Hydrocarbons biogenic

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). 

FIGURE 6.28 Oxidation of squalene to 6-methyl-5-hepten-2-one, acetone, geranyl acetone, and 4-oxopentanal (adapted from Fruekilde et al., f998). 

Some of the reactions, e.g., that of isoprene with OH and NO, were discussed earlier in this chapter. Table 6.26 summarizes some of the major products observed in the gas-phase reactions of several other biogenic hydrocarbons with OH and 03 (Atkinson, 1997a). These products are anticipated, based on the mechanisms described earlier in this chapter. As also expected, the yields of these major products generally do not account for 100% of the reactant lost, and there are a number of other products, including multifunctional species, that are also formed. As an example, the formation of more than 30 individual products has been observed from the reaction of a-pinene with O, in air, some of which are unidentified, and the same is true for the A3-carene reaction (Yu et al., 1998). Products included hydroxy oxoacids, hydroxy dicarbonyls, and dicarbonyls. The formation of low-volatility products that form particles (e.g., Hoffmann et al., 1998 Jang and Kamens, 1999) is likely responsible for a significant fraction of 

This is supported by studies of the aerosol composition in forested areas. For example, Kavouras et al. (1998) identified cis- and tram-pinonic acids as well as pinonaldehyde and nopinone in particles in a forest in Portugal. The diurnal variations of the pinonic acids and formic acid were similar, peaking in the afternoon as expected if they were formed by the reaction of O, with a-pinene. On the other hand, the concentrations of pinonaldehyde, expected from the oxidation of a-pinene by OH, O, and NO, and nopinone, from the oxidation of j3-pinene, were the smallest in the after- 

TABLE 6.26 Some Products and Their Yields Observed in the Gas-Phase Reactions of Some Biogenic Hydrocarbons with OH and 03  

Vegetation naturally releases organic compounds to the atmosphere. In 1960, Went (1960) first proposed that natural foliar emissions of VOCs from trees and other vegetation could have a significant effect on the chemistry of the Earth s atmosphere. 

Measurements in wooded and agricultural areas coupled with emission studies from selected individual trees and agricultural crops have demonstrated the ubiquitous nature of biogenic emissions and the variety of organic compounds that can be emitted. Table 2.12 shows the chemical structures of some of the common biogenic hydrocarbons. Each of the compounds shown in Table 2.12 is characterized by an olefinic double bond that renders the molecule highly reactive in the atmosphere, with the result that the lifetimes of these molecules tend to be quite short. 

Isoprene (2-methyl-1,3-butadiene, C5H8) is unique among the biogenic hydrocarbons in its relationship to photosynthetic activity in a plant. It is emitted from a wide variety of mostly deciduous vegetation in the presence of photosynthetically active radiation, exhibiting a strong increase in emission as temperature increases. Not only do the isoprene and terpenoid emissions vary considerably among plant species, but the biochemical and biophysical processes that control the rate of these emissions also appear to be quite 

TABLE 2.11 Median Mixing Ratio of the 25 Most Abundant Nonmethane Organic Compounds Measured in the Summer 1987 Southern California Air Quality Study 

Median Mixing Ratio in Parts per Billion of Carbon  

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On a local or regional basis, terpenes may contribute significantly to oxidant problems (16,60,85,86,87, ), making control of anthropogenic hydrocarbons problematical. The question of relative controls on hydrocarbon and NOjj emissions from automobiles and industry has been a matter of longstanding controversy and debate in the US, particularly because of the potential role of biogenic hydrocarbons in oxidant production (16,89,90). This controversy is yet unresolved. 

Grosjean D, EL Williams, E Grosjean, JM Andino, JH Seinfeld (1993c) Atmospheric oxidation of biogenic hydrocarbons reaction of ozone with 3-pinene, D-limonene, and rra -caryophyllene. Environ Sci Technol 27 2754-2758. 

Lamb, B D. Gay, H. Westberg, and T. Pierce, A Biogenic Hydrocarbon Emission Inventory for the U.S.A. Using a Simple Forest Canopy Model, Atmos. Environ., 27, 1673-1690 (1993). 

As seen from the rate constants in Table 6.13, the reactions of biogenic hydrocarbons with NO, are quite fast, so this reaction is expected to be a major fate of these biogenics at night and to also contribute to the removal of NO. The lifetime of isoprene with respect to reaction with NO, at 50 ppt, for example, is only about 20 min.. Addition of N03 occurs primarily at the 1-position and leads in the absence of NO to the... 

Table 6.14 gives the rate constants for the reactions of chlorine atoms with some simple alkenes and some biogenic hydrocarbons. As expected, the reactions are... 

The oxidation of isoprene, one of the most common biogenic hydrocarbons, is expected to lead to the formation of some MPAN. Thus, as treated earlier, one of the major products of isoprene oxidation is methacrolein. Its subsequent oxidation by OH is expected to form MPAN ... 

Benjamin, M. T., M. Sudol, D. Vorsatz, and A. M. Winer, A Spatially and Temporally Resolved Biogenic Hydrocarbon Emissions Inventory for the California South Coast Air Basin, Atmos. Environ., 31, 3087-3100(1997). 

Chameides, W. L R. W. Lindsay, J. Richardson, and C. S. Kiang, The Role of Biogenic Hydrocarbons in Urban Photochemical Smog Atlanta as a Case Study, Science, 241, 1473-1475 (1988). 

Drewitt, G. B K. Curren, D. G. Steyn, T. J. Gillespie, and H. Niki, Measurement of Biogenic Hydrocarbon Emissions from Vegetation in the Lower Fraser Valley, British Columbia, Atmos. Envimn., 32, 3457-3466 (1998). 

Grosjean, D., E. L. Williams, II, E. Grosjean, J. M. Andino, and J. H. Seinfeld, Atmospheric Oxidation of Biogenic Hydrocarbons Reaction of Ozone with /J-Pinene, d-Limonene, and trans-Caryophyllene, Em iron. Sci. Techriol., 27, 2754-2758 (1993a). 

Guenther, A., L. Otter, P. Zimmerman, J. Greenberg, R. Scholes, and M. Scholes, Biogenic Hydrocarbon Emissions from Southern Africa Savannas, J. Geophys. Res., 101, 25859-25865 (1996a). 

The reaction between olefins and ozone produces light that can be measured and related to the concentration of the reactants. One of the preferred methods for measuring ambient ozone concentrations utilizes the chemiluminescence generated in the ozone-ethylene reaction for detection. Recently, Hills and Zimmerman (16) described the use of this detection principle for determining hydrocarbon concentrations. They utilized the chemiluminescence created when ozone reacts with isoprene for development of a continuous, fast-response isoprene analyzer. This real-time isoprene system is reported to be linear over three orders of magnitude and to have a detection limit of about 1 ppbv. Because the system doesn t include a preseparation of hydrocarbons, interferences from other olefins (ethylene, propylene, and so forth) could occur. Thus far the chemiluminescent detector has been used to monitor isoprene emissions under conditions in which the concentrations of olefins that could interfere are negligible compared to those of the biogenic hydrocarbon. 

Note that biogenic hydrocarbons, in particular isoprene, dominate the above outlined impact of NMHC on global tropospheric chemistry. Moreover, convection of... 

Hoffmann, T., J. R. Odum, F. Bowman, D. Collins, D. Klockow, R. C. Flagan and J. H. Seinfeld (1997) Formation of Organic Aerosols from the Oxidation of Biogenic Hydrocarbons, J. Atmos. Chem., 26, 189-222. 

Griffin R.J., D. R. Cocker III, J. H. Seinfeld and D. Dabdub (1999) Estimate of global atmospheric organic aerosol from oxidation of biogenic hydrocarbons,... 

Kanakidou M., K. Tsigaridis, F. J. Dentener and P.J. Crutzen (1999) Human activity enhances the formation of organic aerosols by biogenic hydrocarbon oxidation, submitted to J. Geophys. Res.. 

Barr et al. (2003) performed an analysis of the impact of phytogenic aerosol (PhA) which is defined as forming mainly due to monoterpene oxidation (primarily, a- and /3-pinenes), on the radiative regime of the ABL over the forest in the eastern part of Canada. In the forest ecosystem the level of emissions to the atmosphere of biogenic hydrocarbons is moderate, with the concentration of a- and /3-pinenes constituting about 1.6 ppb. NMHC oxidation resulted in the formation of PhA at a number density of particles of about 5 108 cm 3. For a given concentration and size distribution of aerosol, its impact on the short-wave radiation transfer in the ABL was assessed. 

Bieger, T., Abrajano, Jr., T.A., and Hellou, J. (1997) Generation of biogeneic hydrocarbons during a spring bloom in Newfoundland coastal (NW Atlantic) waters. Org. Geochem. 26, 207-218. 

A major component of the reactive hydrocarbon loading are the biogenic hydrocarbons. As previously indicated, the hydrocarbon oxidation chemistry is integral to the production of ozone. Globally, the contribution of NMHC to net photochemical production of ozone is estimated to be about 40%. ... 

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