Troposphere hydrocarbons, lifetimes

HO oxidation of CO is much faster than the reaction with methane, resulting in a mean CO lifetime of about two months, but considerably slower than reaction with the majority of the nonmethane hydrocarbons. Table I gives representative removal rates for a number of atmospheric organic compounds their atmospheric lifetimes are the reciprocals of these removal rates (see Equation E4, below). The reaction sequence R31, R13, R14, R15 constitutes one of many tropospheric chain reactions that use CO or hydrocarbons as fuel in the production of tropospheric ozone. These four reactions (if not diverted through other pathways) produce the net reaction... 

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). 

As seen in Table 6.1, the reactions of the nitrate radical with the simple aromatic hydrocarbons are generally too slow to be important in the tropospheric decay of the organic. However, one of the products of the aromatic reactions, the cresols, reacts quite rapidly with NO,. o-Cresol, for example, reacts with N03 with a room temperature rate constant of 1.4 X 10 " cm3 molecule-1 s-1, giving a lifetime for the cresol of only 1 min at 50 ppt N03. This rapid reaction is effectively an overall hydrogen abstraction from the pheno-... 

Cooper, D.L., Cunningham, T.P., Allan, N.L., McCulloch, A. (1993) Potential CFC replacements Tropospheric lifetimes of C3 hydrofluorocarbons and hydrocarbons and hydrofluoroethers. Atmos. Environ. 27A, 117-119. 

Hydroxyl radical, OH, is the principal atmospheric oxidant for a vast array of organic and inorganic compounds in the atmosphere. In addition to being the primary oxidant of non-methane hydrocarbons (representative examples of these secondary reactions are given in Table 6), OH radical controls the rate of CO and CH4 oxidation. Furthermore, the OH reaction with ozone also limits the destruction of O3 in the troposphere, it also determines the lifetime of CH3CI, CHsBr, and a wide range of HCFC s, and it controls the rate of NO to HNO3 conversion. Concentration profiles for hydroxyl radical in the atmosphere are shown in Fig. 2. 

The primary tropospheric oxidants are OH, O3, and NO3, with "OH and O3 reactions with hydrocarbons dominating primarily during daytime hours, and NO3 reactions dominating at night. Rate constants for the reactions of many different aromatic compounds with each of the aforementioned oxidants have been determined through laboratory experiments [16]. The rate constant data as well as atmospheric lifetimes for the reactions of toluene, m-xylene, p-xylene, m-ethyl-toluene, and 1,2,4-trimethylbenzene appear in Table 14.1. Only these particular aromatic compoimds will be discussed in this review paper, since much of the computational chemistry efforts have focused on these compounds. When considering typical atmospheric concentrations of the major atmospheric oxidants, OH, O3, and NO3 of 1.5 x 10, 7 x 10, and 4.8 x 10, molecules cm , respectively [17], combined with the rate constants, it is clear that the major atmospheric loss process for these selected aromatic compounds is reaction with the hydroxyl... 

International agreements (Montreal Protocol in 1987 and subsequent amendments), as well as national regulations, have strongly limited the production and the use of the CFCs. These chemical compounds have been gradually replaced by partially halogenated hydrocarbons, and specifically by hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). These alternative products are relatively easily destroyed in the troposphere and hence their lifetimes are substantially shorter than those of the CFCs (typically 1-10 yrs as opposed to 10-100 yrs). The ozone depletion potential of the HCFCs is about an order of magnitude smaller than that of the fully halogenated halocarbons. HFCs are not a threat to ozone because they do not contain any chlorine or bromine atoms. 

The oxidizing properties of the troposphere have a strong influence on the lifetime of chemical compounds in the atmosphere, and hence on the probability for a molecule to reach the middle atmosphere. Most hydrocarbons, for example, including hydrogenated halocarbons, are efficiently destroyed by the OH radical in the troposphere before they can penetrate into the stratosphere. Compounds that are not oxidized in the troposphere (e.g., chlorofluorocarbons) or weakly oxidized (e.g., methane) reach the stratosphere more easily. 

Table 6-1. Hydrocarbon Reactivities Rate Coefficients (at 298 K) for Reactions with OH Radicals and Ozone The Corresponding Rates for h( OH) = 5 x 105, n (O,) = 6.5 x 10" molecules/ cm1-, And the Associated Lifetimes of the Hydrocarbons in the Troposphere...

The oxidizing power of the atmosphere has likely decreased significantly, especially in the Northern Hemisphere, as a result of human activities. As a result, the lifetime of methane may have increased by 10-15% since the preindustrial era. At the same time, the abundance of tropospheric ozone has increased perhaps by as much as a factor of 2-3 in the Northern Hemisphere. Enhanced biomass burning fluxes of NO CO, and hydrocarbons from tropical ecosystems are likely to be important. Future changes in tropospheric ozone are predicted to be largest in the tropics (India, China). These projected increases in tropical emissions are likely to have a... 

Tropospheric lifetimes of organic species due to reaction with OH, NO, and O, can be estimated by combining the rate constant data (Appendix B) with estimated ambient tropospheric concentrations of OH, NO and O,. Resulting tropospheric lifetimes of a number of organic species, including several biogenic hydrocarbons, with respect to these gas-phase reactions are given in Table 5.2. An important point to note is that the atmospheric... 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

Table IX-M-I. Summary of the estimated photochemical lifetimes (t) for the hydrocarbon oxidation products and derivatives for the cloudless, lower troposphere (latitude 40°N 298 K 500 m altitude vertical ozone column = 350 DU)... 

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