Abstract
The three‐dimensional global distribution of OH over a year is calculated as a function of temperature, ultraviolet irradiance, and densities of H2O, CO, O3, CH4, and NOt, (defined as NO + NO2 + NO3 + 2N2O5 + HNO2 + HNO4). The concentration of OH is computed within a chemical tracer model (CTM) with an accuracy comparable to that of a detailed photochemical model. Distributions of CO, NOt, O3, CH4, and the density of O3 column were specified on the basis of observations. Meteorological fields were derived from the general circulation model developed at the Goddard Institute for Space Studies. The numerical method for parametrization of chemistry is described in Spivakovsky et al. (this issue). The CTM is used to simulate the global distribution of CH3CCl3. The computed distribution of OH implies a lifetime of 5.5 years for CH3CCl3 (obtained by relating the global burden of CH3CCl3 to the global loss, integrated using simulated three‐dimensional distributions). Analysis of the long‐term trend in CH3CCl3 as defined by observations suggests a lifetime of 6.2 years (consistent with Prinn et al. (1987)), indicating that model levels of OH may be too high by about 13%. This estimate for the lifetime depends on industry data for global emissions and on the absolute calibration of observations. It is argued that seasonal variations of CH3CCl3 provide an independent test for computed OH fields that is insensitive to the uncertainties in the budget of CH3CCl3. The annual cycle of CH3CCl3 from about 25°S to the South Pole is dominated by seasonal changes in OH. Observed seasonal variations of CH3CCl3 indicate that the OH field south of 20°S±4° should be scaled by 0.75±0.25 from computed values, consistent with the result based on long‐term trends. Reactions involving non‐methane hydrocarbons were not included in the current model. These reactions could account for lower concentrations of OH than computed. Seasonal variations of CH3CCl3 in the tropics and in the northern mid‐latitudes are dominated by effects of transport. If use of CH3CCl3 is phased out (as envisioned by the Montreal protocol), the dynamically driven seasonal variations of CH3CCl3 will decrease dramatically, whereas the chemically driven variations will remain proportional to the concentration of CH3CCl3; then the annual cycle of CH3CCl3 in northern mid‐latitudes will provide a measure of OH as does at present the annual cycle in southern mid‐latitudes. The influence of chemistry on the latitudinal distribution of CH3CCl3 is small and at present does not provide a constraint for the globally averaged OH or for the latitudinal distribution of OH. However, if emissions of CH3CCl3 were to cease, the tropical depression in the concentration of CH3CCl3 caused by high levels of OH in the tropics may provide an additional means to test OH models.
Highlights
Observationosf CH3CC13havebeenusedto assessthe globally and annuallyintegratedconcentrationof OH [Singh,1977a, b; The OH radicalplays a criticalrole in the chemistryof the Lovelock,1977; Makide and Rowland, 1981;Logan et al, 1981; loweratmosphereR. eactionswith OH providethedominantpath Chameidesand Tan, 1981; Prinn et al, 1983, 1987; Khalil and for removalof a varietyof halocarbonsh,ydrocarbonsa,ndCO, Rasmussen,1984a; Fraser et al, 1986], since the reaction with andfor conversionof NO2 to nitric acid
The concentrationof OH is sensitiveto the intensity sourcesof CH3CCI andCFCs are locatedin the sameindustrial of solarradiation,to temperatureand to concentrationosf H20, regions,but in contrastto CH CC13,CFCs are inert in the tropo
CTM hasbeenusedtostudytheglobaldispersioonf speciesemit- of CFCs[Pratheret al., 1987]andSSKr[Jacobet al., 1987],is ted mainly in industrializedregions at northemmid-latitudes, tested further as discussed below. chlorofluorocarbo(nCsFCs) [Pratheret al., 1987], and aKr
Summary
Observationosf CH3CC13havebeenusedto assessthe globally and annuallyintegratedconcentrationof OH [Singh,1977a, b; The OH radicalplays a criticalrole in the chemistryof the Lovelock,1977; Makide and Rowland, 1981;Logan et al, 1981; loweratmosphereR. eactionswith OH providethedominantpath Chameidesand Tan, 1981; Prinn et al, 1983, 1987; Khalil and for removalof a varietyof halocarbonsh,ydrocarbonsa,ndCO, Rasmussen,1984a; Fraser et al, 1986], since the reaction with andfor conversionof NO2 to nitric acid. Eactionswith OH providethedominantpath Chameidesand Tan, 1981; Prinn et al, 1983, 1987; Khalil and for removalof a varietyof halocarbonsh,ydrocarbonsa,ndCO, Rasmussen,1984a; Fraser et al, 1986], since the reaction with andfor conversionof NO2 to nitric acid. OH determinestheatmospherircesidencetimesfor manyindustri- CH3CC13is documentedby theindustry[Midgley, 1989]. Recent al compoundsi,ncludingthosecontributingto the destructionof systematicmeasurementosf CH3CCI3 and othertracerssuchas the ozonelayer andto the greenhouseffect. Becauseof the ex- chlorofluorocarbons(CFCs) [Prinn et al, 1987; Fraser et al, treme variability of OH in time and space,one has to rely on 1985a, b, 1987; Cronn et al, 1986; Cunnold et al, 1986] offer modelsratherthanon observationtso providethe globaldistribu- new possibilitiesfor testingphotochemicaml odels. Uchcomparisonfsor a localenvironmen[tPerneret model (CTM), with realisticrepresentationof global circulation al., 1987; Platt et al, 1988], howeverimportant,do not accom- andchemistry,is essentiaflor interpretationof measurementasnd plishthe taskof testinga globalmodelfor OH, whichdepends for simulationof importantfeaturesof thedistributions. (GCMs) to study the global distribution of chemical tracers.
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