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Project ID: EVK2-CT-2001-00114
Finanziato nell'ambito di: FP5-EESD
Paese: Germany

Aerosol yields from the degradation of key oxygenated compounds

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.

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