocarbons are reduced to a limit of 0.0003 mg/m³. The air
flow into the chamber is controlled with a magnetic valve and is measured with a volumetric meter.

In addition to conventional analytical instrumentation (GC, GC-MS, HPLC, O₃, NO_X and NO_Y analysers, actinometers for j(O₃), j(O¹D) and j(NO₂)) the chamber is equipped with state-of-the-art in situ measurement techniques (DOAS, long-path FT-IR, TDL).

For trace gas detection in the infrared range between 400 and 4000 cm^-1 an FT-IR spectrometer (NICOLET, MAGNA550, resolution of 0.5 cm^-1, MCT-detector) in combination with a long-path absorption system is installed. The long path system is a White arrangement with a base length of R=8170 mm. The mirrors of the White system have a diameter of 406 mm and are mounted inside the chamber at a height of 500 mm on large flanges. The maximum optical path-length is 653.6 m, which corresponds to the optimum path-length of this system given by the reflectivity of the mirrors. Two symmetric transfer systems guide the collimated IR beam vertically from the spectrometer to the White system and back to the spectrometer. These transfer systems are mounted under the chamber floor. The second part of the transfer optic is integrated in the spectrometer. It focuses the beam corresponding to the original path in the spectrometer.

In order to measure trace gases and highly reactive species like NO₃ in the UV and the visible a DOAS instrument in combination with a White system is installed. In addition to a common White system three quartz prisms are set up near the front mirror to work as reflectors. These reflection prisms correct misalignment of the mirrors and double the number of passes. The two back mirrors are set up in two motorised gimbal mirror mounts to change the numbers of traversals. In principle 16, 48, 80, 112 and 144 traversals are possible, corresponding to a light path of 128 , 384, 640, 896, 1152 m, respectively. Two sets of mirrors can be used, one with an UV-enhanced aluminium coating with a reflectivity of about 88% in the range from 200 to 700 nm and one with a dielectric coating with a reflectivity of about 99% in the wavelength regions from 300 to 390 nm (O₃, NO₂, HCHO, HONO) and from 600 to 700 nm (NO₃). As light source a high pressure arc lamp is used. The exit beam of the White cell is transmitted with a quartzfiber bundle in front of the entrance slit of a Czerny-Turner spectrometer with a focal length of 0.5 m (Acton Spectra Pro 5). For detecting the spectra a photodiode array detector (PDA) is used.

This image shows one of the two chambers of EUPHORE, some of the instrumentation and some noteworthy features are shown. Note the two co-workers sitting on the protective housing behind the chamber (red circle), to give an idea of the chambers size.

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Project Work plan

This  project is a detailed study of the oxidation of key aromatic compounds and oxidation products, with relevance to the troposphere.  Laboratory work will be complemented by both chamber and modelling studies, to provide improved mechanisms for aromatic oxidation.

The project contains four key objectives:

1. To investigate the kinetics and mechanisms of the oxidation of benzene, toluene, p-xylene and 1,3,5-trimethylbenzene. Where appropriate, studies of the secondary chemistry will be broadened to other xylenes (WP1,2,3)

2.   To construct atmospheric oxidation mechanisms using information gained from (1) and to test specific aspects of them via experiments at EUPHORE (WP4,5)

3. To investigate the formation of secondary organic aerosols formed through the atmospheric oxidation of key aromatic species, through a combination of chamber and modelling studies (WP5,6)

4. To run atmospheric models, based on the new mechanisms, to assess the role of aromatics in regional scale ozone formation in Europe, in secondary aerosol formation and in global atmospheric chemical processes (WP7)

The EXACT project is multidisciplinary and will contain the following elements:

EXACT is split into seven workpackages:

1. Initial stages of aromatic oxidation

2. Secondary chemistry

3. Synthesis of substrates, intermediates and products

4. Experimental design for EUPHORE

5. EUPHORE experiments

6. Secondary organic aerosol formation

7. Model simulations of the effects of aromatic oxidation.

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Workpackage description

Workpackage 1: Initial stages of aromatic oxidation

Recent experiments demonstrated the formation of an intermediate peroxy-radical from benzene-OH adducts and O₂ and the existence of a fast equilibrium between the adduct and the peroxy-radical (Bohn and Zetzsch, 1999). As a consequence both radical species have similar effective lifetimes which are determined by loss reactions of the adduct and/or peroxy-radical. The lifetime of the coupled system is short, preventing reaction of the peroxy-radical with NO under typical tropospheric conditions and  diminishing the importance of this formerly discussed route of reaction. The question arises whether or not other atmospherically important aromatic compounds, behave similarly. Moreover, the temperature dependencies of the equilibrium constants, determined by the reaction enthalpies of peroxy-radical formation, are unknown. In the case of benzene, current theoretical predictions of the reaction enthalpy differ by a factor of ten and an experimental determination is necessary. These data are also required to assess the possibility of a change of product distribution with temperature.

An unknown in the degradation mechanism of aromatic compounds is the role of NO for the formation of ring-cleavage products. In current models these products are formed exclusively after conversion of intermediate peroxy-radicals with NO. However, there is experimental evidence that ring-cleavage products are also formed in the absence of NO and that, in the case of benzene, the yield of ´prompt´ HO₂, i.e. HO₂ formed with no preceding NO-reaction, is larger than the yield of phenol indicating contributions from other channels.  A study of the yield of ´prompt´ HO₂ by observing the formation kinetics of HO₂ as a function of NO concentration can reveal quantitative information on the importance of these reactions. Note that these questions also address the number of ozone molecules formed per aromatic molecule oxidised.

Experimental programme

Hannover

Investigations will be performed with a combination of pulsed-photolysis production of OH (248 nm laser-photolysis of H₂O₂) and detection of OH and aromatic-OH adducts by cw-laser long-path absorption at around 308 nm with a folded path length of 100 m. The main advantage of the method is its sensitivity, especially with respect to OH, which enables investigations under pseudo-first-order conditions for the radical species involved. Moreover, in contrast to methods monitoring OH fluorescence, the absorption technique can be used at total pressures of up to 1 bar in N₂, O₂ or N₂/O₂ mixtures. Aromatic-OH adducts are detectable in the same spectral region as OH, although with reduced sensitivity. To determine the equilibrium constants, decay-curves of aromatic-OH adducts will be recorded as a function of O₂-concentration (at up to 1 bar of O₂) as a function of  temperature for each compound under consideration. Recent measurements with benzene show that decay rates and absorption signals exhibit a nonlinear dependence on O₂ concentration which can be utilised to determine the equilibrium constants. In general a qualitatively similar effect is expected for toluene, and other methyl substituted aromatics. In case of the xylenes and trimethylbenzenes only the most symmetric isomers, i.e. p-xylene and 1,3,5 trimethylbenzene (mesitylene), will be investigated to limit the number of possible isomeric peroxy-radicals. Nevertheless, any trend with increasing number of CH₃-groups at the aromatic ring will become apparent while the range of the key aromatic compounds of importance for photochemical ozone production is still covered.

The questions concerning ´prompt´ HO₂ will be addressed by OH cycling-experiments were the kinetics of secondary HO₂ formation is monitored by recovering OH via HO₂ + NO. OH decay curves will be recorded over a wide range of O₂, NO and aromatic concentrations. Although in these experiments the presence of NO is needed to detect HO₂, an extrapolation of kinetic parameters to [NO] = 0 yields information on the kinetics of HO₂ formation in the absence of NO. Since the underlying kinetics is rather complex, numerical methods will be applied in the analysis of the decay curves. In recent experiments of this kind with benzene, a rate constant of the peroxy radical + NO reaction was determined and a ratio of ´prompt´ [HO₂]: [phenol] > 2 was found, indicating HO₂ forming channels in addition to channel (iii). However, the results were not conclusive because the influence of the peroxy radical + NO reaction could not be sufficiently suppressed. In the case of toluene and the other methyl substituted aromatics the situation is more promising, since the effective lifetime of the peroxy radical is expected to be shorter making the peroxy radical + NO reaction less important. These experiments will be limited to toluene.

Bordeaux

These studies will be augmented by experiments using a broad band light source for absorption spectrometry, providing a wider range of monitoring wavelength and temperature, albeit at reduced sensitivity. Radicals will be generated by photolysing Cl₂/H₂/C₆H₆ mixtures to form H-C₆H₆ radicals and the laser/flash photolysis of H₂O₂/C₆H₆ mixtures for the HO-C₆H₆ radicals. Radical concentrations will be monitored in real time around 300 nm, where cyclohexadienyl-type radicals are known to exhibit a characteristic strong absorption band. While the study of the C₆H₇ radical has no atmospheric interest, experience confirms that valuable information can be obtained by investigating systems, which are close to those of direct interest for the atmosphere. A principal objective will be better characterisation of the equilibrium constant as a function of temperature from 260 K and above. Attempts will also be made to characterise the peroxy absorption between 270 and 300 nm if it can be separated from that of other species. Unfortunately, preliminary attempts with the C₆H₇ radical, indicate that monitoring the HO₂ absorption around 210 - 220 nm is not possible because all other species absorb in this spectral range and deconvolution is not possible.

It is now established that the reaction of the adduct with NO is unimportant in the atmosphere. However, this reaction may have to be taken into account in laboratory experiments, such as the OH cycling experiments by Partner 3. The reaction will be studied with the principal objective of characterising the equilibrium for the HO-C₆H₆ + NO reaction and measuring the rate constants for the forward and back reactions.  In a more speculative series of experiments, attempts will also be made to monitor the peroxy radical directly.

These systems will also be studied theoretically using BAC-MP4 (high level ab-initio plus bond additivity corrections), and density functional theory (DFT). These methods have been tested by calculating the stability of a series of adducts X-C₆H₆; the results are in satisfactory agreement with the available experimental data that have been obtained (Berho et al., 1999). The same methods are used to calculate molecular entropies for a better characterisation of equilibrium parameters.  Quantum calculations will be carried out in order to estimate the stability of the peroxy radical and for the determination of the equilibrium parameters using the Third Law method.

 Experiments by Bordeaux and Hannover complement one another through the determination of rate and thermodynamic parameters using different techniques and under significantly different conditions. The fast equilibrium between the adduct and O₂ has only been observed for benzene using laser absorption and studies over a wider wavelength range would be of value. If attempts to monitor the peroxy radical are successful, then the effects of O₂ and NO can be studied over a much wider range of conditions.

Workpackage 2: Secondary chemistry

Background

The initial chemistry leads to a range of intermediates including phenols, aldehydes and open chain dicarbonyls. This section of the project will investigate the formation and reactions of these species, guided by the results of WP1 and aided by the synthesis of intermediates in WP3. An integrated programme of work will be conducted by Cork and Wuppertal.

Experimental programme

These studies will be carried out in large volume photoreactors using long path UV/VIS and Fourier transform infrared spectroscopy and various chromatography techniques (GC-ECD, GC-FID, GC-MS/MS) for the analyses. One of the photoreactors can be temperature regulated down to -60^oC with an accuracy of ± 0.5^oC, enabling experiments over an extended temperature range. Measurement of the photolysis frequencies will be performed at the EUPHORE photoreactor installed at CEAM, Valencia, Spain.

Alkyl substituted hydroxylated benzenes, i.e. phenolic-type compounds are major products of the OH-radical initiated oxidation of aromatic hydrocarbons (BTX: benzene, toluene and the xylene isomers) accounting for up to 25% of products in the case of benzene. Although the kinetics of the reactions of these compounds with OH and NO₃ have been investigated quite thoroughly, the products of the reactions, particularly with OH, are virtually unknown. Reactions with NO₃ have been shown to produce nitrophenols in large yield; however, these investigations have been performed in high NOx environments and the results may not be transferable to the atmosphere. The products from the reaction of OH with phenol, o-, m- and p-cresol and hydroxylated 1,3,5-trimethylbenzene will be investigated using a combination of in situ FTIR and GC-MS/MS techniques for the product identification. Investigations will be performed in the presence and absence of NO. In some experiments, in order to order to avoid the photolysis of dicarbonyl products, the reaction of ozone with 2-methylbut-2-ene will be used as a dark source of OH radicals. NOx and O₃ analysers will provide additional data for  kinetic modelling of the reactions to be performed for validation purposes.

A key aspect will be study of the OH-initiated photooxidation of phenolic compounds. If the behaviour of the phenolic compounds is analogous to that of the precursor aromatic hydrocarbons then dihydroxy compounds are expected as major products. Virtually nothing is presently known about the atmospheric fate of these compounds, and kinetic and product analyses of these reactions with NO₃ and OH are likely to be a major task in the project.

Unsaturated dicarbonyl compounds such as hexene-2,5-dione and other alkyl substituted butenedials are known reactive intermediates in the photooxidation of aromatic hydrocarbons. The butenedials, in particular, are photochemically labile and are an important source of radicals in the aromatic photooxidation mechanism. The atmospheric photolysis frequencies and the branching ratios for the various photolysis product channels for this type of compound are very poorly characterised at present. Many of the expected dihydroxy aromatic and dicarbonyl products are not commercially available.   In such cases the compounds will be synthesised by Partner 6.

Validation of the reaction mechanisms will be performed through experiments at EUPHORE. In addition it is envisaged that the photolysis of selected unsaturated dicarbonyl species such as 2-methyl-2-butendial and 2-methyl-4-oxo-2-pentenal will be performed using sunlight at EUPHORE. To avoid interference from OH radicals, an excess concentration of a radical trap such as cyclohexane will be added to the system.  Alternatively, the concentration of OH radicals generated from photolysis can be measured in situ and also by the loss rate of a reactive tracer such as butyl ether.

Workpackage 3: Synthesis of substrates, intermediates and products

 Background

The atmospheric oxidation chemistry of aromatic compounds is complex and attempts to elucidate the full mechanism using conventional photoreactor studies have been unsuccessful.

Additional constraints on and information for the deduction of the mechanism can be obtained by synthesising key intermediates and precursors of these intermediates and investigating their reactions directly. This work will be performed by Newcastle, with input from WP1 and WP2.

Experimental programme

Several oxepins, epoxides and aldehydes are intermediates in proposed reaction pathways and are therefore needed for reference purposes.  In addition, specific compounds (e.g. methyl-substituted muconaldehydes) may be needed for further experiments in their own right, e.g. for oxidation with hydroxyl radicals, for investigation of aerosol formation.  The required compounds will be synthesised by adapting an existing methodology for the benzene-related systems of benzene oxide/oxepin and the derived muconaldehyde isomers (Bleasdale et al., 1997, 1999) using oxidation (e.g. using ceric ammonium nitrate) to a 1,6-dioxo-hepta-2,4-diene isomer (see Scheme 1). Each of the different muconaldehyde isomers can be obtained in pure form by selecting appropriate reaction conditions.  To extend this chemistry, for example, to materials relevant to the atmospheric degradation of toluene, 1-methylcycohexa-1,4-diene will be converted into toluene-1,2-oxide/2-methyloxepin, which will be oxidised (e.g. using ceric ammonium nitrate) to a 1,6-dioxo-hepta-2,4-diene isomer.  The intermediate cyclohexadienes in this and related methodology will be oxidised with a peracid to give relevant epoxides.  All of the compounds from this aspect of the study will be relatively stable and transportable to Partner laboratories.  It is the aim to obtain all of the compounds as homogeneous species analysed by HPLC and fully characterised by spectroscopic methods (e.g. ¹H and ¹³C NMR).

Peroxides, hydroperoxides and free radicals are relatively unstable and not transportable to Partner laboratories.  The aim will be to generate relatively stable precursors of these species that can be converted into the required end product by a simple, well-defined process, the final step(s) having been validated in the laboratory of Partner 6.  Peroxides will be obtained by photosensitised addition of oxygen to appropriate cyclohexa-1,3-dienes, while trityl-protected hydroperoxides will be synthesised by nucleophilic displacements employing a trityl hydroperoxide as the nucleophile (Henderson, et al. 1997).  The trityl-protected hydroperoxides should be transportable and then convertible into the free hydroperoxides by a mild acidic hydrolysis.  Relatively stable precursors of hydroxycyclohexadienyl radicals (i.e. adducts from the addition of hydroxyl radicals to benzene, toluene etc.) will be synthesised from benzene oxide and methyl-substituted benzene oxides.  A promising synthetic route to a precursor of the 2-hydroxycyclohexa-3,5-dien-1-yl radical has been devised by Partner 6, which needs further refinement and extension to methyl-substituted radicals.

 Scheme 1: Synthesis of 1,6-dioxo-hepta-2,4-diene isomers

Workpackage 4: Experimental design for EUPHORE

Background

Workpackages 1-3 concentrate on detailed studies of the oxidation mechanisms. The overall aim, of EXACT is to provide mechanisms that contain the best available information on this detailed chemistry, for the purpose of describing the effects of aromatic compounds in the atmosphere. It is therefore important that the mechanisms are tested under conditions which approach those found in the atmosphere. This aim will be achieved through the use of EUPHORE. The aim of WP4 is to design experiments at EUPHORE that fully test key aspects of the mechanisms, which assess the sensitivity of the measurements to model features and to determine model uncertainties to facilitate a proper statistical interpretation of the measurement/model comparison. WP4 will be conducted by Leeds in collaboration with AEA Technology

Programme

The experiments will be planned using simple box models, designed to describe EUPHORE and including a good description of chamber radical sources based on experiments performed at EUPHORE over the last three years. The initial mechanisms will contain all possible channels, with variable channel efficiencies (a_i) and will be used to generate profiles of EUPHORE observables as a function of these efficiencies. The sensitivity of the variables, and especially of ozone formation, to changes in a_i will then be investigated numerically.  As thermokinetic and mechanistic data become available from WP1,2, and the mechanisms are updated, new simulations and sensitivity analyses will be computed, together with the definition of optimal experimental conditions for testing the mechanisms in EUPHORE. The comparison between experiment and model will be supported by a full uncertainty analysis, facilitating a proper statistical assessment of the significance of any model/experiment differences.

Workpackage 5: EUPHORE experiments

Background

The atmospheric chemistry division at CEAM operates a technically advanced photochemical reactor, EUPHORE, which was constructed in collaboration with 7 research institutes from Europe, in order to tackle the complex and challenging environmental problems associated with photochemical smog formation. The outdoor simulation chambers are located on the second level in the southern part of the building and are made from a fluorine-ethene-propene (FEP) foil with a hemi-spherical structure and a volume of about 200 m³. They are equipped with a broad range of analytical instrumentation to measure trace gas concentrations as well as physical parameters. Among the equipment that will be available are advanced long path FTIR and UV/VIS differential optical absorption systems, HPLC adapted to fluorescence measurements of hydroperoxides, HPLC with UV/VIS detector of carbonyl compounds, GC/GC-MS systems with automatic pre-concentration sampling systems, j(O¹D) and j(NO₂) filter radiometer, spectral radiometer and scanning mobility particle sizer (SMPS). The mechanisms developed WP1and 2 will be tested in EUPHORE, using the conditions identified in WP4. WP5 will be conducted by CEAM, with inputs from Leeds, Cork and Wuppertal.

Experimental programme

Single aromatic compounds will be studied via concentration-time profiles of the major reactants, molecular intermediates and products will be measured using the available infrastructure. The particulate phase will be collected on filters and subsequently the organic content characterised. In order to determine the relative importance of individual chemical processes contributing to the production of aerosol particles, the experiments will be performed under very different initial conditions with respect to VOC and NO_X ratios. A newly developed procedure to perform smog chamber runs will be employed. This controlled emission method is able to maintain a constant NO_X concentration and at levels typical of the remote boundary layer.  The implementation of this technique is a further improvement with respect to a realistic representation of tropospheric chemistry in the boundary layer by use of an outdoor simulation chamber.

WP5 contributes directly to WP2,4 and 6. It is anticipated that three campaigns will be conducted of 2,4 and 4 months. The person months allocated to WP5 are restricted to those from CEAM, who will coordinate and run the reactor experiments. Contributions from Leeds, Cork and Wuppertal, to work at EUPHORE, are contained in WP2,4 and 6.

Workpackage 6: Secondary organic aerosol formation

Background

It is now well known that the oxidation of aromatic compounds leads to the formation of secondary organic aerosols (SOA).  The SOA are known to have a deleterious effect on human health and other ecosystems.  What is less well-known, are the pathways by which the SOA are formed, and which aromatics are significant precursors. WP6 will be conducted by Wuppertal.

Experimental programme

Aerosol formation from the aromatics and their oxidation products will be studied.  The oxidation products will include the mainly unsaturated dicarbonyls and possibly also phenolic compounds. Exploratory experiments will be performed in a 1000 l reactor using long path in situ FTIR to monitor the reactants and major products. The aerosol formation (size and number distribution) will be measured using a scanning mobility particle sizer (SMPS) in conjunction with a condensation nuclei counter (CNC). The dependence of reaction conditions on the aerosol yields will be examined, e.g. NO_X, light intensity, temperature. Validation experiments will be performed in EUPHORE under "true" solar conditions since the photolysis rates (reaction path branching ratios, quantum yields) could in some cases be significantly different between laboratory conditions and those experienced at EUPHORE.

Workpackage 7: Model simulations of the effects of aromatic oxidation

Background

The ultimate aim of EXACT is to assess the effects of the oxidation of aromatic compounds in the troposphere. The combined deliverables from WP1-6 are chemical mechanisms and an assessment of SOA formation. WP7 aims to use the mechanisms modify the MCM and to assess the effects of aromatics in the atmosphere; it will be conducted by Leeds in collaboration with AEA Technology.

Programme

The initial aim is to modify the MCM for the aromatic compounds studied directly and to develop an aromatics protocol to permit development of revised schemes for the 18 aromatic compounds in the MCM. These mechanisms will feed back to WP4, the experimental design workpackage. The MCM is very detailed and is not appropriate for some uses. In addition, it is often difficult to interpret the mechanistic implications of simulations. Accordingly, the mechanisms will be reduced objectively using sensitivity analysis. The approach to reduction will depend on the aims of the simulations and the definition of the important variables. The reductions can be substantial, leading to much easier use and interpretation of the mechanism

The mechanisms will also be used, in full or reduced form, in appropriate models, to assess the impact of aromatic oxidation on ozone formation. The updated degradation mechanisms will be incorporated into an existing photochemical trajectory model (PTM) designed to simulate the chemical development of boundary layer air masses over Europe, and used previously to calculate Photochemical Ozone Creation Potentials (POCP) as a measure of the contribution to regional scale ozone formation (Derwent et al., 1996; 1998). The sensitivity of POCP values to a series of systematic variations in the rates and products of reactions of radical intermediates and oxygenated products will be investigated for selected aromatic hydrocarbons. Particular emphasis will be placed on the influence of existing uncertainties in the mechanisms for the early stages of degradation, and to what extent these have been reduced by work performed within the present programme. The role of aromatic hydrocarbons in the generation of other gas-phase secondary pollutants and SOA will also be considered. Finally, attention will be given to the generation of comparatively low reactivity products (e.g. acetone, 2-butanone, biacetyl, organic nitrates) which may potentially be exported to the free troposphere and have an impact on global chemical processes.

In addition, it will be of interest to compare the new mechanisms with the results of field campaigns conducted near Berlin in 1998 (BERLIOZ) and in Birmingham in 1999/2000 (PUMA). These are heavily instrumented campaigns, that include radical measurements. Data from both campaigns are available to us. The aim will be to assess the radical chemistry and rates of formation of ozone and other oxidants using the modified MCM. As far as urban regions are concerned, the greatest uncertainties relate to the aromatic compounds. Thus the mechanisms developed in EXACT will be of great value in interpreting the results of the field campaigns.

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Collaboratory for Aromatic Oxidation Measurements (CAOM)

The CAOM will house a database for the experimental results from EXACT and an easily accessible location for on-going project and sub-project reports. Once they have been validated and agreed by the consortium, kinetic and mechanistic results, overall aromatic oxidation mechanisms and reports and papers will be mounted on the CAOM website. Access to parts of the site are controlled through passwords with selected components restricted to the EXACT consortium only, while more complete datasets and papers etc. will be made freely available (subject to copyright laws). 

Peer reviewed papers

Free access

Restricted access: EXACT members only

1. Preliminary results from product and kinetic studies on the reactions of OH and NO₃ with dihydroxybenzenes and benzoquinones (Romeo Olariu, Wuppertal)

2. HO₂ yields from OH-initiated oxidation of benzene, toluene and toluene d₈ determined in the presence of NO via cw-UV laser absorption of OH (Cornelius Zetzsch, Hannover)

3. Theoretical approach of the hydroxy-cyclohexadienyl + O₂ reaction: preliminary results (Marie-Therese Rayez, Bordeaux)

4. Reactions of cyclohexadienyl-type radicals (Robert Lesclaux, Bordeaux)

5. Reactions of Benzene Oxide-Oxepin (Christine Bleasedale, Newcastle)

6. Primary and secondary formation of glyoxal from BTX (Rainer Volkamer, Heidelberg)

7. OH-Oxidation of dimethylphenols - particle formation and possible reaction products (Lars Thuener, Cork)

8. Study of the particle formation during the photooxidation of toluene in the presence of NO_X (Klaus Wirtz, CEAM)

9. Preliminary data on aerosol formation from OH/NO₃ + phenols: experiments at Wuppertal and Valencia (Romeo Olariu, Wuppertal)

10. Development of aromatic oxidation mechanisms: Routes to observed products and product yield comparison (Mike Jenkin, AEAT)

11. Aromatic mechanism construction and initial comparison with some chamber data for toluene (Sam Saunders, Leeds)

12. Modelling of Toluene Chamber Experiments (Volker Wagner, Leeds)

13. Description of secondary organic aerosol formation from toluene oxidation (Mike Jenkin, AEAT)

14. Synthesis of important intermediates, (Bernard Golding, Newcastle)

1. Aromatic modelling and campaign outline (Volker Wagner, Leeds)

2. Current state of the development of aromatic mechanisms (Mike Jenkin, Imperial College) 

3. Synthesis of intermediates (Alistair Henderson, Newcastle)

4. 2D-GC analysis of intermediates (Jacqui Hamilton, Leeds)

5. Measurement facilities at EUPHORE (Klaus Wirtz, CEAM)

6. Determination of NO2 and glyoxal using DOAS spectrometer (Milagros Rodenas, CEAM)

7. Measurement of cresols using HPLC technology (Manuel Pons, CEAM)

8. Measurement of carbonyls using PFBH-derivatization and GC-MS analysis (Christina Maldonado, CEAM)

9. Review of the chemistry of unsaturated dicarbonyls (Lars Thuener, Cork).

10. Chemistry of cresols and dihydroxybenzenes (Romeo Olariu, Wuppertal)

1. EXACT-1 Campaign at EUPHORE (Klaus Wirtz, CEAM)

2. EXACT-1 Campaign: Model-Experiment Comparisons and Carbon and NOy budget for the 27/09/01 toluene experiment: preliminary results (Volker Wagner, Leeds)

3. An experimental investigation of the early stages of the oxidation of benzene and toluene by OH, part I and part II (David Johnson, Bordeaux)

4. Theoretical approach of the first steps of degradation of benzene hydroxycyclohexadienyl+O₂ reaction (Marie-Therese Rayez, Bordeaux)

5. Appraisal of the peroxy-bicyclic OH-regeneration mechanism and Comments on hydroxy-aromatic degradation chemistry in mechanisms (Mike Jenkin, Imperial)

6. Synthetic work for EXACT (Alistair Henderson, Newcastle)

7. New results on the atmospheric photo-oxidation of phenols: part I (Romeo Olariu, Wuppertal)

8. New results on the atmospheric photo-oxidation of phenols: part II (Alexandre Tomas, Wuppertal)

9. Formation of phenol-type compounds from BTX+OH (Rainer Volkamer, Heidelberg)

10. Formaldehyde formation from toluene+OH (Rainer Volkamer, Heidelberg)

11. Chemistry of xylene oxidation products (John Wenger, Cork)

12. Preliminary results of photolysis experiments on unsaturated dicarbonyls at EUPHORE (Lars Thuener, Cork)

A workshop was held in Leeds (23-31 January 2002) to discuss issues in mechanism development.
Participants : Mike Pilling, Claire Bloss, Volker Wagner. (University of Leeds), Mike Jenkin (Imperial College London), Rainer Volkamer (Institut fuer Umweltphysik at the University of Heidelberg, now at Massachusetts Institute of Technology.)
This collaboration has continued with further discussions on mechanism development and the EXACT-2 campaign.

Report on Leeds Workshop (Rainer Volkamer)

1. Initial stages of aromatic oxidation, experimental results (Robert Lesclaux, Bordeaux)

2. Initial stages of aromatic oxidation, experimental results (Cornelius Zetzsch, Hannover)

3. Initial stages of aromatic oxidation, theoretical calculations (Robert Lesclaux and Marie-Therese Rayez, Bordeaux)

4. The synthesis of EXACT intermediates and trapping of arene oxides as Diels-Alder Adducts (Bernard Golding, Newcastle)

5. Atmospheric Chemistry of Xylene Oxidation Products (Ger Rea, Cork)

6. Development of a code for SOA formation from toluene degradation (Mike Jenkin, Imperial College London)

7. Experimental design for EUPHORE experiments, (Claire Bloss, Leeds)

8. Third EXACT campaign, July 2002 (Klaus Wirtz, CEAM)

9. Summary of GCXGC and FAGE results, (Mike Pilling, Leeds)

10. Model simulations of the effects of aromatic oxidation - model evaluation (Claire Bloss, Leeds)

11. Impacts of aromatic oxidation - POCP calculations (Mike Jenkin, Imperial College London)

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Publications

The following peer reviewed papers have been published on work carried out within EXACT or are of direct relevance to the project. 

Berho, F. and Lesclaux, R. (2001).
Gas phase reactivity of the cyclohexadienyl radical with O-2 and NO and thermochemistry of the association reaction with NO.
Phys. Chem. Chem. Phys. 3(6), 970-979. 

Bohn, B. (2001).
Formation of peroxy radicals from OH-toluene adducts and O-2.
J. Phys. Chem. A 105(25), 6092-6101. 

Bohn, B. and Zetzsch, C. (1999).
Gas-phase reaction of the OH-benzene adduct with O-2: reversibility and secondary formation of HO2.
Phys. Chem. Chem. Phys. 1(22), 5097-5107. 

Henderson, A. P., Mutlu, E., Leclercq, A., Bleasdale, C., Clegg, W., Henderson, R. A. and Golding, B. T. (2002).
Trapping of benzene oxide-oxepin and methyl-substituted derivatives with 4-phenyl- and 4-pentafluorophenyl-1,2,4- triazoline-3,5-dione.
Chem. Commun.(17), 1956-1957. 

Hurley, M. D., Sokolov, O., Wallington, T. J., Takekawa, H., Karasawa, M., Klotz, B., Barnes, I. and Becker, K. H. (2001).
Organic aerosol formation during the atmospheric degradation of toluene.
Environ. Sci. Technol. 35(7), 1358-1366. 

Jenkin, M. E., Saunders, S. M., Derwent, R. G. and Pilling, M. J. (2002).
Development of a reduced speciated VOC degradation mechanism for use in ozone models.
Atmos. Environ. 36(30), 4725-4734. 

Jenkin, M. E., Saunders, S. M., Wagner, V. and Pilling, M. J. (2003).
Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part B): tropospheric degradation of aromatic volatile organic compounds.
Atmos. Chem. Phys. 3, 181-193. 

Johnson, D., Raoult, S., Rayez, M. T., Rayez, J. C. and Lesclaux, R. (2002).
An experimental and theoretical investigation of the gas-phase benzene OH radical adduct plus O-2 reaction.
Phys. Chem. Chem. Phys. 4(19), 4678-4686. 

Klotz, B., Volkamer, R., Hurley, M. D., Andersen, M. P. S., Nielsen, O. J., Barnes, I., Imamura, T., Wirtz, K., Becker, K. H., Platt, U., Wallington, T. J. and Washida, N. (2002).
OH-initiated oxidation of benzene - Part II. Influence of elevated NOx concentrations.
Phys. Chem. Chem. Phys. 4(18), 4399-4411. 

Olariu, R. I., Barnes, I., Becker, K. H. and Klotz, B. (2000).
Rate coefficients for the gas-phase reaction of OH radicals with selected dihydroxybenzenes and benzoquinones.
Int. J. Chem. Kinet. 32(11), 696-702. 

Olariu, R. I., Klotz, B., Barnes, I., Becker, K. H. and Mocanu, R. (2002).
FT-IR study of the ring-retaining products from the reaction of OH radicals with phenol, o-, m-, and p-cresol.
Atmos. Environ. 36(22), 3685-3697. 

Tomas, A., Olariu, R., Barnes, I. and Becker, K. H. (2003).
Kinetics of the Reaction of O3 with Selected Benzenediols.
Int. J. Chem. Kinet. 35, 223-230. 

Volkamer, R., Klotz, B., Barnes, I., Imamura, T., Wirtz, K., Washida, N., Becker, K. H. and Platt, U. (2002).
OH-initiated oxidation of benzene - Part I. Phenol formation under atmospheric conditions.
Phys. Chem. Chem. Phys. 4(9), 1598-1610. 

Volkamer, R., Platt, U. and Wirtz, K. (2001).
Primary and secondary glyoxal formation from aromatics: Experimental evidence for the bicycloalkyl-radical pathway from benzene, toluene, and p-xylene.
J. Phys. Chem. A 105(33), 7865-7874. 

Wagner, V., Jenkin, M. E., Saunders, S. M., Stanton, J., Wirtz, K. and Pilling, M. J. (2003).
Modelling of the photooxidation of Toluene: conceptual ideas for validating detailed mechanisms.
Atmos. Chem. Phys. 3, 89-106. 

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For further information ...

Please contact Claire Bloss or Mike Pilling.

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Useful links

1. Atmospheric Chemistry at the University of Leeds 

2. Centre for Atmospheric Science, University of Cambridge 

3. AIRSITE, University of North Carolina 

4. Atmospheric Chemistry Group, Jülich 

5. Recent publications from the Atmospheric Chemistry Modelling Group at Harvard University 

6. The UK National Air Quality Information Archive 

7. NOAA trajectory and dispersion model site 

8. University of Wuppertal atmospheric research 

9. JPL kinetics group

10. John Seinfeld's site at CALTECH

11. UCLA atmospheric science

12. Air pollution research at University of California, Riverside

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References

1. Atkinson R. (1994), “Kinetics and mechanisms of the gas phase reactions of the hydroxyl radical with organic compounds”, J. Phys. Chem. Ref. Data, Monograph 2, 47.
2. Atkinson R. and S.M. Aschmann (1994), “Products of the gas-phase reactions of aromatic hydrocarbons: Effect of NO₂ concentration”, Int. J. Chem. Kinet., 26, 929-944.
3. Behro F., M.T. Rayez and R. Lesclaux (1999), “UV absorption spectrum and self reaction kinetics of the cycohexadienyl radical, and stability of a series of cyclohexadienyl-type radicals”, J. Phys. Chem., 103, 5501-5509.
4. Bjergbakke E., A. Sillesen and P. Pagsberg (1996), “UV spectrum and kinetics of hydroxycyclohexadienyl radicals”, J. Phys. Chem., 100, 5729-5736.
5. Bleasdale C., R. Cameron, C. Edwards, and B. T. Golding (1997), “Dimethyldioxirane converts benzene oxide / oxepin into (Z,Z)-muconaldehyde and sym.-oxepin oxide: modeling the metabolism of benzene and it photo-oxidative degradation”, Chem Res Toxicol, 10, 1314-1318.
6. Bleasdale C., B. C. Gilbert, B. T. Golding, A. Henderson, and A. Whitwood (1999), "Stereospecific formation of  (E,Z)-muconaldehyde by one electron oxidation of benzene oxide-oxepin with CAN; EPR characterisation of intermediate radicals", paper in preparation.
7. Bohn B. and C. Zetzsch (1999), “Reversible peroxy radical formation from OH-benzene adducts and O₂ and secondary HO₂ production”, First International Symposium on Atmospheric reactive Substances, Bayreuth.
8. Derwent R.G., M.E. Jenkin and S.M. Saunders (1996), " Photochemical ozone creation potentials for a large number of reactive hydrocarbons under European conditions", Atmos. Environ., 30, 181-199.
9. Derwent R.G., M.E. Jenkin, S.M. Saunders and M.J. Pilling (1998) “Photochemical ozone creation potentials for organic compounds in North West Europe calculated with a master chemical mechanism”, Atmos. Environ., 32, 2419-2441.
10. Forstner H.L., R.C. Flagan, and J.H. Seinfeld (1997), “Secondary aerosol from the photo-oxidation of aromatic hydrocarbons: molecular composition”, Environ. Sci. Technol., 21, 1345-1358.
11. Ghigo G. and G. Tonachini (1998), “Benzene oxidation in the troposphere. Theoretical investigation on the competition of three postulated reaction channels”, J. Am. Chem. Soc., 120, 6753-6757.
12. Heiden A.C., K. Kobel and J. Wildt (1999), " Toluene emissions from plants", Geophysical Research Abstracts, 1, 496.
13. Henderson, A.P., J. Riseborough, C. Bleasdale, W. Clegg, M.R.J. Elsegood, and B.T. Golding (1997), “4,4’-Dimethoxytrityl and 4,4’,4’’-Trimethoxytrityl as Protecting Groups for Amino Functions; Selectivity for Primary Amino Groups and Application in ¹⁵N-labelling”, J. Chem. Soc. Perkin Trans. 1, 3407-3413.
14. Houweling S., F. Dentener and J. Lieleveld (1998), “The impact of nonmethane hydrocarbon compounds on tropospheric photochemistry”, J. Geophys. Res., 103, 10,673-10,696.
15. Jenkin M.E., G.D. Hayman, R.G. Derwent, S.M. Saunders, N. Carslaw, S. Pascoe, and M.J. Pilling (1999), “Tropospheric chemistry modelling: Improvements to current models and applications to policy issues”, Final Report (Reference AEA/RAMP/20150/R004 Issue 1) prepared for the Department of the Environment, Transport and the Regions on contract EPG 1/3/70, March 1999.
16. Klotz B., I. Barnes, and K.H. Becker (1995), “Atmospheric chemistry of benzene oxide/oxepin: a possible intermediate in the photo-oxidation of aromatic hydrocarbons”, in “Homogeneous and heterogeneous chemical processes in the troposphere”, European Commission Air Pollution Research Report 57, EUR 16766 EN.
17. Klotz B., S. Sørensen, I. Barnes, K.H. Becker, T. Etzkorn, R. Volkamer, U. Platt, K. Wirtz and M. Martín-Reviejo (1998), “Atmospheric oxidation of toluene in a large-volume outdoor photoreactor: In situ determination of ring-retaining product yields”, J. Phys. Chem., 102, 10289-10299.
18. Klotz B., I. Barnes and K. H. Becker (1999), “Atmospheric degradation pathways of alkylbenzenes”, First International Symposium on Atmospheric reactive Substances, Bayreuth.
19. Knipsel R., R. Koch, M. Siese and C. Zetzsch (1990) “Adduct formation of OH radicals with benzene, toluene and phenol and consecutive reactions of the adducts with NOx and O₂”, Ber. Bunsenges. Phys. Chem., 94, 1375-1379.
20. Kwok E.S.C, S.M. Aschmann, R. Atkinson, and J. Arey (1997), “Products of the gas-phase reactions of o-, m- and p-xylene with the OH radical in the presence and absence of NO_X”, J. Chem. Soc., Faraday Trans., 93, 2847-2854.
21. Lay T.H., J.W. Bozzelli and J.H. Seinfeld (1996), “Atmospheric photochemical oxidation of benzene: Benzene plus OH and the benzene-OH adduct (hydroxy-2,4-cyclohexadienyl) plus O₂”, J. Phys. Chem., 100, 6543-6554.
22. Liang C.K., J.F. Pankow, J.R. Odum and J.H. Seinfeld (1997), “Gas/particle partitioning of semivolatile organic compounds to model inorganic, organic, and ambient smog aerosols”, Environmental Science and Technology, 31, 3086-3092.
23. Odum J.R., T.P.W. Jungkamp, R.J. Griffin, R.C. Flagan and J.H. Seinfeld (1997), “The atmospheric aerosol-forming potential of whole gasoline vapor”, Science, 276, 96-99.
24. Pan X.M. and C. von Sonntag (1990), “OH-radical-induced oxidation of benzene in the presence of oxygen - R.+O₂ = RO₂. Equilibria in aqueous-solution – A pulse-radiolysis study”, Z. Naturforsch., B45, 1337-1340.
25. PORG (1997) “Ozone in the United Kingdom”, Report prepared by the UK Photochemical Oxidants Review Group, HMSO Publications, London.
26. Seinfeld J.H. and S.N. Pandis (1998), “Atmospheric Chemistry and Physics: From Air Pollution to Climate Change”, John Wiley and Sons Inc., New York.
27. Yu J.Z., H.E. Jeffries, and K.G. Sexton (1997), “Atmospheric photooxidation of alkylbenzenes .1. Carbonyl product analyses”, Atmos. Environ., 31, 2261-2280.

28. Yu J.Z., and H.E. Jeffries (1997), “Atmospheric photooxidation of alkylbenzenes .2. Evidence of formation of epoxide intermediates”, Atmos. Environ., 31, 2281-2287.

[Back to the top]  [Consortium members and web links]  [Introduction]  [Innovation]  [The EUPHORE chamber]  [Project work plan]  [Workpackage description]  [Collaboratory for Aromatic Oxidation Mechanisms]  [For further information...]  [Useful links]  [References]

Last modified on February 15th, 2002

Web pages are maintained by Claire Bloss, School of Chemistry, University of Leeds