Biogenic VOCs oxidation products

Although some of the biogenic VOCs are relatively simple compounds such as ethene, most are quite complex in structure (e.g., Figs. 6.22 and 6.26). Furthermore, they tend to be unsaturated, often with multiple double bonds. As a result, they are very reactive (see Chapter 16.B) with OH, 03, NO, and Cl atoms (e.g., Atkinson et al., 1995a). In addition, because they are quite large and of relatively low volatility, their polar oxidation products are even less volatile. This makes elucidating reaction mechanisms and quantifying product yields quite difficult. For a review of this area, see Atkinson and Arey (1998). 

Atmospheric Profiles of Biogenic VOCs and Their Oxidation Products... 

In a second part of the work, a Structure-Activity Relationship has been developed for the addition of OH to (poly)alkenes. It was shown that the total rate constants can be expressed in very good approximation by a sum of partial, site-specific rate constants for addition to a given (double-bonded) carbon atom, the values of these partial rate constants depending solely on the stability-type of the ensuing p-hydroxy radical. The SAR is particularly useful in that it also allows the prediction of the detailed primary product distributions of (poly)alkene + OH reactions. Therefore, the SAR is a powerful tool in the modeling of the tropospheric OH-initiated oxidation of biogenic VOC. 

SOA material formed by either pathway may be surface-active. For example, organic acids including c -pinonic acid, which is formed via the oxidation of the biogenic VOC a-pinene, are traditional SOA products which have been shown to be surface-active [92-96]. Alkene ozonolysis has also been shown to yield water-soluble surface-active organic compounds [97]. 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

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