Atmospheric Chemistry of Biogenic Hydrocarbons

The reaction sequences for the simpler aliphatic alcohols under atmospheric conditions are known (Atkinson 1989 Mellouki et al. 2003) these involve H-atom abstraction, mainly from the a C—H bonds. For example, the methanol-OH reaction is 

Free tropospheric concentrations of methanol range from about 700 ppt at northern midlatitudes to about 400 ppt at southern latitudes (Singh et al. 1995). In general, ethanol abundance in the free troposphere is an order of magnitude lower than that of methanol. Average lifetimes of CH3OH and C2H5OH in the atmosphere are on the order of 16 days and 4 days, respectively. 

A great variety of organic compounds are emitted by vegetation. These biogenic compounds are highly reactive in the atmosphere. They are basically alkenes or cycloalkenes, and their atmospheric chemistry is generally analogous to that of alkenes. Because of the presence of C=C double bonds these molecules are susceptible to attack by 03 and N03, in addition to the customary reaction with OH radicals. 

TABLE 6.5 Estimated Tropospheric Lifetimes of Organic Compounds 

If one had to single out the most important biogenic hydrocarbon in atmospheric chemistry, it would be isoprene. Isoprene has the chemical formula 

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Of the atmospheric chemistry of the biogenic hydrocarbons, by far the most is known about isoprene. Whereas rate constants of many other biogenic hydrocarbons with OH, O3, and NO, have been measured, comparatively little is known about the distribution of products. Hatakeyama et al. (1989, 1991) and Arey et al. (1990) have measured some of the products of the a-pinene-O, reaction, including pinonaldehyde, norpinonaldehyde, formaldehyde, CO, and COj. Considerable work remains to be done in elucidating the atmospheric oxidation mechanisms of biogenic hydrocarbons. 

Hoffmatm, Thorsten, Jay R. Odum, Frank Bowman, Donald Collins, Dieter Klockow, Richard C. Flagan, and John H. Seinfeld, Formation of Organic Aerosols from the Oxidation of Biogenic Hydrocarbons, Journal of Atmospheric Chemistry, 26, 189-222 (1997). 

Why are vegetation-induced increases in NMHC of concern Biogenic hydrocarbons play an important role in generating pollutants such as O3, CO, and organic peroxides, while influencing hydroxyl radical (OH ) chemistry to reduce atmospheric oxida-... 

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. 

This book covers homogeneous gas-phase kinetics important in the atmosphere, which has been almost established, and provides the solid scientific bases of oxidation of trace gases and oxidant formation. Nevertheless, unresolved problems remain, for example, unsatisfactory reproduction of observed OH/HO2 mixing ratio by model simulation under certain conditions, and oxidation mechanisms involving isoprene, terpenes and other biogenic hydrocarbons, and anthropogenic aromatic hydrocarbons. Therefore, descriptions of these topics are not completed in the book. Heterogeneous reaction chemistry is not covered well except for the chemistry on polar stratospheric clouds (PSCs) and reactive uptake coefficients of selected... 

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