Exploitable results
The detailed and simplified degradation schemes developed from the laboratory investigations have been validated using the outdoor EUPHORE smog chamber facility in Valencia, Spain.
The mechanism development can be achieved by simulating the chemical processes in an outdoor smog chamber. This has certain advantages over the use of field studies or small laboratory chambers with artificial light sources. Physical parameters like temperature, relative humidity and solar light intensity can be controlled and meteorological effects can be eliminated, thus greatly simplifying in most cases the interpretation of the data obtained during the photochemical experiments. The smog chamber facility at Valencia consists of two large (200 m3) outdoor chambers, equipped with a range of analytical methods. In situ concentration measurements are made using FTIR and UV-visible spectrometers fitted with multiple pass optical systems (path length 300-500m).
The chambers are fitted with ports for withdrawing samples for analysis by gas chromatography coupled mass spectrometry. Continuous sampling of NO, NO2 and O3 is also possible. The degradation studies of individual oxygenated VOCs can be carried out at sub-ppm levels of reactants with a minimum of interference from heterogeneous reactions at the chamber walls. It cannot be necessarily assumed that models developed under urban conditions with high NOx concentrations can be satisfactorily applied for cleaner areas with respect to the NOx content. The environmental chamber allows to work at ambient levels with respect to the reactive nitrogen oxides concentrations thus it is possible to evaluate mechanisms under the low NOx concentrations as found in rural and downwind of urban areas.
Work using these large chambers has been directed at testing the detailed degradation mechanism derived for single oxygenated compounds from laboratory data. The results allowed optimisation of unknown parameters and feedback to the laboratory studies and they have been used to validate degradation schemes for all the generic oxygenated compounds investigated. Aerosol size and number distribution will be measured during the photosmog runs using a scanning mobility particle sizer SMPS connected to a condensation nuclei counter (CNC).
The major results are quality controlled concentration-time series of reactants and products and physical parameters like humidity, temperature and spectral resolved solar light intensity obtained during the photochemical degradation of individual oxygenated compounds applicable to validate mechanisms for atmospheric conditions. Aerosol size and number distribution for individual simulation chamber runs.
Vinyl Ethers are used in different industries, particularly as solvents, as motor oil additives and for coatings. They are released into the atmosphere where they are available for photochemical transformation. The atmospheric oxidation of these oxygenated compounds can contribute to the formation of ozone and other secondary pollutants. In the troposphere, these unsaturated compounds react with OH and NO3 radicals and with O3. Kinetic and mechanistic data are needed in order to determine the lifetimes of these compounds in the troposphere and to assess their contribution to urban and regional pollution.
Within the MOST project, rate coefficients and product yields for the gas phase reactions of a series of vinyl ethers with the most important atmospheric oxidants have been determined (for the first time for most reactions). The ozonolysis, OH and NO3 reactions lead to the formation of alkyl formates in high yield. For the vinyl and divinyl ethers HCHO was found to be the major co-product in the reactions with OH. In the case of the ozone reactions with vinyl and divinyl ethers the co-products are mainly determined by reactions of the CH2OO Criegee intermediate. The obtained results provide information about the atmospheric fate and impact of the investigated compounds. The rate data obtained contribute to better define their tropospheric lifetime. The unsaturated ethers have lifetimes of few hours; they are removed from the atmosphere in the gas phase by reaction with OH, NO3 and O3. Their oxidation leads mainly to formates and aldehydes. Aerosol formation was observed in the ozonolysis of all vinyl ethers and ethyleneglycol vinyl ethers.
The rather short-lived unsaturated ethers could have a local impact, which however is not only defined by the persistence of these compounds but also by the fate of their oxidation products. Aldehydes will be removed by photolysis or reaction with OH and have lifetimes of less than a day while alkyl formates will be oxidised mainly by reaction with OH radicals leading to acids and anhydrides which are highly soluble and may be rapidly incorporated into cloud droplets. Alkyl formates have lifetimes of weeks, which may reduce significantly the local impact of unsaturated ethers.
The global model TM3 is already able to simulate the O3/HxOy/NOx and C1-C5 chemistry (182 gas phase chemical reactions). This chemistry has been extended by a highly-reduced chemical scheme suitable for incorporation in global models containing 45 additional gas phase chemical reactions (11 compounds (Decane, Acetone, MEK, MIBK, Toluene, Xylene, Benzene, Methanol, Ethanol, Ethyl acetate and 2-butoxy ethanol) and 19 reactions for the solvents in use and 10 compounds (Dipropylene glycol monoethylether (DPM), Octadecyl vinyl ether (ODVE), Dodecyl vinyl ether (DDVE), Diethyleneglycol monovinyl ether (MVE-2), Cyclohexanedimethanol divinyl ether (CHDVE), Diethyleneglycol divinyl ether (DVE-2), Triethyleneglycol divinyl ether (DVE-3), 2,4 pentadione, 2,5 hexadione) and 26 reactions for the new solvents). This reduced chemical mechanisms considers oxidation of the solvents from all three major oxidants (ozone, hydroxyl radical and nitrate radical). It has been produced by the mechanism generation program CHEMATA and was built at the top of the base chemistry described by Poisson et al (2000). This version of TM3 model developed for the MOST project contains a total of 281 chemical reactions involving 133 species.
The highly reduced gas phase chemistry scheme obtained by CHEMATA for solvent degradation has been evaluated by comparison with the explicit degradation scheme of the Master Chemical Mechanism by performing box model simulations using the FACSIMILE software. For the base case scenario the percentage difference in the average ozone concentration that is calculated by the two mechanisms after five days of simulation, remains below 3% for all solvents considered.
Two different criteria have been used in order to rank the individual VOCs according to their capability to generate ozone. The first one is the maximum ozone concentration during the five days of the model run. This value is generally obtained in the late afternoon of the last day of the simulation. The second one corresponds to the O3 production efficiency of the organic species (AO3/AHC) and represents the quantity of ozone produced during the five days of the simulation as a result of the degradation of the corresponding quantity of the hydrocarbon reacted.
Global emissions: For realistic scenarios it is necessary to have good estimates for the origin and the chemical composition of the solvents in use. For the global model this information for the current solvent-use emission inventories is based on the EMEP/CORINAIR Emission Inventory Guidebook (http://reports.eea.eu.int/ technical_report_2001_3/en/group06.pdf) which provides information on the relative contribution of the human activities (9 main categories) to solvent emissions in W. Europe as well as the major constituents emitted per activity (chemical speciation of emissions - 8 major chemical groups with several individuals per group) and on the emission inventory EDGAR which is available for the years 1990 and 1995 on the TNO / RIVM web site (ftp://info.rivm.nl/pub/lae/EDGARV20/ DATA/details/NMV/) on a 1ox1o resolution. Future emission inventories by changes in the amounts emitted and in the chemical speciation of the emissions require knowledge of the properties of the new compounds with regard to the specific usage they are planned. To detour this problem we have performed a number of sensitivity studies that allow the evaluation of potential maximum gain we could have from solvent replacement.
Global model components: The TM3 model modified to be able to simulate emissions and fate of solvents in the troposphere. For this, in addition to the implementation of emissions and the gas phase chemistry that has been developed in the frame of the project, the aerosol module of Tsigaridis and Kanakidou (2003) has been incorporated in the model. Dry and wet deposition processes for the solvents in use are also taken into account in TM3.
The potential maximum benefit for air quality from changes in solvent use emissions has been investigated with the global model by comparing the base case simulation (BC) considering all solvents in use (represented in the global model by the 11 chosen compounds that are mentioned earlier) with a simulation without any solvent emissions (NS). Note that the solvent in use emissions constitute about 7% of the total VOC emissions of the BC scenario.
The work has delivered rate coefficients for the reactions of OH and NO3 radicals and O3 with a series of alkyl vinyl ethers, ethyleneglycol vinyl ethers, and pentandiones. This body of data can be used to calculate the gas phase atmospheric lifetimes with respect to degradation with these oxidants. The various types of vinyl ether are very reactive towards OH, NO3 and O3 and in the atmosphere such compounds will have lifetimes of a few hours or less. This implies that their chemistry will occur at the local and regional scale. The chamber studies also showed that vinyl ethers can readily under acid catalysed reactions on surfaces which cause breakdown of the vinyl ether to an alcohol and aldehyde.
Detailed products studies on the reaction of OH and NO3 radicals and O3 with the vinyl ethers have shown that organic formates and formaldehyde are the major products. These yields have been quantified and allow the postulation of near-explicit mechanisms in the cases of the OH and O3 reactions and semi-explicit mechanisms in the case of NO3 where only total bulk yields for the different nitrates formed in the systems can be given. The database allows the postulation of one-equation mechanistic representations in many cases, which are required for global models.
The work has shown that 2,4-pentandione also known as acetylacetone (CH3C(O)CH2C(O)CH3) exists mainly in its enolic form (CH3C(O)CH=C(OH)CH3) in the gas phase. Due to enolic double bond character of the compound it reacts fast with OH radicals and has an atmospheric lifetime of approximately 3.8h due to reaction with this oxidant. The reactions with O3 and NO3 are too slow to be of atmospheric importance. The addition of OH radicals to alkenes results in the formation of 1,2-hydroxyalkoxy radicals. The further fate of these radicals is decomposition via C-C fission to form carbonyls (aldehydes and ketones) or reaction with O2 to form dihydroxycarbonyls. For small alkenes decomposition dominates. The product studies on OH + acetylacetone suggest that in this case C-C is a minor process and that the major pathway is reaction to form a mixture of the vicinal triketone 2,3,4-pentantrione (CH3COCOCOCH3) and hydrated analogues, e.g. pentan-2,3-dione-4-diol (CH3COCOC(OH)2CH3). In the atmosphere the trione will exist in the hydrated form and will be quickly taken up into the aqueous phase.
Several classes of oxygenated solvents have been investigated for aerosol formation. Aerosol formation has been observed from the ozonolysis of vinyl ethers. The aerosol yields from the ozonolysis of small alkyl vinyl ethers was quite small (~1%) but quite substantial yields were observed from the ozonolysis of ethyleneglycol vinyl ethers. An example is shown in the diagram for ethyleleneglycol divinyl ether. Aerosol was also observed in the OH reaction of these compounds but it is difficult to decouple this from the O3 reactions, which also occur in the systems used to generate OH. The mechanism of the aerosol formation is still not clear but is thought to involve reactions of the Criegee intermediate, formed in the ozonolysis reaction with the vinyl ether.
These results are arising from the investigation of the uptake rate kinetics and measurement of aqueous phase oxidation reactions representative of in-cloud processes of oxygenated. This allowed t o construct mechanisms describing the in-cloud oxidation of these species and test these reaction schemes via steady-state techniques in an aqueous phase irradiation chamber.
The reactivity of hydroxyl radical with oxygenated organic compounds was investigated by three partners using different experimental approaches. A very good agreement in the determination of the rate constants for the intercomparison compounds were achieved. For the first time the ionic strength, dependent measurements of OH reactions were carried out. The absorption of acetone, 2-butanone, 2,3-butanedione, and 2-oxo-propanal into aqueous drops was observed using a flow tube reactor. Finally, the kinetic data set obtained by these investigations will be implemented as a module in an extended version of tropospheric multiphase model CAPRAM 2.5.
In addition to the inherent cross checking of the quality of reported data within the project, we compiled all results to construct a final scientifically sound database for the development of structure-activity relationships. These are extremely useful for prediction purposes and have therefore direct potentials to be applied by industries and/or policy makers.
It is evident that an understanding of the complete oxidation mechanisms for oxygenates is essential if a meaningful and accurate classification of these compounds in terms of their ozone formation potential and impact on the oxidising capacity of the troposphere are to be made. The development of SARs is accompanied by a broadband application field.
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