Troposphere alkenes

Atkinson, R. (1997) Gas-phase tropospheric chemistry of volatile organic compounds l. Alkanes and alkenes. J. Phys. Chem. Ref. Data 26, 215-289. 

To pare the list of VOC oxidations down to the most important processes, we can calculate the effective lifetimes of organics with respect to reactions with each of the oxidants listed in the previous section. Since these natural lifetimes are defined as r = 1 / [X], we also need to assume an average concentration for the oxidant, [X]. We can therefore take a typical organic from each of the major classes (alkane, alkene, aromatic, etc.) and compare the individual lifetimes for reaction with OH, 03, N03, etc. Those reactions having very long lifetimes are insignificant with respect to their contribution to tropospheric chemistry and hence can be ignored for the purposes of this discussion. 

The room temperature rate constants for the reactions of 03 with some alkenes are given in Table 6.9. While the values are many orders of magnitude smaller than those for the corresponding OH reactions, the fact that tropospheric ozone concentrations are so much larger makes these reactions a significant removal process for the alkenes. 

The generation of OH in 03-alkene reactions has important implications for tropospheric chemistry. Thus the 03-alkene reactions could be important free radical sources at dusk and during the night when pho-tolytic sources of OH are minimal (e.g., Paulson and Orlando, 1996 Bey et al., 1997 Paulson et al., 1998). For example, Paulson and Orlando (1996) predicted that 10-15% of the total radical production may be from 03 alkene reactions in a typical rural area in the southeastern United States. As seen in Fig. 6.6, this reaction is expected to be most important at night. 

In short, the concentration of N03 in the troposphere can vary from very small, low-ppt concentrations to several hundred ppt, depending on the particular air mass. As discussed in Chapters 7 and 10, at typical tropospheric levels, it is believed to play a major role in nighttime chemistry, in some cases rivaling daytime OH for the net oxidation of certain organics, particularly alkenes (e.g., see Aliwell and Jones, 1998) as well as certain gaseous PAH. 

Atkinson Gas-Phase Tropospheric Chemistry of Volatile Organic Compounds 1. Alkanes and Alkenes [15]... 

Tropospheric chemistry models have to take into account a significant number of chemical reactions required to simulate correctly tropospheric chemistry. In the global background marine troposphere, it seems reasonable to consider a simplified chemistry scheme based on O3/ NOx/ CH, and CO photochemical reactions. However, natural emissions of organic compounds from oceans (mainly alkenes and dimethyl sulphide-DMS) might significantly affect the marine boundary layer chemistry and in particular OH concentrations. Over continental areas both under clean and polluted conditions,... 

The mechanism and kinetics of the atmospheric oxidation of alkynes, initiated by OH radicals, have been studied particularly to determine the role of alkyne oxidation in tropospheric ozone formation. A general mechanism for OH-initiated oxidation of alkynes has been developed with the aid of thermodynamic calculations. In general, the significance of atmospheric alkynes to the formation of tropospheric ozone was found to be smaller than for alkanes and alkenes, due to the absence of the hydroxy peroxy-forming product channel in the OH-initiated atmospheric oxidation of alkynes.227... 

In contrast to the water phase the HO radicals can have a much longer lifetime in gaseous media, i.e. up to 1 s for the OH and 60 s for the HO radical, respectively (Fabian, 1989). Despite the low concentration of OH radicals of about 10 molecules per cm in the sunlit troposphere (Ehhalt, 1999) they play an important role in controlling the removal of many organic natural and manmade compounds from the atmosphere (Eisele et al., 1997, Eisele and Bradshaw, 1993). Even in indoor environments, the formation of hydroxyl radicals is possible by ozone/alkene reactions (Atkinson et al., 1995). Steady-state indoor hydroxyl radical concentrations of about 6.7x10 ppb equivalent to 1.7x10 molecules cm were calculated at an ozone concentration of 20 ppb (Weschler and Shields, 1996). 

Alkenes are removed from the troposphere via reaction with 0(3P) atoms, OH radicals, N03 radicals, and 03. Of these, reaction with 0(3P) atoms is essentially negligible under atmospheric conditions. 

Reactions with ozone are competitive with the daytime OH radical reactions and the nighttime NO-, radial reactions as a tropospheric loss process for the alkenes. These reactions have been shown to proceed via initial O, addition to the olefinic double bond, followed by rapid decomposition of the resulting molozonide (Atkinson, 1990) ... 

As the product molecule AB becomes more complex, the value of k, decreases because the combination energy is distributed among more and more vibrational modes. The concentration of the third body, [M], is usually related directly to the pressure since in the atmosphere M is the sum of N2 and 02. The concentration of M at which the reaction rate behavior changes from third-order to second-order is lower the more complex the product molecule. Combination of two hydrogen atoms to form H2 is third-order all the way up to 104atm. On the other hand, addition of the OH radical to the alkene, 1-butene, C4H8, is second-order at all tropospheric pressures. 

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