BOREAS TF-01 SSA-OA Weekly Tower CH4 and N2O Flux Summary The BOREAS TF-01 team collected various trace gas and energy flux data in its efforts to characterize the temporal energy and gas exchanges that occurred over the SSA-OA site. This data set contains methane (CH4) and nitrous oxide (N2O) fluxes that were measured at the BOREAS SSA-OA site. These fluxes were measured from 16-Apr to 16-Sep-1994. The data were averaged to weekly values and are available 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 13) Future Modifications and Plans 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 TF-01 SSA-OA Weekly Tower CH4 and N2O Flux 1.2 Data Set Introduction Fluxes of CH4 and N2O were measured at the BOReal Ecosystem-Atmosphere Study (BOREAS) Southern Study Area (SSA) Old Aspen (OA) site. These fluxes were measured through the growing season of 1994. The tower-based fluxes were determined using a high-resolution tunable diode laser Trace Gas Analysis System (TGAS) together with micrometeorological techniques. The fluxes were small and required long averaging times to be resolved. Half-hour averages were determined and these were averaged to weekly values. 1.3 Objective/Purpose The global budgets of CH4 and N2O are poorly constrained, and several priority trace gas research areas have been specified, including the boreal forest. The objectives of this research were to quantify the exchange of CH4 and N2O at the aspen site and to better understand the factors that control their exchange. 1.4 Summary of Parameters and Variables Half-hour observations were combined to produce weekly CH4 and N2O flux values. The data set includes weekly average, standard error, and number of half-hourly values used to calculate the average, for both CH4 and N2O. By convention, a positive flux indicates trace gas emission, and a negative flux indicates uptake. 1.5 Discussion Boreal forests have been reported as net sinks of CH4, although the uptake of CH4 by soils has not been well characterized. CH4 oxidation in moist soils may be a negative feedback on atmospheric CH4 increases. Generally, there is a lack of N2O for all northern ecosystems. 1.6 Related Data Sets BOREAS TF-01 SSA-OA Undercanopy Flux, Meteorological, and Soil Temperature Data BOREAS TF-02 SSA-OA Tower Flux, Meteorological, and Precipitation Data BOREAS TF-04 CO2 and CH4 Chamber Flux Data from the SSA 2. Investigator(s) 2.1 Investigator(s) Name and Title George Thurtell University of Guelph Grant Edwards University of Guelph 2.2 Title of Investigation Boreal Forest Atmosphere Interactions: Exchanges of Energy, Water Vapor, and Trace Gases 2.3 Contact Information Contact 1 Isobel Simpson Chemistry Department University of California, Irvine Irvine, CA (949) 824-6024 (949) 824-2905 (fax) isimpson@uci.edu Contact 2 Grant Edwards University of Guelph Department of Land Resource Science Guelph, Ontario Canada (519) 824-5730 (fax) gedwards@net2.eos.uoguelph.ca Contact 3 George Thurtell University of Guelph Department of Land Resource Science Guelph, Ontario Canada (519) 824-4120 (519) 824-5730 (fax) gthurtel@lrs.uoguelph.ca Contact 4 K. Fred Huemmrich University of Maryland NASA GSFC Greenbelt, MD (301) 286-4862 (301) 286-0239 (fax) Karl.Huemmrich@gsfc.nasa.gov 3. Theory of Measurements The tower-based fluxes were determined using the flux-gradient method. Measurements were made of the concentration differences of CH4 and N2O between 26.8 and 37.5 m above the forest floor. The canopy height was 22 m. Air was alternately sampled from either level every 4 seconds, and the resulting concentration differences were averaged over half an hour. Micrometeorological data collected at 39.1 m using a sonic anemometer were used to determine a transfer coefficient. Together, the concentration differences and stability- corrected transfer coefficient were used to determine the flux of CH4 and N2O in the following flux-gradient relationship: - [ ustar k (conc2 - conc1) ] Flux = ------------------------------------------- (z2 - d) 0.74 [ ln { -------- } - psih2 + psih1 ] (z1 - d) where ustar is the friction velocity [m/s]; k is von Karman's constant (taken as 0.4); z2 and z1 are the upper (37.5 m) and lower (26.8 m) sampling heights, respectively; conc2 and conc1 are the respective concentrations [ng/m3] at z2 and z1; d is the zero plane displacement; and psih2 and psih1 are the respective scalar diabatic correction functions at z2 and z1. A value for the zero plane displacement was determined before, during, and after leaf-out using sonic anemometer data collected during times of neutral atmospheric stability. 4. Equipment 4.1 Sensor/Instrument Description 4.1.1 Collection Environment Data were collected through the growing season of 1994. The site was burned approximately 80 years ago, and the primary forest cover was trembling aspen (Populus tremuloides) with some balsam poplar (Populus balsamifera). The aspen canopy architecture features an open trunk space with the crown concentrated at the top 5-6 m of the trees. The maximum aspen leaf area index in 1994 was 2.4. There was a rich understory comprising 60% hazelnut (Corylus cornuta) and 15% wild rose (Rosa woodsii). The soil is classified as an Orthic Gray Aluvisol, featuring a well- to moderately well-drained loam to clay loam till and an organic layer depth less than 8 cm. 4.1.2 Source/Platform Measurements were made from a 37-m double-scaffold walk-up tower. Two Campbell Scientific tunable diode laser TGAS were used to measure the concentration differences of CH4 and N2O between 26.8 and 37.5 m above the forest floor. Air was alternately sampled from either level every 4 seconds, and the resulting concentration differences were averaged over half an hour. Micrometeorological data collected at 39.1 m using a 20-cm-path Kaijo-Denke sonic anemometer were used to determine a transfer coefficient. 4.1.3 Source/Platform Mission Objectives The purpose of the tower was to suspend instruments to measure trace gas, energy fluxes, and meteorological variables above a mature aspen stand. 4.1.4 Key Variables Half-hour observations were combined to produce weekly CH4 and N2O flux values. The data set includes weekly average, standard error, and number of half-hourly values used to calculate the average, for both CH4 and N2O. By convention, a positive flux indicates trace gas emission, and a negative flux indicates uptake. 4.1.5 Principles of Operation Laser-based trace gas analysis is based on absorption spectroscopy, whereby each molecule is known to absorb radiation in a characteristic spectrum. In the TGAS unit, a lead-salt laser is housed in a liquid nitrogen-cooled Dewar. The laser beam is sent through a single-path, 1.5-m sample tube, and then is split into sample and reference gas cells, each housing a Peliter-cooled cadmium-mercury- telluride detector. Air sampled from outside is pumped through the sample tube, where it encounters the laser beam. A good absorption line is selected based on its strength and the absence of interference by other absorbing gases. The laser emission is tunable, and the laser is modulated to scan back and forth across the entire absorption feature 500 times a second. The area under the absorption feature is integrated to determine the concentration of the trace gas within the air sample. Concentration readings are output at 10 Hz, and each reading is calibrated in real time against a reference gas (in this case CH4 or N2O) of known concentration. 4.1.6 Sensor/Instrument Measurement Geometry Air intakes were located on the SSA-OA flux tower at 0.5, 5.0, 18.1, 26.8, and 37.5 m above the ground. Concentration differences were determined in 4-hour cycles between each pair of intakes with an emphasis on the above canopy measurements (the 26.8- and 37.5-m intakes). The air from the upper and lower intakes passed through an equal amount of 0.5-inch inner diameter (i.d.) high- density polyethylene tubing to a common Perma Pure dryer located on the tower. A total of four dryers were installed on the tower, one for each pair of intakes. 4.1.7 Manufacturer of Sensor/Instrument TGAS units: Campbell Scientific, Inc. 815 West 1800 North Logan, UT 84321-1784 (435) 753-2342 (435) 750-9540 (fax) info@campbellsci.com 4.2 Calibration 4.2.1 Specifications Concentration readings from TGAS were output at 10 Hz. Each reading is calibrated in real time against a reference gas (in this case CH4 or N2O) of known concentration. 4.2.1.1 Tolerance See Section 9.2.1. 4.2.2 Frequency of Calibration Calibration against a reference gas was done in real time. 4.2.3 Other Calibration Information None. 5. Data Acquisition Methods A suite of four concentration differences was measured between five levels both above and within the aspen canopy. The concentration differences were determined in a 4-hour cycle, with emphasis on the above-canopy measurements. The TGAS units operated 24 hours a day over the 5 months of measurement, with about 1 hour of data edited out each day during liquid nitrogen fills. A five-way Whitey ball valve (model SS-45ZF8-42ACZ) with an electric actuator (model MS-142ACZ) was used to switch the sampling among air intakes. For a given pair of intakes, a Numatics solenoid valve (model 152SS-400K) located on the tower was used to alternately draw sample air from the upper and lower intakes every 4 s. Rapid switching between the two levels acts as a high-pass filter and therefore reduces the low-frequency noise. The air from the upper and lower intakes passed through an equal amount of 0.5- inch i.d. high density polyethylene tubing to a common Perma Pure dryer (model PD-1000-SS48) located on the tower. A total of four dryers were installed on the tower, one for each pair of intakes. Drying is necessary to avoid corrections to the flux measurements for density effects caused by water vapor transfer. The dried air then was pumped at low pressure through tubes from the tower to a hut housing the TGAS units. Travel through a common long tube ensured that the upper and lower air samples had a common temperature by the time they reached the TGAS units. A Whitey needle valve (model SS-3NRM4) positioned at the end of each dryer was used to create a pressure drop, and the pressure in each TGAS unit was maintained at 75 mbar. The concentration readings recovered from the solenoid valve switch in 0.6-0.7 s, and the first 0.8 s of data were discarded following each 4-s switch. Delay times to account for the travel time down the tubes were also set in the TGAS units. The sample air was split and diverted through the two TGAS units in parallel,. The air was then exhausted through a Busch pump (model RA-0040-A005-1002) at 22 actual ft3 min-1. 6. Observations 6.1 Data Notes None. 6.2 Field Notes The TGAS concentration differences were edited out during liquid nitrogen fills (twice daily), power surges, laser alignment, etc. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage All data were collected at the BOREAS SSA-OA site. North American Datum of 1983 (NAD83) coordinates for the site are latitude 53.62889º N, longitude 106.19779º W, and elevation of 600.63 m. 7.1.2 Spatial Coverage Map Not applicable. 7.1.3 Spatial Resolution Large eddy simulations using characteristics appropriate for the aspen site indicated that under neutral atmospheric stability, 90% of the measured trace gas flux, was expected to originate from within a distance of 2 km upwind of the tower, with a peak influence distance of 200 m. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage Fluxes were measured from 16-Apr to 16-Sep-1994. The data were averaged to weekly values. 7.2.2 Temporal Coverage Map None. 7.2.3 Temporal Resolution The fluxes were small and required long averaging times to be resolved. Half- hour averages were determined, and these were averaged to weekly values. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (tf1ch4.def). 7.4 Sample Data Record Sample data format shown in the companion data definition file (tf1ch4.def). 8. Data Organization 8.1 Data Granularity All of the TF-01 SSA-OA Weekly Tower CH4 and NO2 Flux Data are contained in one dataset. 8.2 Data Format The data files contain numerical and character fields of varying length separated by commas. The character fields are enclosed with single apostrophe marks. There are no spaces between the fields. Sample data records are shown in the companion data definition file (tf1ch4.def). 9. Data Manipulations 9.1 Formulae 9.1.1 Derivation Techniques and Algorithms The fluxes of CH4 and N2O were determined in the following flux-gradient relationship: - [ ustar k (conc2 - conc1) ] Flux = ------------------------------------------- (z2 - d) 0.74 [ ln { -------- } - psih2 + psih1 ] (z1 - d) where ustar is the friction velocity [m/s]; k is von Karman's constant (taken as 0.4); z2 and z1 are the upper (37.5 m) and lower (26.8 m) sampling heights, respectively; conc2 and conc1 are the respective concentrations [ng/m3] at z2 and z1; d is the zero plane displacement; and psih2 and psih1 are the respective scalar diabatic correction functions at z2 and z1. 9.2 Data Processing Sequence 9.2.1 Processing Steps The editing criteria applied to the 39.1-m sonic anemometer data in this preliminary analysis include: (1) Measured horizontal windspeed (Umeas) > 1.5 m/s. (2) Calculated horizontal windspeed (Ucalc) > 1.5 m/s. Ucalc was determined from the logarithmic wind profile equation using ustar from the sonic anemometer. (3) Zeta ( [z-d]/L ) between -10 and 2, where L is the Monin-Obukhov length. (4) The ratio of Umeas and Ucalc between 0.8 and 1.2. BORIS staff processed these data by: 1) Reviewing the initial data files and loading them online for BOREAS team access. 2) Designing relational data base tables to inventory and store the data. 3) Loading the data into the relational data base tables. 4) Working with the team to document the data set. 5) Extracting the data into logical files. 9.2.2 Processing Changes None. 9.3 Calculations 9.3.1 Special Corrections/Adjustments None. 9.3.2 Calculated Variables See Section 9.1.1. 9.4 Graphs and Plots None. 10. Errors 10.1 Sources of Error Wind speed editing was introduced because sonic anemometers can give error at low wind speeds due to flow distortion around the transducers. 10.2 Quality Assessment 10.2.1 Data Validation by Source Quality checks were made as part of the data processing to remove poor data values. See Section 9.2.1. 10.2.2 Confidence Level/Accuracy Judgment The half-hour above-canopy trace gas concentration differences were often close to the resolution of the TGAS units for both CH4 and N2O. 10.2.3 Measurement Error for Parameters None. 10.2.4 Additional Quality Assessments None. 10.2.5 Data Verification by Data Center BOREAS Information System staff reviewed the data and documentation for clarity and consistency. 11. Notes 11.1 Limitations of the Data The sonic anemometer and the TGAS units operated 24 hours a day, and the edited data include half hours collected in both the daytime and the nighttime. 1994 was a record warm, frost-free year in the BOREAS SSA. 11.2 Known Problems with the Data None. 11.3 Usage Guidance None. 11.4 Other Relevant Information Overall, the preliminary analysis indicates a small net upwards N2O flux of 1.5±0.7 ng m-2 s-1 from April to September 1994. N2O is produced by soil bacteria during nitrification and denitrification, and these initial results suggest that this forest is efficient in its use of nitrogen. The data also show an overall CH4 emission of 24±5 ng m-2 s-1 during the measurement period. Well-aerated soils in temperate and tropical regions have been shown to consume CH4, and we had also expected to see CH4 consumption at the OA site. We have carefully investigated the CH4 results, and we believe the upwards CH4 fluxes to be real. We are continuing to investigate the observed emission of CH4 from the OA stand. However, at this time we expect that CH4 emissions from warm, wet areas located throughout the footprint were greater than CH4 uptake in drier areas, to give a net upwards flux of CH4 during the measurement period. 12. Application of the Data Set These data are useful for the study of nitrogen and carbon exchange in a mature aspen forest. 13. Future Modifications and Plans We are continuing to assess our data selection criteria, and we remind BORIS users that these results are preliminary only and may change after further analysis. 14. Software 14.1 Software Description None given. 14.2 Software Access None given. 15. Data Access 15.1 Contact for Data Center/Data Access Information These BOREAS data are available from the Earth Observing System Data and Information System (EOS-DIS) 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 15.2 Procedures for Obtaining Data BOREAS data may be obtained through the ORNL DAAC World Wide Web site at http://www-eosdis.ornl.gov/ or users may place requests for data by telephone, electronic mail, or fax. 15.3 Output Products and Availability Requested data can be provided electronically on the ORNL DAAC's anonymous FTP site or on various media including, CD-ROMs, 8-MM tapes, or diskettes. The complete set of BOREAS data CD-ROMs, entitled "Collected Data of the Boreal Ecosystem-Atmosphere Study", edited by Newcomer, J., et al., NASA, 1999, are also available. 16. Output Products and Availability 16.1 Tape Products None. 16.2 Film Products None. 16.3 Other Products These data are available on the BOREAS CD-ROM series. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation None. 17.2 Journal Articles and Study Reports Newcomer, J., D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J. Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers, eds. 2000. Collected Data of The Boreal Ecosystem-Atmosphere Study. NASA. CD- ROM. 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., F. Hall, and 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. Sellers, P.J., F.G. Hall, R.D. Kelly, A. Black, D. Baldocchi, J. Berry, M. Ryan, K.J. Ranson, P.M. Crill, D.P. Lettenmaier, H. Margolis, J. Cihlar, J. Newcomer, D. Fitzjarrald, P.G. Jarvis, S.T. Gower, D. Halliwell, D. Williams, B. Goodison, D.E. Wickland, and F.E. Guertin. 1997. BOREAS in 1997: Experiment Overview, Scientific Results and Future Directions. Journal of Geophysical Research 102(D24): 28,731-28,770. Simpson, I.J., G.C. Edwards, G.W. Thurtell, G. den Hartog, H.H. Neumann, and R.M. Staebler. 1997. Micrometeorological measurements of methane and nitrous oxide exchange above a boreal aspen forest. Journal of Geophysical Research 102(D24): 29,331-29,341. 17.3 Archive/DBMS Usage Documentation None. 18. Glossary of Terms None. 19. List of Acronyms ASCII - American Standard Code for Information Interchange BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOREAS Information System CD-ROM - Compact Disk-Read-Only Memory DAAC - Distributed Active Archive Center EOS - Earth Observing System EOSDIS - EOS Data and Information System GIS - Geographic Information System GMT - Greenwich Mean Time GSFC - Goddard Space Flight Center HTML - Hyper-Text Markup Language i.d. - Inner diameter IFC - Intensive Field Campaign NAD83 - North American Datum of 1983 NASA - National Aeronautics and Space Administration NSA - Northern Study Area OA - Old Aspen ORNL - Oak Ridge National Laboratory PANP - Prince Albert National Park SSA - Southern Study Area TF - Tower Flux TGAS - Trace Gas Analysis System URL - Uniform Resource Locator 20. Document Information 20.1 Document Revision Date Written: 03-Jun-1999 Revised: 22-Nov-1999 20.2 Document Review Date(s) BORIS Review: 10-Jun-1999 Science Review: 20.3 Document ID 20.4 Citation When using these data, please include the following acknowledgment: Data were collected and processed by Isobel Simpson, Grant Edwards, George Thurtell. The results are described in the paper Simpson et al., 1997. If using data from the BOREAS CD-ROM series, also reference the data as: Simpson, I.J., G.C. Edwards, G.W. Thurtell, and T.A. Black, "Boreal Forest Atmosphere Interactions: Exchanges of Energy, Water Vapor, and Trace Gases." In Collected Data of The Boreal Ecosystem-Atmosphere Study. Eds. J. Newcomer, D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J. Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers. CD-ROM. NASA, 2000. Also, cite the BOREAS CD-ROM set as: Newcomer, J., D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J. Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers, eds. Collected Data of The Boreal Ecosystem-Atmosphere Study. NASA. CD-ROM. NASA, 2000. 20.5 Document Curator 20.6 Document URL Keywords: Methane Nitrous Oxide Aspen TF01_CH4_N2O_Flux.doc 11/22/99