Abstract
The atmospheric distributions of CH4, C2H6, C3H8, C2H2, and C2Cl4 and their annual chemical removal rates in steady state are determined versus latitude using a modified version of the Oslo two‐dimensional global tropospheric photochemical model. A photochemically calculated hydroxyl radical distribution, which has been validated with methylchloroform data, and seasonally varying surface measurements of the title species are used to compute their respective global annual surface source strengths and steady state lifetimes. Computed annual surface source strengths of CH4, C2H6, C3H8, C2H2, and C2Cl4 are 490, 10.4, 8.4, 3.1 Tg (1 Tg = 1012 g), and 432 kT (1 kT = 109 g), respectively. The calculated annual chemical removal rates of these compounds show distinct latitudinal distributions. Because their steady state global lifetimes are less than the model interhemispheric exchange time (about 1 year), the calculated north to south ratios of the deduced surface emission strengths of C2H6, C3H8, C2H2, and C2Cl4 probably reflect the locations of their sources. Within the limits of previously estimated industrial emissions of C2Cl4 (3–4 kT) for the southern hemisphere, our calculations indicate that about 47 kT of additional southern hemispheric source of C2Cl4 is required for 1989–1990 to attain steady state mass balance in this region. There are two possibilities for this needed source: either other industrial sources are missing, or there are unidentified natural sources of C2Cl4. So far, oceans have been suggested as a natural source. Normalization of monthly varying ratios of hemispherically averaged calculated surface mixing ratios of C2H6, C3H8, and C2H2 and their respective observed mixing ratios with respect to those for C2Cl4 indicates that the sources of these hydrocarbons are seasonal in nature. It is also shown that convective transport effectively redistributes these short‐lived species but their calculated surface source strengths are relatively independent of this transport process.
Highlights
Surface Measurements of Light Hydrocarbons and C2C14One of the goalsof thispaperis to estimatesurfacesource strengthsof light hydrocarbonsand C2C14S. urfaceemission estimatesof CH4 from varioussourcesare reasonablywell constrainedwithin a certain range [Ciceroneand Oreroland, 1988;Crutzen,1991],but for NMHCs, asstatedearlier,there stillexistmanyuncertaintiesin the identities,andhencein the magnitudesof their sourcestrengthsW. ith the exceptionof
Port of light hydrocarbonsand C2C14and for their chemical The chemicalpart of the two-dimensionaml odel equation lossesdue to prescribeddistributionsof OH and C1radicals. was solvedusingthe time-flux operator splittingmethod and Monthly averageddistributionsof C1and OH radicalsfor the "quasisteadystate approximation(QSSA)" [Hesstvedett al., extendedregion were taken from the Oslo two-dimensional 1978].To maximizemassconservationand to uselongertime tropospheric-stratosphermicodel.for the marinebound- stepsfor integration,the chemicalfamilytechniquewasusedin ary layer (MBL), monthlyvaryingdistributionsof C1radicals addition to constrainingthe sum of concentrationsof rapidly were adopted from the Oslo model
The calculatedannual to smooth out the excessivegradient distribution causedby steadystatesourcestrengthof C2C14is 432 kT with a north to convection.Becauseour convectivetransportschemeemploys south ratio of 7.5
Summary
One of the goalsof thispaperis to estimatesurfacesource strengthsof light hydrocarbonsand C2C14S. urfaceemission estimatesof CH4 from varioussourcesare reasonablywell constrainedwithin a certain range [Ciceroneand Oreroland, 1988;Crutzen,1991],but for NMHCs, asstatedearlier,there stillexistmanyuncertaintiesin the identities,andhencein the magnitudesof their sourcestrengthsW. ith the exceptionof. GCM) [Plumband Mahlman, 1987].The transportfeaturesof surfaceconcentrationswere used as the lower boundaryconthis model have been tested with observed distributions and dition To investigatethe seasonalityin their sources,the caltrendsof CFC-11,CFC-12[Cunnoldet al., 1994],and85Kr culated annual latitudinal surface emissionswere uniformly [Weisset al., 1983] tracersyielding an interhemisphericex- applied on per time step basis at the lower boundary.For changetime of about 1 year [Gupta, 1996]. Was solvedusingthe time-flux operator splittingmethod and Monthly averageddistributionsof C1and OH radicalsfor the "quasisteadystate approximation(QSSA)" [Hesstvedett al., extendedregion were taken from the Oslo two-dimensional 1978].To maximizemassconservationand to uselongertime tropospheric-stratosphermicodel.for the marinebound- stepsfor integration,the chemicalfamilytechniquewasusedin ary layer (MBL), monthlyvaryingdistributionsof C1radicals addition to constrainingthe sum of concentrationsof rapidly were adopted from the Oslo model In this model the only cyclingspeciessuchasOH/HO2, HO2/HO2NO2, and NO/NO2 source of chlorine radicals in the MBL is due to reaction of [Berntsenand Isaksen,1994].For all the simulationst,ypically.
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