Troposphere atmospheric 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... 

Air calculated lifetimes x = 4.6 h due to reaction with 03 in 24-h period, x = 4.6 h with OH radical during daytime, and x = 20 min for N03 radical during nighttime for clean atmosphere x = 1.4 h for reaction with 03 in 24-h period, x = 2.3 h with OH radical during daytime, and x = 2 min with N03 radical during nighttime in moderately polluted atmosphere (Atkinson et al.1984a, Winer et al. 1984) calculated atmospheric lifetimes of 5.6 h, 5.1 h and 11 min for reaction with 03, OH and N03 radicals respectively for clean tropospheric conditions at room temp. (Atkinson et al. 1986) calculated tropospheric lifetimes of 3.4 h, 4.6 h and 2.0 h due to reactions with OH radical, 03 and N03 radical respectively at room temp. (Corchnoy Atkinson 1990). 

Air t1/2 = 6 h with a steady-state concn of tropospheric ozone of 2 x 10-9 M in clean air (Butkovic et al. 1983) t/2 = 2.01-20.1 h, based on photooxidation half-life in air (Howard et al. 1991) calculated atmospheric lifetime of 11 h based on gas-phase OH reactions (Brubaker Hites 1998). Surface water computed near-surface of a water body, tl/2 = 8.4 h for direct photochemical transformation at latitude 40°N, midday, midsummer with tl/2 = 59 d (no sediment-water partitioning), t,/2 = 69 d (with sediment-water partitioning) on direct photolysis in a 5-m deep inland water body (Zepp Schlotzhauer 1979) t,/2 = 0.44 s in presence of 10 M ozone at pH 7 (Butkovic et al. 1983) calculated t,/2 = 59 d under sunlight for summer at 40°N latitude (Mill Mabey 1985) t,/2 = 3-25 h, based on aqueous photolysis half-life (Howard et al. 1991) ... 

Second, in bi- and termolecular reactions, tl/2 and r depend on the concentration of other reactants this is particularly important when interpreting atmospheric lifetimes. For example, as discussed earlier, reaction with the OH radical is a major fate of most organics during daylight in both the clean and polluted troposphere. However, the actual concentrations of OH at various geographical locations and under a variety of conditions are highly variable for example, its concentration varies diurnally since it is produced primarily by photochemical processes. Finally, the concentration of OH varies with altitude as well, so the lifetime will depend on where in the troposphere the reaction occurs. 

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. 

OH increases through reaction R2 can reduce the atmospheric lifetime of CH4 and therefore its concentration in the atmosphere. Note that approximately 20% of the OH destruction in the troposphere proceeds via this reaction. Consequently, R7 together with the subsequent reaction... 

The rather fast reaction rate of halomethanes with Cl atoms suggests that this process may play a primary role in the removal of halomethanes from the troposphere and results in the formation of HC1 or 1C1 molecules. These degradation pathways do not lead to bromine or iodine atoms but to relatively stable molecules, which may initiate a different bromine and iodine cycles in the marine boundary layer. The atmospheric lifetime of IC1 is probably controlled by its sunlight photodissociation to iodine and chlorine atoms. Another possible degradation pathway of IC1 may be the hydrolysis to hypoiodous acid IOH, which may further be dissolved in seawater. 

In addition to photolysis (Chapter 15) and chemical reactions (see the next section), wet and dry deposition also can remove gas- and particle-phase chemical compounds from the troposphere (Eisenreich et al., 1981 Bidleman, 1988). Thus to completely characterize the atmospheric loss processes and overall lifetime of a chemical, we must understand its atmospheric lifetime due to dry and/or wet deposition. Wet deposition refers to the removal of the chemical (or particle-associated chemical) from the atmosphere by precipitation of rain, fog, or snow to earth s surface). Dry deposition refers to the removal of the chemical or particle-assodated chemical from the atmosphere to the Earth s surface by diffusion and / or sedimentation. 

Oxidation rate constant k, for gas-phase second order rate constants, kOH for reaction with OH radical, kN03 with N03 radical and kG3 with 03 or as indicated, data at other temperatures see reference k03 < 0.04 x 10-18 cm3 molecule-1 s-1 at 296 K with a tropospheric lifetimes x > 290 d and x 3 d due to reactions with 03 and OH radical at room temp. (Atkinson et al. 1982) k0H = (2.95 0.15) x 10 cm3 molecule-1 s-1 with a atmospheric lifetime x = 3.9 d at room temp, (relative rate method, Edney etal. 1986)... 

The direct photolysis of compounds such as HONO, 03, HCHO, and N02 in the tropospheric gas phase is a very important source of reactive species, which are then involved in the transformation of organic compounds. Additionally, some organic molecules including organic pollutants undergo photolysis as a significant or even the main process of removal from the atmosphere. It is for instance the case for nitronaphthalenes, the atmospheric lifetime of which can be as low as a couple of hours because of direct photolysis [11, 12]. 

Air atmospheric lifetimes of 2.3 h in clean troposphere and 1.2 h in moderately polluted atmosphere, based on gas-phase reaction with hydroxyl radical at room temp. atmospheric lifetimes of 15.0 d in clean troposphere and 5.0 d in moderately polluted atmosphere, based on gas-phase reaction with 03 at room temp. (Atkinson et al. 1987)... 

The flux of UV light, O3, and H20 vapor combine to give a potent source of OH radicals. OH radicals react with almost everything emitted into the atmosphere. The atmospheric lifetimes of many pollutants are determined by their reactivity towards OH radicals. The generation of OH radicals is the primary mechanism by which the atmosphere cleanses itself. Only compounds such as CFCs, Halons, and N20 which are inert towards OH radical attack survive transport through the troposphere into the stratosphere where they can damage the ozone layer. 

The use of 3D computer models to calculate atmospheric lifetimes is a rather cumbersome approach and access to such models is limited. A simpler technique to estimate the tropospheric lifetime of compound X with respect to OH attack is to scale the tropospheric lifetime of CCI3CH3 by the rate constant ratio k(OH+CCl3CH3)/k(OH+X). 

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