Global Mean OH Concentration

The short lifetime of OH implies that its concentration is highly variable. Deriving the atmospheric lifetimes of gases removed by oxidation by OH requires an estimate of OH concentrations averaged appropriately over time and space. Mass-balance arguments for proxy species with known sources can assist for this purpose. The most successful application, first proposed by Singh (1977) and Lovelock (1977), has been the use of the industrial solvent CH3CC13 to estimate the global mean OH concentration. The source of CH3CC13 is exclusely anthropogenic, and its historical trend is well known from industrial data. Production of CH3CC13 has been banned since 1996 as part of the Montreal Protocol. The dominant sink of CH3CC13 is oxidation by OH in the troposphere (photolysis in the stratosphere and uptake by the oceans are small additional sinks). Tropospheric mixing ratios of CH3CC13 are relatively uniform, so that a mass-balance analysis for CH3CC13 yields a global mean OH concentration weighted by atmospheric mass and by the temperature dependence of the CH3CC13 + OH reaction. The global mean OH concentration obtained in this manner can then be used to infer the lifetimes of other long-lived gases removed by reaction with OH, such as CH4 and hydrogenated halocarbons (HCFCs) (Prather and Spivakovsky, 1990).

The most recent use of CH3CC13 observations to constrain the global mean OH concentration has been by Krol et al. (1998) and Spivakovsky et al. (2000). These authors derive a CH3CC13 lifetime of 5.5 years in the troposphere against oxidation by OH, corresponding to a global mean OH concentration of (1.1 ±0.2) x 106 molecules/cm3. Spivakovsky et al. (2000) point out that the magnitude of the CH3CC13 interhemispheric gradient implies that the difference between the mean OH concentrations in the Northern and Southern Hemispheres is no more than 50%.

OH [10s molecules cm"*] January

OH [10s molecules cm"*] January

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OH [10' molecules cm'1] July

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OH [10' molecules cm'1] July

Figure 3 Longitudinally averaged monthly mean OH concentrations.

Mass-balance arguments for other chemical tracers oxidized by OH including 14CO, CHF2C1, CH2C12, and hydrocarbons have been used to confirm the above estimate of the global mean OH concentration and to provide additional constraints on the geographical and seasonal distribution of OH (Volz et al., 1981; Mak et al., 1992; Goldstein et al, 1995; Spivakovsky et al, 2000).

Simulation of the CH3CC13 lifetime has long been a standard test for evaluating the global mean OH concentration computed in tropospheric chemistry models, starting from the work of Crutzen and Fishman (1977). In these models, the OH concentrations are computed from a global simulation of 03 - NOv - CO-hydrocarbon chemistry that treats emissions, transport, chemistry, and deposition in a self-consistent way (e.g., Wang et al, 1998a). The current generation of models reproduces the atmospheric lifetime of CH3CC13 to within typically 25%.

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