Troposphere isoprene reaction

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

Trees and shrubs contain a group of fragrant compounds called terpenes. The simplest terpene is isoprene. All other terpenes are built around carbon skeletons constructed from one or more isoprene units. Plants emit terpenes into the atmosphere, as anyone who has walked in a pine or eucalyptus forest will have noticed. The possible effect of terpenes on the concentration of ozone in the troposphere has been the subject of much debate and has led to careful measurements of rates of reaction with ozone. 

Since isoprene is emitted from vegetation only during daylight hours, the reaction with OH is expected to be the dominant tropospheric removal pathway. The... 

The data presented have important implications in the behavior of tropospheric nonanthropogenic ozone, aerosol, and other trace constituents. Observational and experimental data have been reported by Rip-perton et al. 20) indicating the natural synthesis of ozone in the troposphere. Considering this study, the ubiquitous presence of various terpenes (21), isoprene (22), and oxides of nitrogen (20) suggest that some ozone is synthesized in the lower troposphere by the reaction NO2 + a-pinene + hv. Conversely, the destruction of ozone in the troposphere is partially ascribed to reactions with the terpenes and intermediates of the photochemical mixture. 

Ruppert, L., Barnes, I, and Becker, K. H. (1995) Tropospheric reactions of isoprene and oxidation products kinetic and mechanistic studies, in Tropospheric Oxidation Mechanisms, edited by K. H. Becker. European Commission, Report EUR 16171 EN, Luxembourg, pp. 91-102. 

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. 

Evidence for epoxide formation from NO3 reactions with alkenes has been reported (Hjorth, Schindler). These results show that oxirane formation occurs via a radical adduct, in which rotation about the C-C bond can occur before elimination of NO2. Reactions of NO3 with isoprene and 2-butene at low pressure and at low O2 concentrations gave mainly oxiranes, whereas in air at atmospheric pressure, oxirane yields were negligible. However, even at atmospheric pressures, the reaction of NO3 with 2,3-dimethyl-2-butene gave an oxirane yield of around 20 %. Thus, it is apparent that, at least with some alkenes, oxirane formation may be important under tropospheric conditions. 

Tropospheric reactions of isoprene and oxidation products Kinetic and mechanistic studies,... 

On a global scale, the influx of ozone from the stratosphere is estimated to contribute about 600 Tg yr to the budget of tropospheric ozone. This is about 1% of that produced in the stratosphere. Total photochemical production of ozone within the troposphere amoimts to about 4200 Tg yr . The reaction of HO2 with NO makes the largest contribution, about 3100 Tg yr, the reaction of CHsOO- with NO adds 800 Tg yr, and the reaction of other RO2 radicals, derived largely from the oxidation of isoprene, adds another 300 Tg yr . The total production of ozone is clearly dominated by processes in the troposphere, but the import from the stratosphere carmot be neglected. The downward flux features a maximum in later winter, and it is partly resportsible for the spring maximum of tropospheric ozone. 

Others such as isoprene (CsHg), or the terpenes (CioHie) and related compounds (responsible for many plant smells), emitted by plants and trees have chemical lifetimes measured in hours, minutes or seconds and therefore never travel more than a few kilometres from their source, depending on the wind speed. In fact, the principal limitation on the lifetimes of shorter-lived organic species in the troposphere is their reaction with OH, which, although its background mixing ratio is in the 10 range, is therefore critical to tropospheric chemistry. 

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