Abstract
A global three‐dimensional chemical tracer model of the distribution and seasonal cycles of the surface concentration of CH3CCl3 is compared with surface observations from the Atmospheric Lifetime Experiment (ALE) for the years 1980–1985. Two‐dimensional OH distributions calculated by a photochemical model are empirically adjusted from observed trends in the global average and the interhemispheric ratio of methyl chloroform. The effects of the recently discovered ocean sink for methyl chloroform were investigated. The model simulates the 5‐year record of observations made at the five ALE sampling sites to generally within ±5% of the observed mean. The calculated average global lifetime of methyl chloroform is 5.7 ± 0.3 years. The estimated global mean OH concentration is 6.5 ± 0.4 × 105 cm−3. However, the inclusion of the ocean sink does not significantly improve the simulation of the observed interhemispheric gradient of methyl chloroform. Atmospheric transport dominates the simulated CH3CCl3 seasonal cycle throughout the northern hemisphere but is less important in the southern hemisphere.
Highlights
The globaldistributioonf sourceosf greenhousgeasesinvites sourcesof CH3CC13.Beforethe recentdiscoveryof an apparent theuseof generacl irculatiomn odels(GCMs)of theatmosphereocean sink for methyl chloroform [Butler et al, 1991], it was to examinethe 3-dimensionadlistributionstr,ends,andseasonalthoughtthat the soletroposphericsink of CH3CC13was through cyclesof thesetracegasesA. lthoughtherearequestionasbouthe reaction with OH radicals [Lovelock, 1977]
Becausethe flux of al., 1990].Theyhavebeenusedtosimulatteheglobadl istributionCH3CC13to the tropospherecan be closely approximatedby of inerttracegasessuchasCFC-11,whichhasnotonlya well- industrialproductionfiguresand its destructionthroughreaction known,solelyhumansourcebut alsoan extendedhistoryof with OH canbe expressedassimplebimoleculareactionrate,the atmosphericmeasurementsfor validation [Kao et al, 1992; global distribution,seasonalcycle, and trend of CH3CC13have
Thesolechemicarleactionwithinthetroposphertheatdestroys methylchloroformisthereactionwithOH radicalsP. hotolysibsy We began with monthly average two-dimensional OH
Summary
X. TIE,1'2C.-Y. KAO3, ANDE. J.MROZ3, R. J.CICERON4EF, . N. ALYEA5, ANDD. M. CUNNOL5D A globalthree-dimensionaclhemicaltracermodelof the distributionandseasonacl yclesof the surface concentratioonf CH3CC13is comparedwith surfaceobservationfsrom theAtmospheriLcifetimeExperiment (ALE) for theyears1980-1985.Two-dimensionaOlH distributioncsalculatedby a photochemicaml odelare empiricallaydjuste'dfromobservetdrendsin theglobaal veragaendtheinterhemispherraitcioof methyl chloroformT. he effectsof the recentlydiscoveredoceansinkfor methylchloroformwereinvestigatedT.he modelsimulatesthe5-yearrecordof observationmsadeat thefive ALE samplingsitesto generallywithin+5% of theobservedmean.The calculatedaveragegloballifetimeof methylchloroformis 5.7 + 0.3 years.The estimatedglobalmeanOH concentratioisn6.5 + 0.4 x 105cm-3.However,theinclusionof theoceansinkdoes not significantlyimprovethe simulationof the observedinterhemispherigcradientof methylchloroform. AtmospheritcranspordtominatetshesimulatedCH3CC1s3easonaclyclethroughoutht enorthernhemispherbeut is lessimportantin thesouthernhemisphere.
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