高纬苔原中甲烷检测方案(在线自动监测系统)

natureVol 456|4 December 2008|doi：10.1038/nature07464 LETTERSNATURE Vol 456 4 December 2008 Large tundra methane burst during onset of freezing Mikhail Mastepanov'， Charlotte Sigsgaard²， Edward J. Dlugokencky， Sander Houweling4.5， Lena Strom，Mikkel P. Tamstorf & Torben R. Christensenl Terrestrial wetland emissions are the largest single source of thegreenhouse gas methane. Northern high-latitude wetlands con-tribute significantly to the overall methane emissions from wet-lands， but the relative source distribution between tropical andhigh-latitude wetlands remains uncertain23. As a result， not all theobserved spatial and seasonal patterns of atmospheric methaneconcentrations can be satisfactorily explained， particularly forhigh northern latitudes. For example， a late-autumn shoulder isconsistently observed in the seasonal cycles of atmospheric meth-ane at high-latitude sites， but the sources responsible for theseincreased methane concentrations remain uncertain. Here wereport a data set that extends hourly methane flux measurementsfrom a high Arctic setting into the late autumn and early winter，during the onset of soil freezing. We find that emissions fall to alow steady level after the growing season but then increase signifi-cantly during the freeze-in period. The integral of emissions dur-ing the freeze-in period is approximately equal to the amount ofmethane emitted during the entire summer season. Three-dimen-sional atmospheric chemistry and transport model simulations ofglobal atmospheric methane concentrations indicate that theobserved early winter emission burst improves the agreementbetween the simulated seasonal cycle and atmospheric data fromlatitudes north of 60°N. Our findings suggest that permafrost-assoc-iated freeze-in bursts of methane emissions from tundra regionscould be an important and so far unrecognized component of theseasonal distribution of methane emissions from high latitudes. Methane emissions from permafrost dominated tundra regionsare well documented- and also recognized as considerable contri-butors to the dynamics of high-latitude atmospheric methane con-centrations. The scale and dynamics of growing-season methaneemissions from tundra settings have been documented mostlythrough flux measurements made with low time resolution usingmanual chambers5，6，10 together with some at higher time resolutiontaken only during the growing season’11，12. Here we report a data setthat extends hourly CH4 flux measurements from a high Arctic set-ting into the frozen season. The measurement site is located inZackenberg Valley， northeast Greenland， 74.30°N 21.00°W. Sixautomated chambers provided flux measurements once per hour，in a typical fen area dominated by graminoids Eriophorum scheuch-zeri， Dupontia psilosantha and Arctagrostis latifolia. Methane concen-tration in the chambers was measured by a laser off-axis integrated-cavity output spectroscopy analyser (Fast Methane Analyser， LosGatos Research). The instrument sensitivity is better than 10 p.p.b.；time resolution of the primary concentration data is 1 s. As part of the field season of the 2007 International Polar Year， theZackenberg research station was kept open two months longer thannormal. This gave us a chance to observe autumn and early-winterfluxes， which showed some surprisingly high emissions (Fig. 1； Supplementary Table 1). This very high and variable flux happenedwhen the active layer was gradually freezing， so CH4 that had accu-mulated in this layer was probably squeezed out through the frostaction. This feature has not been observed in studies at lower lati-tudes， possibly because the permafrost bottom is necessary to preventCH4 from diffusing downwards. The autumn fluxes varied greatlyover small distances (chambers were less than 1 m apart)， probablybecause peat and vegetation structure provided pathways for emis-sion to the atmosphere. A late-autumn increase in methane emis-sions was observed in one of the early tundra flux studies13， but itlacked the time resolution needed to quantify the relative importancefor the annual flux budget. The observed growing season emission dynamics are comparableto earlier work at the same and at similar tundra sites. Integratedsummer season emissions， roughly 4.5g CH4 mfor the season， alsomatch well with previous estimates for the same climatic and ecosys-tem setting.7. Emissions decreased during September until they reached the pre-sumed low winter emission level (Fig.1). However， at the onset of soilfreeze-in， a substantial increase in emissions was observed and wassustained for several weeks， corresponding to the time required for acomplete freeze-in of the entire soil and root zone profile. Freeze-inemissions were much more variable than summer emissions. Peakemissions during the freeze-in period in individual chambers reachedlevels of 112.5mgCH m-h， which to our knowledge are thehighest rates reported from tundra ecosystems (excluding hotspotemissions from thermokarst lakes4)， and they appear at a time whenprevious assumptions would put tundra emissions at a negligiblelevel (see Supplementary Information for further discussion). Earlier studies have indicated the possibility of a spring burst fromtrapped methane during the winter5，16. We have early-season fluxdata from Zackenberg for 2006 (M. Mastepanov et al.， manuscript inpreparation) showing that spring emissions amounted to less than2% of summer emissions (Fig. 1 insert； Supplementary Table2)， withsummer emissions being very similar for 2006and2007(Supplementary Tables 1 and 2). Emissions of methane during springfrom this type of tundra environment are therefore not considered asa major contributor to annual methane emissions. To investigate the potential importance of the observed methaneemissions during freezing of the permafrost surface layer at largescales， we carried out model simulations of atmospheric transportand compared them with observations. Model-simulated methaneconcentrations were sampled at the times and locations when mea-surements were taken at selected background monitoring sites of theNOAA Earth System Research Laboratory's cooperative air samplingnetwork. Average seasonal cycles were constructed from air samplescollected over the 4-year simulation period. Furthermore， back-ground sites were averaged into two latitudinal bands： 25-55°N ( ' GeoBiosphere Science Centre， Ph y sical G e o graphy and E cosystems Analys i s， Lund University，So l vega t an 12，22362 ， L u nd， Sweden . In s titute o f Geogra p hy and Geology， University ofCopenhagen， Oster Voldgade 10， DK-1350 Copenhagen，Denmark. NOAA Earth System Research Laboratory， 3 2 5 B r oadway， Boulder， Colorado 80305， USA. SRON N etherlands T L Institute for Space Research， Sorbonnelaan 2， 3584 CA Utrecht， The Netherlands. I n stitute for Marine and Atmospheric R e search Utrecht (IMAU)， Utrecht University， Princetonplein 5 ， 3584 CC Utrecht， The Netherlands. N a tional Environmental R e search Institute， University of Aarhus， Frederiksborgvej 399， 4000 Roskilde， Denmark. ) Figure 1|Full-season methane emission and soil temperature. Soiltemperatures at three depths shown as a coloured area between dailyminimum and daily maximum values (5， 10 and 15 cm depth as red， greenand blue). The arrows of the same colour show the date offreezing of eachhorizon. Soil temperatures from the nearby climate station (light blue) areshown for the period when on-site data are lacking. Site-average fluxes areshown as daily mean values averaged over six individual chambers. The error and 55-85°N (see Methods for a list of sites and where to access theCH4 data). The averages represent 30-day running means of all sam-ples (either modelled or measured)， calculated in 5-day intervals. There is a very reasonable agreement between the reference emis-sion scenario (SC1) and the measurements (Fig. 2). At mid latitudesof the Northern Hemisphere， both model scenarios nicely reproducethe seasonal amplitude， although the phase lags the measurements byabout a month in the second part of the year. At high northernlatitudes the differences between model and measurements are more Figure 2|Comparison of measured and model-simulated latitudinallyaveraged seasonal cycles of methane. Black， measurements with 2 sigmauncertainty intervals； red， the model simulation using the referencescenario； green， the model simulation including a representation ofadditional emissions from freezing permafrost (see text). bars show standard error of mean between the chambers. The lower insertedpanel shows early-season emission in 2006 during the corresponding periodrelative to the date of snowmelt in 2007 (yellow arrows indicate date ofsnowmelt in the two years). The onset of the second emission peak coincideswith freezing of the upper horizon and continues to reach a maximum whensoil freezes down to-15cm. pronounced， highlighting a deficiency of the reference model insimulating the timing of the concentration increase from summerto winter. Interestingly， the largest deviations occur in October whenthe unrepresented emissions from permafrost are highest. The dif-ference between the two model simulations confirms that the influ-ence of the simulated permafrost emissions is considerable and doesimprove the simulated seasonal cycle. Significant differences remainbetween model SC2 and the measurements， but it should be kept inmind that the underlying parameterization is only a preliminaryextrapolation of the actual flux measurements. Therefore， once addi-tional information on permafrost freeze-in emissions become avail-able， confirmation of our model results is needed on the basis of amore sophisticated emission parameterization. Nevertheless， theseresults show that CH4 emissions from the freezing active layer inpermafrost areas may be an important missing process that limitsmodel performance at high northern latitudes. We also investigated whether there has been a change in the shapeof the seasonal cycle in recent years by comparing observed seasonalcycles for the periods1992-95 and2002-05. The results(Supplementary Fig.3) demonstrate that both seasonal cycles (25-55°N and 55-85°N) were remarkably constant over these periods，indicating that the signature of permafrost emissions in the observedseasonal cycle is not a recent phenomenon. The flux measurements and atmospheric transport model resultspresented here are likely to be of a general nature， as there is nothingunique or artificial about this study site. It is situated in one of themost pristine environments in the world (the National Park ofnorth-east Greenland) and there is no reason why such a physical mech-anism should not happen everywhere that there are similarecosystems. This study benefited from the unique opportunitythrough the International Polar Year effort to keep the Zackenberg station open for longer than usual， and thus to observe a phenom-11n，enon that has most likely been missed in other measurements aroundthe circumpolar north because of the difficulties of maintaining fluxmeasurements into the frozen season at remote high-emitting wettundra sites. Ifthe fluxes measured at Zackenberg are applied to all of0.88×10m of wet meadow tundra" (disregarding possible sim-ilar emissions from mesic tundra which covers even greater areas)，itwill amount to a pulse of ~4Tg CH4 from the highest latitudes atwhat was previously thought to be an inactive time of year in terrest-rial ecosystems.This is in agreement with a corresponding estimatebased on the three-dimensional modelling which amounts to3.9 Tg CH4 (see Methods). This does not greatly increase emissionestimates from high northern latitudes， but it revises our view of theseasonal distribution of known emissions. METHODS SUMMARY Methane emissions were measured by an automatic chamber method； flux wascalculated from the increase in the chamber CH4 concentration， corrected by airtemperature and pressure.Erroneous measurements (for instance，during strongsoutherly winds that tend to cause improper closing of the chambers) werefiltered out and no artificial corrections or gap filling were applied. Global methane concentrations were simulated using an atmospheric chem-istry and transport model， which includes a dedicated representation of themethane cycle. Calculations were performed with and without a parameteriza-tion of methane emissions from freezing permafrost. Simulated concentrationswere compared with high precision methane measurements representative ofthebackground conditions at mid to high northern latitudes. Full Methods and any associated references are available in the online version ofthe paper at www.nature.com/nature. Received 25 April； accepted 18 September 2008. Mikaloff Fletcher， S. E.，Tans， P. P.， Bruhwiler， L. M.， Miller， J. B.& Heimann，M. CH4sources estimated from atmospheric observations of CH4 and its 13c/C isotopicratios： 1. Inverse modeling of source processes. Glob. Biogeochem. Cycles 18，GB4004(2004). 2. Dlugokencky， E. J.et al. Atmospheric methane levels off： Temporary pause or anew steady-state? Geophys. Res. Lett. 30，1992(2003). 3. Miller， J. B. et al. Airborne measurements indicate large methane emissions fromthe eastern Amazon basin. Geophys.Res. Lett. 34， L10809 (2007). ( 4 . Dlugokencky， E. J.，S t eele， L. P.， Lang， P. M. & M asarie， K. A. The growth rate and d istribution of atmospheric methane. J. Geophys. Res . 99 ， 17021-17043 (1994). ) 5. Reeburgh， W. S. et al. A CH4 emission estimate for the Kuparuk River basin，Alaska.J. Geophys. Res. 103， 29005-29013 (1998). 6. Christensen， T. R. et al. Trace gas exchange in a high-arctic valley 1. Variations inCO2 and CH4 flux between tundra vegetation types. Glob. Biogeochem. Cycles 14，701-713(2000). 7. Friborg， T.， Christensen， T.R.， Hansen， B. U.， Nordstroem， C. & Soegaard， H. Tracegas exchange in a high-arctic valley 2. Landscape CH4 fluxes measured andmodeled using eddy correlation data. Glob. Biogeochem. Cycles 14， 715-723(2000). .8. Gedney， N.， Cox，P. M. & Huntingford， C. Climate feedback from wetland methaneemissions. Geophys. Res. Lett. 31， L20503 (2004). 9 Bousquet， P. et al. Contribution of anthropogenic and natural sources to atmospheric methane variability. Nature 443， 439-443 (2006). 10.C(hristensen， T. R. Methane emission from Arctic tundra. Biogeochemistry 21，117-139(1993). 11 Fan， S. M. et al. Micrometeorological measurements of CH4 and CO2 exchangebetween the atmosphere and subarctic tundra. J. Geophys.Res. 97，16627-16644(1992). 12.Corradi， C.， Kolle， O.， Walter， K.， Zimov， S. A. & Schulze， E.-D. Carbon dioxide andmethane exchange of a north-east Siberian tussock tundra. Glob. Change Biol. 11，1910-1925(2005). 13.Whalen，S. C. & Reeburgh， W. S. A methane flux time series for tundraenvironments.Glob. Biogeochem. Cycles 2， 399-409 (1988). 144.Walter， K. M.， Zimov， S. A.， Chanton， J. P.， Verbyla，D. & Chapin， F. S. III. Methanebubbling from Siberian thaw lakes as a positive feedback to climate warming.Nature 443， 71-75(2006). 15.Hargreaves， K. J.， Fowler， D.， Pitcairn， C. E. R. & Aurela， M. Annual methaneemission from Finnish mires estimated from eddy covariance campaignmeasurements. Theor. Appl. Climatol. 70，203-213(2001). 6.Tokida，T. et al.Episodic release of methane bubbles from peatland during springthaw. Chemosphere 70， 165-171(2007). 7.Bliss， L. C. & Matveyeva， N. V. in Arctic Ecosystems in a Changing Climate： AnEcophysiological Perspective (eds Chapin， F.S. III， Jefferies， R. L.， Reynolds， J.F.，Shaver，G. R. & Svoboda， J.)59-89 (Academic，1992). Supplementary Information is linked to the online version of the paper atwww.nature.com/nature. Acknowledgements This work was carried out under the auspices of the GeoBasisprogramme and part of the Zackenberg Ecological Research Operations funded bythe Danish Ministry of the Environment and the ISICaB project funded by theCommission for Scientific Research in Greenland (KVUG).ASIAQ-GreenlandSurvey provided climate data. The work was also supported by the SwedishResearch Councils VR and FORMAS. We thank P. Bergamachi (JRC) and J.-F.Meirink (KNMI) for providing the TM5 model setup. T. Tagesson helped with thefield work in Zackenberg. We are grateful for comments on earlier versions of thismanuscript from A. Lindroth and B. Christensen. Author Contributions T.R.C.， M.P.T.， M.M.， C.S. and L.S. designed the fieldresearch； M.M. designed， constructed and set up the automatic measurementsystem in Zackenberg； C.S. operated the system and performed manualmeasurements； M.M. performed data analysis； E.D. and S.H. provided atmosphericCH4 data and designed and ran the atmospheric transport model experiments；T.R.C.， M.M.， S.H. and E.D. drafted the manuscript. Author Information Reprints and permissions information is available atwww.nature.com/reprints.Correspondence and requests for materials should beaddressed to T.R.C. (Torben.Christensen@nateko.lu.se). METHODS Zackenberg site description. The Zackenberg Valley is situated at 74°30'N，21°00'W in the National Park of northeast Greenland. A research station(Zackenberg Research Station) was established in 1997 and offers basic logisticalfacilities (an airstrip，laboratories，satellite-based communication systems and soforth) necessary for carrying out efficient research. The site has a short record ofmeteorological observations， and 1996 was the first full year when basicmeteorological variables were registered continuously. Mean annual temper-ature in the first 10-year period of the station ranged from -8.5℃ to-10.1℃ with July as the warmest month (mean monthly air temperature of5.8C) and February as the coldest month (mean monthly air temperature of-22.4℃). The average frost-free period during these 10 years was 35 days，last-ing from mid July to late August. In Daneborg， situated on the outer coast 22 kmsoutheast of Zackenberg and with a longer period of meteorological measure-ments， the mean annual temperature for the period 1960-90 was -10.3°C. Thewarmest month，July， had a mean of3.8 °C and February， the coldest month，hada monthly mean of -17.6℃. The valley is dominated by minerotrophic sedge-grass-rich fens mixed with elevated areas of dwarf shrub heaths with Cassiopetetragona and Salix arctica as dominant species. A slightly sloping fen area is themain study area here. The peat layer in the fen is 20-30 cm thick， typical of higharctic fen ecosystems8. Onset of peat accumulation has been 4C-dated to AD1290-1390 in a neighbouring fen area， and the surrounding Little Ice Age nivalfans and nivation basins primarily contain organic material deposited from AD1420 to 1500-158019. The active layer depth specifically on the measurement site reached 50-56 cm(near different chambers) before soil freezing in 2007. Despite the low tempera-tures， the snow cover was mosaic until 20 October， and then was no more than3 cm deep until the first snowstorm on 26 October. A large body of background information from the Zackenberg ResearchStation has recently been summarized in a book volume celebrating the first10 years of activities at the research station. Methane flux measurements. Automatic chambers were deployed in August2005 and the first seasonal data set was obtained in 2006 (3 July to 26 August). In2007 an extended season was carried out (26 June to 25 October). Six chambershave been aligned in a row from the periphery of the fen towards its central part.The distance between the chambers is 30-80cm. The chambers are made ofPlexiglas with aluminium corners. Each chamber is 60×60cm and about30 cm height (depending on microtopography). The chamber lid stays openfor 55 min per hour and closes for five minutes for the measurements. Air ismixed in a closed chamber by a fan； the same fan ventilates a chamber when it isopen. Air from the chamber passes through 30 m of tubing (internal diameter4mm) to the analytical box and after the non-destructive analysis it goes back tothe chamber. The analytical box contains a methane analyser (Fast MethaneAnalyser， Los Gatos Research)， CO2 analyser (SBA-4， PP Systems) and solenoidvalves. The concentration data are collected at 1 Hz rate； data acquisition startsthree minutes before a chamber closes， continues for five minutes while it isclosed， and then two minutes after the chamber opens， so the full cycle of sixchambers takes one hour. Although we did not make direct measurements of soil temperature andhumidity inside the chambers， to avoid extra disturbance， the visual controldoes not give any evidence of the construction affecting the temperature andwater regime inside the chambers. Visible water table and the snow level (duringthe snowfall and snowmelt) is the same inside and outside the chambers. The CH4 fluxes are calculated from the slope of concentration change in theclosed chamber； if the increase was not linear during five minutes of closure， themost linear part of this time is arbitrarily chosen. The air temperature andpressure for flux calculations are obtained from Zackenberg micrometeorologi-cal station located about 1 km from the site. We made additional measurementsof water table level， active layer depth， PAR， soil temperature and humidity nextto the chambers. Atmospheric CH4measurements and chemical transport model. AtmosphericCH measurements are from weekly samples collected at sites in the NOAA EarthSystem Research Laboratory's cooperative global air sampling network. Wedetermined methane dry-air mole fractions by gas chromatography with flameionization detection against the WMO CH4 mole fraction standard scale. Overthe period of this study， repeatability of the measurements (1o) was ~2 p.p.b.Latitudinal averages contained the following sites： 25-55°N contained ‘mid'，bme’， bmw， uta’， mhd'， ask'，nwr’， pta’， azr’， izo’， pocn30'and‘pocn25'，and 55-85°N contained alt'， shm'， brw'， ice’， zep’， stm’， cba’and ‘sum’(seewww.esrl.noaa.gov/gmd/ccgg/flask.html for a list of site codes and ftp：//ftp.cmdl.noaa.gov/ccg/ch4/flask/event for access to data). We made atmospheric transport model calculations using the TM5 modelfor the period 2002-05， at a spatial resolution of 6°×4° and 25 vertical sigma-pressure levels. Two scenarios of methane sources and sinks were applied： areference scenario (SC1) and a scenario including emissions from permafrostfreeze-in (SC2). The methane sources and sinks of SC1 correspond with the apriori assumptions that were used in the inverse modelling calculations ofref. 21，with the exception of wetlands. Wetland emissions were taken from ref. 22. andrescaled to a global total of 175 Tg CHayrand high-latitude (50-90°N) emis-sions of 20Tg CH4yr. SC2 is the same as SC1 except for additional emissionsfrom freezing permafrost. For lack of any detailed information on this process，we followed a highly simplified procedure， assuming that emission started whenthe diurnal mean temperature dropped below -2C and continued for a periodof 1 month. This process was only active in those model grid boxes that wereclassified as continuous or discontinuous permafrost according to the CAPScircumpolar permafrost map2. The annual emission of freezing permafrostwas assumed to be the same as the (summer time) wetland emission in eachmodel grid box for which the process is active. This procedure introduces anadditional source of 3.9Tg CH4yr， which moves from north to south duringautumn and reaches maximum global emissions in October. 18. Meltofte， H.， Christensen， T. R.， Elberling， B.， Forchhammer， M. C. & Rasch， M.(eds)High-Arctic Ecosystem Dynamics in a Changing Climate. Advances in EcologicalResearch Vol. 40 (Elsevier， 2008). 19C.hristiansen， H. H. et al. Holocene environmental reconstruction from deltaicdeposits in northeast Greenland. J. Quat. Sci. 17， 145-160 (2002). 20. Krol， M. C. et al. The two-way nested global chemistry-transport zoom modelTM5： algorithm and applications. Atmos. Chem.Phys. 5，417-432(2005). 21.Bergamaschi， P. et al. Satellite chartography of atmospheric methane fromSCIAMACHY on board ENVISAT. 2. Evaluation based on inverse modelsimulations. J. Geophys. Res. 112， D02304 (2007). 22.. Walter， B. P.， Heimann， M. & Matthews，E. Modeling modern methane emissionsfrom natural wetlands. 2. Interannual variations 1982-1993.J. Geophys. Res. 106，34207-34217(2001). 23. Brown，J.， Ferrians，O.J. Jr， Heginbottom，J. A. & Melnikov， E. S. Circum-arctic Mapof Permafrost and Ground-Ice Conditions. USGS Circum-Pacific Map Series CP-45(scale 1：10，000 000) (US Geological Survey， 1997). Macmillan Publishers Limited. All rights reserved