Terpenes, tropospheric

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. 

Worth, and Jeffries (211) obtained emissions 2 to 10 times larger than any previous estimate. Rasmussen (199) has concluded that, while the identity of the emissions is well known, a quantitative estimate of the worldwide terpene emission rate is not yet possible. It would appear that the natural emissions are much larger than that estimated for man s activities, 27 x 10 tons yr Ripperton et al. have suggested that the reaction of ozone with terpenes provides an important, if not dominant, sink for both compounds in the troposphere. While large terpene mixing ratios (ppm or more) have been measured locally in isolated areas [Ripperton et al. (211)], no global estimate is available. 

The tropospheric chemistry of carbon compounds is summarized in Figure 5. CH4 is the dominant input of reactive carbon in the atmosphere, and the main action of atmospheric chemistry is to oxidize it to C02. Atmospheric H2C=0 and CO are produced along the way. Combustion is only a small source of CO, and the total CH contribution to atmospheric carbon is less than 1% of the atmospheric carbon exchanged by the C02 cycle. The contribution of reactive hydrocarbons such as terpenes is not clear, although they may provide a significant source of CO and... 

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. 

Zinc exchange between plant canopy and ground level troposphere is also responsible for the input of this trace metal to the air pool. One square meter of the canopy may annually exude up to 9 kg of Zn as a terpene component (Beaufort et al, 1975). During bacterial biometallization in sub-aquatic coastal areas, the formation and corresponding volatilization of organozinc species take place however, at present there is no quantitative parameterization of these processes in regional or global scale. 

Oxidation of methane is one of the sources of atmospheric CO. Another internal source of importance is the oxidation of terpenes and isoprenes emitted by forests (Crutzen, 1983). The carbon monoxide concentration in the atmosphere ranges from 0.05 to 0.20 ppmv in the remote troposphere (with considerable differences between the northern and southern hemispheres), which means that about 0.2 Pg of carbon is present as CO in the atmosphere. 

In the troposphere and in the lower stratosphere, the chemical production of H2 is due to the photolysis of formaldehyde produced by methane oxidation. About 13 Tg H2/yr are produced by this mechanism (Schmidt et al., 1980). Oxidation of isoprene and other terpenes in the lower troposphere leads to an additional production of about 10-35 Tg H2/yr. A precise determination of these quantities requires a... 

In the troposphere, the production of ozone results from the day-time oxidation of methane, nonmethane hydrocarbons, and carbon monoxide in the presence of nitrogen oxides. Under natural conditions, methane, produced in oxygen-deficient environments, is released primarily by wetlands, lakes, and rivers. Nonmethane hydrocarbons, such as isoprene and terpenes, are emitted by various types of trees. Nitric oxide is released by soils as a result of microbial activity and is produced in the atmosphere by lightning in thunderstorm systems. 

Combination of the available rate constant data with estimates of the atmospheric concentrations of O3, NO3 and OH indicates that the reaction of NO3 with substituted alkenes and terpenes may be the dominant tropospheric oxidation process for these species. Formation of carbonyl nitrates in the reactions could be important in the long-range transport of odd nitrogen. 

In the troposphere, the lowest layer of atmosphere, volatile terpenes perform a dual action, depending on the presence of anthropogenic pollutants such as oxides of nitrogen (NOx). When NOx concentrations are low, volatile terpenes react with ozone to reduce the atmospheric concentration of this ph5hotoxic gas. When air is contaminated with NOx formed e.g., by combustion, volatile terpenes participate in reactions which results in formation 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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