Oct 30, 1992 - cell cooled thermoelectrically to about 0ÑC, and filled with glass ...... the available data would be required, however, to significantly ira- data.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. D15, PAGES 16,627-16,643, OCTOBER 30, 1992
MicrometeorologicalMeasurementsof CH4 and CO2 Exchange Betweenthe Atmosphereand SubarcticTundra S. M. FAN, S.C. WOFSY, P.S. BAKWIN, and D. J. JACOB Divisionof AppliedSciences,Harvard University,Cambridge,Massachusetts
S. M. ANDERSON,P. L. KEBABIAN,J. B. MCMANUS, and C. E. KOLB Centerfor ChemicalandEnvironmentalPhysics,AerodyneResearch,Inc., B dlerica,Massachusetts
D. R. FIIZJARRALD
Atmospheric Sciences ResearchCenter,StateUniversityof New York-Albany Exchanges of methaneandcarbondioxidebetweentheatmosphere andtheArctictundraweremeasured continuouslynearBethel,Alaska(61ø05.41'N,162ø00.92'W),for 5 weeksduringJuly and August1988. Fluxes wereobtaineddirectlyusingeddycorrelation at 12-maltitude,andconcentrations weremeasured sequentially at eightaltitudesbetween0 and 12 m. A prototypedifferentialinfraredabsorption methaneinstrument basedon a Zeeman-split HeNelaserwasusedfor determination of methaneanda flameionizationdetectorfor totalhydrocarbons(TI-IC). Methanewas found to accountfor nearly all the THC concentrations and fluxes.Methane fluxesat thetowersitewereapportioned to variousmethane-producing habitats,usinga satelliteimageto classify surfacevegetation at 20 x 20 m resolution.The "footprint"of the towerwascomputed usinga Gaussian plumemodelfor dispersion in the surfacelayer. Grid cellsclassified asdry tundra(waterlevel 5 cm belowsur-
face)emitted methane atanaverage rateof 11+3(standard error)mgCI-!4/m 2/d,andwetmeadow tundra (water levelnearsurface)emitted 29-•_3 mgCH4/m2/d. Fluxes fromlakesdepended onwindspeed, averaging 57+_6 mgCH4/m 2/datthesite,where themean windspeed was5 ms-1. Themean emission ratefortundra (includinglakes) around thetower was25+1mgCHa/m2/d, notably smaller thanadopted forboreal weftands in recent inventories of globalmethanesources.Emissions from majorhabitatsderivedfrom the towermeasurements werein reasonable agreement withdatafromchamber studies.Errorsof a factorof-2 accrued in scaling upthe
chamber data,representing 1m2 plots, tothefootprint ofthetowermeasurements (103m),because thesatellite couldnot fully resolveheterogeneous methane-producing habitats.Fluxesobtainedat the towersitewerein harmony withfluxesfromaircraftoverflights. Theaircraftdatarepresent mainlyafternoon periods withgood flyingweather,conditions associated with maximumCI-!4fluxesin thetowertime series.Mean fluxesfrom the
aircraft areconsequently -2 x higherthanseasonal means fromtheregion.Solarirradiance provided theprimarycontrolon thenetecosystem exchange (NEE)of carbondioxide.Themeanmaximumuptakenearthe
localsolar noon was1.4_+0.2 gC/m2/d, andnocturnal respiration averaged 0.73:L'0.18 gC/m2/d. Netuptake of carbon dioxide averaged 0.30gC/m2/d (0.1tonsC/hectare) during theperiod oftheArctic Boundary Layer Experiment (ABLE3A). About6%oftheseasonal netuptake wasreturned totheatmosphere asmethane.
1. INTRODUCTION
source[Matthewsand Fung, 1987; Ciceroneand Oremland,1988; Aselmanand Crutzen, 1989]. The currentuncertaintyin the emis-
Concentrations of atmospheric methanedoubledduringthepast sionratefromnaturalwetlandsis believedto be approximately a
200years[CraigandChou,1982;Stauffer etal., 1985;Khaliland factorof 2. Datafornorthern wetlands, primarily tundraandboRasmussen, 1987],andincreases of 1% per yearcontinue at realbogsandfensbetween 50ø and90ø N (primarily in Alaska, present [Steele et al., 1987;BlakeandRowland, 1988;Zanderet Canada, andtheUSSR)arenotablysparse, eventhoughthese al., 1989;KhalilandRasmussen, 1990].Methane haslongbeen biomes comprise overonehalfof theEarth's areaof natural wetrecognized asa criticalreactant in thephotochemistry of boththe lands[Matthews andFung,1987]. troposphere andthestratosphere [Levy,1971;McConnell et al., Fluxmeasurements madeat northern sitesusingsmallenclo-
1971;Ehhalt, 1974;Wofsy, 1976]andhasbeenidentified asamajot greenhouse gas,accounting forabout 12%of theincrease in global mean radiative forcing during thepastdecade [Hansen, et al.,1989]. Microbial decayof organic matterin natural wetlands is be-
sures (
