OH in the troposphere

The C-H bond strength is largest for primary C-H bonds at —101 kcal mol-1, decreasing to 98 kcal mol-1 for secondary and 96 kcal mol-1 for tertiary C-H bonds (Lide, 1998-1999). Hence one expects that, all else being equal, a tertiary C-H will react faster than a secondary C-H, which in turn will react faster than a primary C-H. Greiner (1970), whose measurements of the absolute rate constants for OH reactions in the mid-1960s provided the first clue of the potential importance of OH in the troposphere, suggested that... 

FIGURE 8.25 Overview of oxidation of DMS by OH in the troposphere (note that many of the reactions after the first step are the same in DMS reactions with S03, Cl, etc.). 

While there are a variety of other chlorinated organics such as methylchloroform (CH3CC13) that are emitted, these have relatively short tropospheric lifetimes because they have an abstractable hydrogen atom (e.g., see WMO, 1995). For example, while the stratospheric lifetime of methylchloroform is estimated to be 34 7 years (Volk et al., 1997), its overall atmospheric lifetime is only 5-6 years, primarily due to the removal by OH in the troposphere (toii 6.6 years), with a much smaller contribution from uptake by the ocean (roi i an 85 years) (WMO, 1995). 

These data also demonstrate the impact of bromine chemistry on the stratosphere (see Chapter 12.D). The initial ODP for methyl bromide is 15, due primarily to the large a factor associated with bromine chemistry. However, since it is removed by reaction with OH in the troposphere as well as by other processes such as hydrolysis in the oceans and uptake by soils and foliage (see Chapter 12.D), it has a short atmospheric lifetime of 1.3 years and hence the ODP decreases rapidly with time, toward a long-term steady-state value. 

The replacements and alternates for the CFCs (Table 13.1) are characterized by having abstractable hydrogen atoms, and hence they are removed to varying extents by reaction with OH in the troposphere before reaching the stratosphere. The HFCs do not contain chlorine at all, so that their ODPs are very small, essentially zero (Table 13.3). In this section we discuss briefly the tropospheric chemistry of HCFCs and HFCs. 

Because many of the alternates and replacements for CFCs have an abstractable hydrogen atom, reaction with OH in the troposphere dominates their loss. Table 13.4 gives some rate constants for the reaction of OH with these compounds the kinetics summary of De-More et al. (1997) should be consulted for other compounds. It is seen that the rate constants at 298 K are typically in the range of 10-l3-10-ls cm3 molecule-1 s-1, depending on the degree of halogen substitution and the nature of the halogen, e.g., F, Cl, or Br. Typical A factors are of the order of 1 X 10 12 cm3 molecule-1 s-1 per H atom (DeMore, 1996). 

Because HC(0)CFC12 has an abstractable hydrogen atom, it reacts with OH in the troposphere ... 

While we have focused here on CFC replacements, similar chemistry applies to replacements for the bromine-containing halons. For example, CF2 BrH is a potential halon substitute that will react with OH in the troposphere (DeMore et al., 1997). Through the subsequent reaction with 02 and then NO, the alkoxy radical CF2BrO is formed. This decomposes via scission of the weak C-Br bond to form COF2 (Bilde et al., 1996). 

Cox (246) estimates the concentration of HN02 to be 109 molec cm"3 in the daytime natural troposphere. The photolysis of HN02 may be an important source of OH in the troposphere, since HN02 absorbs the sun s radiation above 3000 A. The reactions of OH with hydrocarbons (either hydrogen abstraction from paraffins or addition to the double bond in olefins) in the troposphere are known to be the initial steps for photochemical smog formation [see Section VIII-2, p. 333],... 

The fraction of 0( D) atoms that form OH is dependent on pressure and the concentration of H2O typically in the marine boundary layer (MBL) about 10% of the 0( D) generate OH. Reactions (2.7 and 2.8) are the primary source of OH in the troposphere, but there are a number of other reactions and photolysis routes capable of forming OH directly or indirectly. As these compounds are often products of OH radical initiated oxidation they are often termed secondary sources of OH and include the photolysis of HONO, HCHO, H2O2 and acetone and the reaction of 0( D) with methane (see Figure 9). Table 2 illustrates the average contribution of various formation routes with altitude in a standard atmosphere. 

Because of the central role of OH in tropospheric chemistry any variation in the concentration of OH in the troposphere is cause for concern. A change in OH levels would probably lead to changes in the concentration of a variety of greenhouse gases thereby causing a perturbation in the climate. The rates of formation of acids and atmospheric oxidants would... 

Another important result of our calculations concerns the concentrations of OH and O3. It appears that the lower CHa levels in the 16th century imply higher OH and lower O3 concentrations. In the case of OH the variation is predicted to have been quite significant, with about 20-50% more OH in the troposphere in the 16th century as compared to conditions typical of the present-day atmosphere. The change in O3 levels is predicted to be more modest — only about 10-15%. 

Photochemistry of the OH radical controls the trace gas concentration. The photochemistry of the free hydroxyl radical controls the rate at which many trace gases are oxidized and removed from the atmosphere. Processes that are of primary importance in controlling the concentration of OH in the troposphere are indicated by solid lines in the schematic diagram those that have a negligible effect on OH levels but are important because they control the concentrations of associated reaction and products are indicated by broken lines. Circles indicate reservoirs of species in the atmosphere arrows indicate reactions that convert one species to another, with the reactant or photon needed for each reaction indicated along each arrow. Multistep reactions actually consist of two or more sequential elementary reactions. HX = HQ, HBr, HI, or HF. CxHy denotes hydrocarbons. (From Chameides and Davis, Chem. Eng. News 60 (40) 38-52, 1982. Copyright American Chemical Society.)... 

The atmospheric lifetime for methylchloroform, CH3CCI3, is about 9.7 years. Assume that the only reaction destroying methylchloroform is reaction with OH in the troposphere. (In fact, a sizeable fraction is transported to the stratosphere.) The rate constant for the OH -I- CH3CCI3 reaction is given by lr = 5.1 X 10" exp(—1800/T)cm /molecs. Make any reasonable assumptions and estimate an average tropospheric OH concentration. 

Laser-induced fluorescence measurements of atmospheric OH have been carried out now for several years, and shown to be capable of detecting extremely low concentrations of the radical. It has been pointed out however that interference from laser-generated OH could affect the results considerably " the wavelength used for OH excitation, 282 nm, generates 0( Z)) from O3 photolysis, and this reacts with H2O to form OH in the troposphere in a time ( 1 ns) which is shorter than the laser pulse width. Calculations and experimental assessments of the importance of this effect have been described. The reaction of OH with CS2... 

The stratospheric ozone-depleting potential of a compound emitted at the Earth s surface depends on how much of it is destroyed in the troposphere before it gets to the stratosphere, the altitude at which it is broken down in the stratosphere, and chemistry subsequent to its dissociation. Halocarbons containing hydrogen in place of halogens or containing double bonds are susceptible to attack by OH in the troposphere. (We will consider the mechanisms of such reactions in Chapter 6.) The more effective the tropospheric removal processes, the less of the compound that will survive to reach the stratosphere. Once halocarbons reach the stratosphere their relative importance in ozone depletion depends on the altitude at which they are photolyzed and the distribution of halogen atoms, Cl, Br, and F, contained within the molecule. 

Methane is the most abundant hydrocarbon in the atmosphere. Table 2.9 summarizes the global sources of CH4, which are estimated at 535 Tg(CH4) yr (range 410 to 660) (IPCC, 1995). Of the estimated global annual emissions, 160 Tg(CH4) yr is attributed to natural sources, with the most prominent contribution being emissions from wetlands. Of the estimated 375 Tg(CH4) yr from anthropogenic sources, 100 Tg(CH4) yr comes from fossil fuel combustion, and the remainder from biospheric sources. Methane is removed from the atmosphere through reaction with hydroxyl radicals (OH) in the troposphere, estimated at 445 Tg(CH4) yr, and by reaction in the stratosphere, estimated at 40 Tg(CH4) yr. Microbial uptake in soils contributes an estimated 30 Tg(CH4) yr removal rate. The im-... 

Soil uptake (55 Tg yr i) represents a major loss process for H2 and contributes 80% of the total destruction. H2 oxidation by OH in the troposphere contributes the remainder. The global burden of H2 in the atmosphere is 136 Tg. Its overall lifetime in the atmosphere is 1.9 years (Hauglustaine and Ehhalt 2002). H2 is rather well mixed in the free troposphere. However, its distribution shows a significant seasonal variation (Fig. 2.90) in the lower troposphere where soil uptake dominates. This loss process shows a strong temporal variability and is maximal during summer. 

The uptake of N2O5 to water droplets and aerosols forms nitric acid HONO2 (HNO3), which removes NOx form the gas phase. Since the process could have large effects on the production rate of O3 and OH in the troposphere, many studies have been conducted recently. 

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