BOREAS TGB-01 CH4 Concentration and Flux Data from NSA Tower Sites Summary The BOREAS TGB-01 team made numerous measurements of trace gas concentrations and fluxes at various NSA sites. This data set contains half-hourly averages of ambient methane (CH4) measurements and calculated fluxes for the NSA-TF in 1996 and the NSA-BP and NSA-OJP tower sites in 1994. The purpose of this study was to determine the CH4 flux from the study area by measuring ambient CH4 concentrations. This flux can then be compared to the chamber flux measurements taken at the same sites. The data are provided in tabular ASCII files. Table of Contents * 1. Data Set Overview * 2. Investigator(s) * 3. Theory of Measurements * 4. Equipment * 5. Data Acquisition Methods * 6. Observations * 7. Data Description * 8. Data Organization * 9. Data Manipulations * 10. Errors * 11. Notes * 12. Application of the Data Set * 14. Software * 15. Data Access * 16. Output Products and Availability * 17. References * 18. Glossary of Terms * 19. List of Acronyms * 20. Document Information 1. Data Set Overview 1.1 Data Set Identification BOREAS TGB-01 CH4 Concentration and Flux Data from NSA Tower Sites 1.2 Data Set Introduction Ambient CH4 was measured at two heights on a tower at the Northern Study Area (NSA) Tower Fen (TF), Beaver Pond (BP), and Old Jack Pine (OJP) sites during the summers of 1994 and 1996 by the BOReal Ecosystem-Atmosphere Study (BOREAS) Trace Gas Biogeochemistry (TGB)-01 team. Gradients were calculated and fluxes were determined from the available meteorological data. 1.3 Objective/Purpose The purpose of this study was to determine the CH4 flux from the study area by measuring ambient CH4 concentrations. This flux can then be compared to the chamber flux taken at the same sites. 1.4 Summary of Parameters Ambient CH4 concentrations in parts per million (ppm), CH4 fluxes in micromoles/m2/sec, air temperature in oC, and air pressure in kPa are reported. 1.5 Discussion The ambient CH4 measurements at the tower were collected semicontinuously once every 3 minutes over the field period from 15-Apr through 22-Oct-1996 at theTF site. The measurements were collected every 6 minutes over the field period from 03-Jun trhough 17-Sep-1994 at the BP site and from 29-May through 19-Sep- 1994 at the OJP site. The tower measurements of ambient methane are used to calculate an integrated flux from a larger area than that which is measured with the chambers. The tower flux calculations can be compared with the chamber flux measurements to show plant community effects on flux rates. The towers had some problems throughout the field season that are addressed in Section 6. The data for the TF site are in the file tf_dat.txt. 1.6 Related Data Sets BOREAS TGB-01 NSA CH4 and CO2 Chamber Flux Data BOREAS TGB-01 CH4 and CO2 Chamber Flux Data from NSA Tower Sites BOREAS TGB-03 CH4 and CO2 Chamber Flux Data from NSA Tower Sites 2. Investigator(s) 2.1 Investigator(s) Name and Title Dr. Patrick M. Crill Research Associate Professor University of New Hampshire 2.2 Title of Investigation Magnitude and Control of Trace Gas Exchange in Boreal Ecosystems 2.3 Contact Information Contact 1 Dr. Patrick M. Crill Institute for the Study of Earth, Oceans, and Space Complex Systems Research Center University of New Hampshire Durham, NH (603) 862-3519 (603) 862-0188 (fax) Contact 2 Ruth K. Varner Research Scientist Institute for the Study of Earth, Oceans, and Space Complex Systems Research Center University of New Hampshire Durham, NH (603) 862-2939 (603) 862-0188 (fax) Contact 3 Sara K Golightley NASA GSFC Greenbelt, MD (301) 286-2624 sgolight@pop900.gsfc.nasa.gov 3. Theory of Measurements Tower measurements were completed to determine the turbulent exchange of energy and mass between the atmosphere and a variety of surface types as well as the processes controlling these fluxes. The towers measured radiation, heat, water, CO2, and CH4 fluxes. Using the meteorological data, the flux of trace gases like CH4 can be determined and then compared with the chamber flux data. Refer to TF-10, Harry McCaughey, and TF-08, David Fitzjarrald, for tower information at the TF and OJP sites. Refer to BOREAS TGB-04 Water Table and Sediment Temperature, Nigel Roulet, for tower information at the BP site. 4. Equipment 4.1 Sensor/Instrument Description CH4 was quantified with a Shimadzu GC-MINI2-Gas Chromatograph (GC) with a flame ionization detector (FID) operated at 125 degrees C after separation on a HayeSepQ column at 40 degrees C using ultrapure (99.999%) N2 as a carrier gas flowing at 30 mL/min. Analog signals (0-1 V) from the detectors were digitized at 10 Hertz (Hz) with a Hewlett Packard (HP) 35000D A/D board and quantified and logged using HP ChemStation software. Rotary valve switching and chromatography start/stop were controlled with the HP board and software and a relay driver. 4.1.1 Collection Environment The samples were collected with pumps pulling air through 1/8-inch Nyaflow tubing that ran from the tower to the shack that housed the MINI2-GC. The equipment was operated under a range of ambient atmospheric conditions during the period. 4.1.2 Source/Platform Rohn towers located at NSA TF, OJP, and BP sites. 4.1.3 Source/Platform Mission Objectives The mission objective was to measure surface fluxes at the NSA TF, OJP, and BP sites. 4.1.4 Key Variables Ambient CH4 was measured at two different heights on a tower at the two sites. 4.1.5 Principles of Operation The Shimadzu MINI2-GC is equipped with a hydrogen FID. The FID usess a hydrogen flame in an air atmosphere to burn components as they exit the column. In the flame, carbon-carbon bonds are fragmented so that various organic ions and free electrons exist. Application of a voltage across a collector electrode over the flame causes an ion current to flow, which is amplified and then measured as the output signal. This single signal output is for a data processor, integrator, recorder, or computer (Instruction Manual: MINI2-GC; Shimadzu Corporation, Kyoto, Japan). 4.1.6 Sensor/Instrument Measurement Geometry Not applicable. 4.1.7 Manufacturer of Sensor/Instrument Shimadzu Scientific Instruments, Inc. 7102 Riverwood Drive Columbia, MD 21046 (410) 381-1227 Hewlett Packard 4.2 Calibration A 1.898-ppmv (NSA-BP) or a 2.462-ppmv (NSA-OJP) working standard (1994) was run after each sample. Working standards were calibrated against two Canadian Atmospheric Environment Services (AES) certified primary standards by plumbing the standards into the sampling system and running the system for 12 hours. The working standard was also calibrated in the lab before deployment. 4.2.1 Specifications None given. 4.2.1.1 Tolerance The FID's maximum sensitivity is 3E*10-12 g/s for diphenyl. 4.2.2 Frequency of Calibration Calibration gases were run after every set of high and low samples. 4.2.3 Other Calibration Information Not applicable. 5. Data Acquisition Methods Sampling ports were suspended at 6.65 m and 3.59 m on the NSA-TF Tower and at 0.25 and 1.5 m above ground level on the NSA-BP. The tower was located 88.0 m into the fen area from the access road and 150 m into the beaver pond. A continuous stream of air was pulled through 1/8-inch outer diameter (o.d.) nylon tubing at 2.5 L/min with diaphragm pumps. Using electronically actuated rotary valves, a subsample of this flow is diverted through a 1-m perma-pure drier (to ensure consistent humidity over a run period), then into the 1 mL sample loop, and then injected into the GC. A 1.898-ppmv working standard was run after each sample. The OJP ambient CH4 sampling was completed in much the same manner as the BP sampling. The sampling ports were suspended at 13.5 m and 30 m above ground level on the tower. A continuous stream of air was pulled through 1/8- inch o.d. nylon tubing at 2.5 L/min with diaphragm pumps. Using electronically actuated rotary valves, a subsample of this flow was diverted through a 1-m hydropurge tube in the NSA-TF and through a 1-m perma-pure drier (to ensure consistent humidity over a run period), then into the 1-mL sample loop, and then injected into the GC. At the NSA-TF, a 1.519-ppmv working standard (14-Apr through 22-Aug), and a 1.517-ppmv working standard (23-Aug through 22-Oct) was used. A 1.898-ppmv CH4 working standard was run after each sample at the BP site. The standard for the OJP site was 2.462-ppmv CH4. 6. Observations 6.1 Data Notes None given. 6.2 Field Notes TF: 14-Apr-1996 - Hooked up gases and instrument at TF tower site. 15-Apr-1996 - MINI2-GC up and running. 30-Apr-1996 - Replaced valve fitting of CH4 standard due to small leak. 02-May-1996 - Generator turned off for maintenance at 09:00. 06-May-1996 - Changed regulator on CH4standard. 16-May-1996 - Generator turned off for maintenance at 11:30. 25-May-1996 - Column temperature of MINI2-GC reduced to 45 degrees C at 11:22 run#00048. 30-May-1996 - Generator turned off for maintenance at 10:11. Replaced fallen air intake tubes on tower in their original places. Restarted at 11:47. 02-Jun-1996 - Generator off for unknown reason. Restarted at 18:15. 05-Jun-1996 - Shut off CH4 standard at 10:19 and moved standard cylinder outside of fen hut. Restarted at 10:54. 07-Jun-1996 - Generator down at 11:11. Turned off CH4 standard, H2, air compressor, MINI2-GC, and unplugged pumps. Started back up at 13:15. 10-Jun-1996 - Switched the pump intakes from high point (6.65 m) to low point (3.59 m). 13-Jun-1996 - Generator turned off for maintenance at 10:15. Restarted at 14:08. 23-Jun-1996 - Turned off MINI2-GC at 10:53. 24-Jun-1996 - Generator down for maintenance at 12:50. Switched intake pumps back to original configuration. Relit GC 14:21. 27-Jun-1996 - Generator down for maintenance at 10:15. Restarted at 12:40. 11-Jul-1996 - Generator down for maintenance. Restarted at 12:45. 14-Jul-1996 - At run #01323 computer clock is 14:03, and 'real' time is 13:53. 21-Jul-1996 - GC not operating, last file recorded was run#1532 on 19-Jul-97 at 02:57. Restarted sequence on run #01533 at 15:59. Computer clock reads 16:11. 25-Jul-1996 - Generator down for maintenance at 13:41. Started up at 14:30. 27-Jul-1996 - Stopped sequence at run #01801, column temperature of MINI2-GC raised to 120 degrees C at 12:44. Turned column temperature to 45 degrees C at 13:55. Restarted with run #01802 at 13:59. 01-Aug-1996 - Generator water pump replacement, system down at 12:30. Restarted for run #02030. Shut down again at 13:45, run #2031. 08-Aug-1996 - Shut down system for generator maintenance at 12:47. Restarted at 13:55. 22-Aug-1996 - Generator down for maintenance at 12:47. Stopped sequence at run #02979. New CH4 standard, 1.517 ppm at site now. At 14:30 quality standard peaks begin to appear due to the CH4 standard regulator pressure. 23-Aug-1996 - Upper tower intake moved horizontally but kept at the same level. Tower work done in the vicinity of the intake from 21:15 to 23:00. 25-Aug-1996 - Upper tower intake moved to original position at 20:20. 29-Aug-1996 - Changed Hydro Purge cylinder. Restarted sequence at 11:55. 07-Sep-1996 - Generator stopped due to power surge, restarted on 09-Sep-97 at 11:00. Last run recorded was run #03622. 09-Sep-1996 - Small fire in generator hut, system down at 11:15. 11-Sep-1996 - Bad filter in generator, system down for a few days. Fuse blew in MINI2-GC. 13-Sep-1996 - Replaced fuse in MINI2-GC. Started sequence at 11:15 with run #03623. Looks good! 19-Sep-1996 - Generator down for maintenance at 09:30. Restarted at 13:45. 04-Oct-1996 - Generator down for maintenance at 12:08. 17-Oct-1996 - Generator down for maintenance at 12:00. Restarted at 12:30 with run #05097. 22-Oct-1996 - Sequence stopped after run #05328 (last run). Tape backup of data files, system shut down for season at 13:40. BP: 02-Jun-1994 - Hooked up gases at BP tower. 03-Jun-1994 - MINI2-GC up and running. 04-Jun-1994 - Generator was down, back up at 15:16. Power down again at 16:57 in the middle of run #45. Power back on line at 17:15. Started sequence with run #45 at 18:41. 05-Jun-1994 - Power down again. Started with run #45 again. Switched out drierite column. 06-Jun-1994 - Generator down again. Instruments back up and running at 14:16. Start with run #66. 08-Jun-1994 - Found machines off. Generator maintenance. Turned everything on at 14:24. Run sequence started at 14:28. 10-Jun-1994 - Problem with automatic backup onto tape drive. Had delayed program for 51 minutes. Copied *.txt files, don't like the look of peaks. Paused sequence after run #254. Started sequence at 18:02. 13-Jun-1994 - Had stopped after run #268, 06-11-94, 01:11. Lit flame using a similar set-up to OJP for fuel air. Restart at 18:22. 14-Jun-1994 - Power off. The standard was off. Power bar got moved. Appears to be working beautifully. 15-Jun-1994 - Snowed last night. Everything was down since 17:30 yesterday. Last run was #311. Even the pump was switched off. Got everything up, replaced Isobar with another surge suppressor. Heated ovens and detectors to 150 degrees C and 165 degrees C respectively, for 2 hours. Capped air flow to second detector. Started sequence at run #312 at 12:35. 16-Jun-1994 - Generator down at 17:30. Started back up at 18:50, run #1847. 18-Jul-1994 - Mini2-GC down for a week due to generator failure. Flow turned on at 11:00. 20-Jul-1994 - Generator was down. Run started at 12:43, Run #1604. 04-Jul-1994 - Restart of system. 21-Jul-1994 - Compressor went down at 15:03, back up at 15:05 with run #1552. 25-Jul-1994 - Oil addition to generator. 02-Aug-1994 - Generator off. Start-up at 15:40. 05-Sep-1994 - Switched generators and added oil to big one, 17:27. 14-Sep-1994 - Sequence stopped, restarted at 15:29. 15-Sep-1994 - Calibrating field standard with AES calibrated standard. 19-Sep-1994 - Breakdown of system, stopped running at 09:53. OJP: 28-May-1994 - GC on at 14:00. Started sequence at 14:26. 29-May-1994 - Pumps off briefly. 6-Jun-1994 - Having problems with pumps; finally up and running at 18:00. 09-Jun-1994 - Down for generator maintenance. 19-Jul-1994 - Very noisy baseline. Radio transmission appears to interfere with GC baseline trace. 04-Aug-1994 - Problem with GC; stopped sequence and rebooted. At 15:26 started up again. 06-Aug-1994 - Aborted sequence at Run #2961. Spike outs in baseline. Baked out column at 105 degrees C for 26 min. Started up again at 12:40. Not much flow. 07-Aug-1994 - Run #3007 screwed up due to messing with pumps to fix flow problem. 09-Aug-1994 - Baked out column again due to messy baseline; back up at 11:55. 29-Aug-1994 - Peaks look good; inject valve sounds bad!!! 01-Sep-1994 - Auto GC looks good. 02-Sep-1994 - Generator went down; restarted and relit at 15:02. 04-Sep-1994 - Generator has been down for maintenance this morning. Baked out column for 2 hrs. at 180oC. Restarted at 14:05. 05-Sep-1994 - Baseline looks good! 10-Sep-1994 - Shut down. Restarted at 11:22. 15-Sep-1994 - Running calibration of cylinders through automatic system. Started at 20:00. 19-Sep-1994 - Shut down entire system for the season! 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage The spatial positions of the measurement sites vary depending on the prevailing winds. NSA-OJP Tower (55.93 degrees N, 98.62 degrees W) NSA-BP Tower (55.84 degrees N, 98.03 degrees W) NSA-TF Tower (55.91481 degrees N, 98.42072 degrees W) 7.1.2 Spatial Coverage Map Not Available. 7.1.3 Spatial Resolution The spatial resolution of the area of the measurement sites varies depending on the prevailing winds. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage The automatic sampling of ambient CH4 at the TF sites was approximately every 3 minutes, and at the BP and the OJP sites was approximately every 6 minutes, semicontinuously over the period from 15-Apr through 22-Apr-1996, and 28-May through 19-Sep-1994. 7.2.2 Temporal Coverage Map TIME PERIOD SITE COLLECTION NOTES 15-Apr-1996 Tower Fen Start of semicontinuous data collection IFC-1 Tower Fen Semicontinuous data collection June Tower Fen Semicontinuous data collection July Tower Fen Semicontinuous data collection IFC-2 Tower Fen Semicontinuous data collection August Tower Fen Semicontinuous data collection September Tower Fen Semicontinuous data collection October Tower Fen Semicontinuous data collection IFC-3 Tower Fen Semicontinuous data collection 22-Oct-1996 Tower Fen End of semicontinuous data collection 7.2.3 Temporal Resolution The ambient CH4 at the towers was measured approximately every 3 minutes in 1996 and every 6 minutes in 1994. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (tgb1cfd.def). 7.4 Sample Data Record Sample data format shown in the companion data definition file (tgb1cfd.def). 8. Data Organization 8.1 Data Granularity All of the CH4 Concentration and Flux Data from NSA Tower Sites are contained in one dataset. 8.2 Data Format(s) The data files contain numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe marks. There are no spaces between the fields. Sample data records are shown in the companion data definition files (tgb1cfd.def). 9. Data Manipulations 9.1 Formulae 9.1.1 Derivation Techniques and Algorithms TF: Ambient CH4 mixing ratios were determined at 3-minute intervals at 6.65m (ppm_CH4_6.65_m) and 3.59 m (ppm_CH4_3.59_m) over the NSA-TF site. BP: Ambient CH4 mixing ratios were determined at 6-minute intervals at 0.25m (ppmv_CH4_25_cm) and 1.5 m (ppmv_CH4_1.5_m) over the NSA-BP site. The concentrations were determined by comparing the unknown ambient peak areas with the known standard peak area that was run immediately following each set of ambient samples: Rf = Cstd / Astd Cs = Rf * As Rf = Response factor Astd = Standard peak area Cstd = Concentration of the standard Cs = Concentration of the sample As = Peak area of sample The response factor was calculated for the GC and then multiplied by the ambient air peak to determine the concentration of the tower sample. The concentration gradients were calculated by subtracting the lower sampling port CH4 concentrations from the higher sampling port CH4 concentrations and dividing by the difference in height of the sampling ports (3.06m, 1996; 1.25m, 1994). G = (CH4low - CH4high) / (Porthigh - Portlow) G = Gradient CH4low = Methane mixing ratio at the lower sampling port CH4high = Methane mixing ratio at the upper sampling port Porthigh = Height of the upper sampling port above the ground Portlow = Height of the lower sampling port above the ground Half-hourly averages were determined and used to calculate CH4 emissions (CH4_flux) by comparing the concentration gradient to the transfer coefficient (K), air temperature at 1 meter, and air pressure. NOTE: The transfer coefficient (K), air temperature at 1 meter, and air pressure data were calculated and provided by N. Roulet and N. Comer, McGill University, BOREAS TGB-03 CO2 and CH4 chamber flux data over the NSA. See BOREAS TGB-03 CO2 and CH4 chamber flux data over the NSA for details on flux calculation. OJP: Ambient CH4 concentrations were determined at 6-minute intervals at 13.5 m and 30 m over the NSA-OJP site. The concentrations were determined by comparing the unknown ambient peak areas with the known standard peak area that was run immediately following each set of ambient samples using the above-described equations. The concentration gradients were calculated by subtracting the lower sampling port CH4 concentrations from the higher sampling port CH4 concentrations and dividing by the difference in height of the sampling ports (16.5 m) as per above. Half-hourly averages were determined. 9.2 Data Processing Sequence 9.2.1 Processing Steps None given. 9.2.2 Processing Changes None. 9.3 Calculations 9.3.1 Special Corrections/Adjustments None given. 9.3.2 Calculated Variables None given. 9.4 Graphs and Plots None given. 10. Errors 10.1 Sources of Error None given. 10.2 Quality Assessment 10.2.1 Data Validation by Source None given. 10.2.2 Confidence Level/Accuracy Judgment None given. 10.2.3 Measurement Error for Parameters The analytical precision of the GC is 0.2% for CH4 at near-ambient conditions. 10.2.4 Additional Quality Assessments None given. 10.2.5 Data Verification by Data Center Data were examined for general consistency and clarity. 11. Notes 11.1 Limitations of the Data The analytical precision of the GC is 0.2% for CH4. 11.2 Known Problems with the Data Periodic power failures may cause incongruities in the data. 11.3 Usage Guidance None given. 11.4 Other Relevant Information None given. 12. Application of the Data Set These data sets may be used in comparison with the chamber flux data to determine if the tower flux calculations agree with the small-scale flux calculations. These data can also be compared with the larger scale aircraft studies. 13. Future Modifications and Plans None. 14. Software 14.1 Software Description HP ChemStation. 14.2 Software Access Contact Hewlett Packard. 15. Data Access 15.1 Contact Information Ms. Beth Nelson BOREAS Data Manager NASA GSFC Greenbelt, MD (301) 286-4005 (301) 286-0239 (fax) beth@ltpmail.gsfc.nasa.gov 15.2 Data Center Identification See Section 15.1. 15.3 Procedures for Obtaining Data Users may place requests by telephone, electronic mail, or fax. 15.4 Data Center Status/Plans These data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The BOREAS contact at ORNL is: ORNL DAAC User Services Oak Ridge National Laboratory (865) 241-3952 ornldaac@ornl.gov ornl@eos.nasa.gov 16. Output Products and Availability 16.1 Tape Products None. 16.2 Film Products None 16.3 Other Products Tabular American Standard Code for Information Interchange (ASCII) files. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation Instruction Manual: MINI2-GC Shimadzu Corporation, Kyoto, Japan. 17.2 Journal Articles and Study Reports Sellers, P.and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 94). Sellers, P.and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1996-2.0, NASA BOREAS Report (EXPLAN 96). Sellers, P.and F. Hall. 1997. BOREAS Overview Paper. JGR Special Issue. Sellers, P.and F. Hall, K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPS DOC 94). Sellers, P., F. Hall and K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere Study: 1996 Operations. NASA BOREAS Report (OPS DOC 96). Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. Wickland. 1995. The boreal ecosystem-atmosphere study (BOREAS): an overview and early results from the 1994 field year. Bulletin of the American Meteorological Society. 76(9):1549-1577. 17.3 Archive/DBMS Usage Documentation None. 18. Glossary of Terms None given. 19. List of Acronyms AES - Atmospheric Environment Services ASCII - American Standard Code for Information Interchange BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOREAS Information System BP - Beaver Pond CGR - Certified by Group CMDL - Climate Monitoring and Diagnostics Laboratory CPI - Checked by PI CPI-??? - CPI but Questionable DAAC - Distributed Active Archive Center ECD - Electron Capture Detector EOS - Earth Observing System EOSDIS - EOS Data and Information System FID - Flame Ionization Detector GC - Gas Chromatograph GMT - Greenwich Mean Time GSFC - Goddard Space Flight Center HP - Hewlett Packard NASA - National Aeronautics and Space Administration NOAA - National Oceanic and Atmospheric Administration NSA - Northern Study Area OBS - Old Black Spruce OJP - Old Jack Pine ORNL - Oak Ridge National Laboratory PANP - Prince Albert National Park PRE - Preliminary SSA - Southern Study Area TCD - Thermal Conductivity Detector TF - Tower Fen TGB - Trace Gas Biogeochemistry URL - Uniform Resource Locator YJP - Young Jack Pine 20. Document Information 20.1 Document Revision Dates Written: 22-Nov-1994 Last updated: 08-May-1998 20.2 Document Review Date BORIS Review: 6-Oct-1997 Science Review: 20.3 Document ID 20.4 Citation 20.5 Document Curator 20.6 Document URL Keywords CO2 FLUX CH4 FLUX CARBON DIOXIDE METHANE TGB01_CH4_TOWER.doc 05/26/98