Troposphere alkanes

The concentration of NO determines the relative importance of reaction 3, and the formation of NO2 is particularly significant since this is readily photolyzed to produce 0( P) that reacts with oxygen to produce ozone. This alkane-NO reaction may produce O3 at the troposphere-stratosphere interface ... 

Altshuller, A. P. (1991) Chemical reactions and transport of alkanes and their products in the troposphere. J. Atmos. Chem. 12, 19-61. 

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

FIGURE 1.4 Typical sequence of elementary reactions in which OH initiates the oxidation of an alkane in the troposphere. 

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 first thing that stands out in Table 6.2 is that the OH-CH4 rate constant, 6.2 X 10 15 cm3 molecule 1 s-1, is much smaller than those for the higher alkanes, a factor of 40 below that for ethane. This relatively slow reaction between OH and CH4 is the reason that the focus is on non-methane hydrocarbons (NMHC) in terms of ozone control in urban areas. Thus, even at a typical peak OH concentration of 5 X 106 molecules cm 3, the calculated lifetime of CH4 at 298 K is 373 days, far too long to play a significant role on urban and even regional scales. Clearly, however, this reaction is important in the global troposphere (see Chapter 14.B.2b). 

While these reactions are much slower than the corresponding OH reactions, the nighttime peak concentrations of NO, under some conditions are much larger than those of OH during the day, 400 ppt vs 0.4 ppt. Even given the differences in concentration, however, as seen from the lifetimes in Table 6.1, the nitrate radical reaction is still relatively slow. While the removal of the alkanes by NO, is thus not expected to be very significant under most tropospheric conditions, reaction (20) can contribute to HNO, formation and the removal of NOx from the atmosphere. 

At 298 K and atmospheric pressure with 50% relative humidity, about 0.2 HO" are produced per O( D) atom formed. Photolysis of 03 in the presence of water vapor is the major tropospheric source of HO", particularly in the lower troposphere where water vapor mixing ratios are high (for an explanation of the term mixing ratio see below). Other sources of HO" in the troposphere include the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other carbonyls in the presence of NO, and the dark reactions of 03 with alkanes. Note that all these processes involve quite complicated reaction schemes. For a discussion of these reaction schemes we refer to the literature (e.g., Atkinson, 2000). 

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

RO and R02 (R C4) Reaction. It is presently well established that the only significant gas phase loss process for the alkanes in the troposphere is reaction with HO radicals [88]. In general, the reaction mechanism for the... 

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... 

As well as atmospheric sources, pyrolysis of fluorine-containing polymers, which may occur in engine oil additives, non-stick cookware or incinerated medical equipment (i.e. syringes) and household waste, may also produce TFA. This process may also produce perfluorinated alkanes and cycloalkanes, which have significant GWP, and have estimated tropospheric half-lives of more than 2000 years. Trifluoroacetate may also be produced by metabolism of trifluoromethyl-containing drugs such as Prozac, and anaesthetics including halothane and iso-fluorane [4],... 

Aromatic, olefinic, and acetylenic hydrocarbons, but especially saturated hydrocarbons belong to persistent pollutants difficult to eliminate from the troposphere. The only exceptions are some aromatic compounds that undergo direct photodegradation in result of solar irradiation. Alkanes are undoubtedly much less reactive than other organic compounds including unsaturated hydrocarbons, surely because they are more completely saturated and their activation involves cleavage of the relatively strong C—H bond (s = 415 kJ). 

There has been considerable debate as to what degree Cl, like Br, radicals might play a role in Arctic springtime tropospheric chemistry. To obtain information on this, Jobson et al. (1994) collected daily air samples at Alert (82.5° N, 62.3° W) from January 21 to April 19, and on an ice floe 150 km north of Alert during April 2-15, 1992. They derived information on the concentrations of OH, Cl, and Br from the different decay rates of a suite of non-methane hydrocarbons the so-called hydrocarbon clock method. Besides some removal of alkanes by reaction with OH during ODEs, additional alkane losses, consistent with removal by reaction with Cl, were measured. 

The presence of halogen-containing radicals can play a substantial role in chemical processes in the atmosphere. Chlorine atoms react much faster with alkanes than OH, the effect of which has been clearly observed in the Arctic troposphere in connection with the sunrise ODEs (Ramacher et al., 1997 Ariya et al., 1997 Song and Carmichael, 1999). The high reactivity of chlorine with hydrocarbons may explain the relatively high CH2O volume mixing ratios of... 

The largest sink for alkanes in the atmosphere is reaction with OH and NO3 radicals. The formation of photochemical smog is described in detail in (Chapter 9.11, Sillman). Mono-aromatic hydrocarbons react only slowly with O3 and NO3 radicals in the troposphere. The only important atmospheric processes for mono-aromatic hydrocarbons, and naphthalene and dinaphthalenes are reactions with OH radicals (Atkinson, 1990). The products of these reactions include aldehydes, cresols, and, in the presence of NO, benzylnitrates. Methane can be an important contributor to ozone formation, especially in the remote troposphere, as described in (Chapter 9.11, Sillman). 

The simplest alkane is methane (CH4). Methane oxidation is the essential chemistry of the background troposphere (Logan et al., 1981 Thompson and Cicerone, 1986). Ice-core records show that methane concentrations in the atmosphere have more than doubled since preindustrial times (Khalil and Rasmussen, 1987), reaching a rate of increase of 1% yr-1 in the last decade (Khalil et al., 1989). Methane is emitted to the atmosphere by ruminants, wetlands, tundra, open waters, termites, rice paddies, biomass burning, natural gas production, and coal mining [see Jacob (1991) for a review of the literature on methane sources] the principal sink of CH4 is reaction with OH. 

Under tropospheric conditions, the alkanes react with OH radicals during daylight hours and with the N03 radical during nighttime hours, with the latter process being of minor (< 10%) importance as an atmospheric loss process. 

Eichmann, R., P. Neuling, G. Ketseridis, J. Hahn, R. Jaenicke, and C. Junge (1979). n- Alkane studies in the troposphere—I. Gas and particulate concentrations in North Atlantic air. Atmos. Environ. 13, 587-599. 

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