Southern Great Plains
Site
CO2 Flux Measurement Systems at SGP:
SGPco2flx
and
Portable Flux Measurement Systems at SGP:
SGPp1flx
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General PurposeThe SGP carbon dioxide flux (CO2flux) measurement systems provide 1/2 hour average fluxes of CO2, H2O (latent heat), and sensible heat. The fluxes are obtained by the eddy covariance technique, which computes the flux as the mean product of the vertical wind component with CO2 and H2O densities, or estimated virtual temperature. A three-dimensional sonic anemometer is used to obtain the orthogonal wind components and the virtual (sonic) temperature. An infrared gas analyzer is used to obtain the CO2 and H2O densities. A separate sub-system also collects 1/2 average measures of meteorological and soil variables from separate 4m towers. |  |
Primary Quantities Measured with SystemThe CO2flux systems measurement systems provide from 1/2 to 4 hr mean estimates of the fluxes of CO2, H2O (latent heat), and sensible heat from a variable area (footprint) of the land surface upwind of the instrument. In rough terms, the extent of the footprint, which depends on the mean wind speed and the degree of turbulent mixing in the atmosphere, varies from 5 - 500 times the height of the sensors above the land surface. For example, the instrument located at 60 m on the Central Facility (CF) tower detect land surface fluxes at distances between approximately 0.3 - 30 km from the tower depending upon meteorological conditions. The fluxes are computed from the following directly measured data. Orthogonal components of the wind velocity, u, v, and w (m s-1), and sonic temperature (K), which is approximately equal to virtual temperature are measured by the sonic anemometer at 10 Hz. CO2 and H2O densities (mmol m-3) are measured by an infrared gas analyzer (IRGA). Overall Uncertainties for Primary Quantities MeasuredUncertainties in the measurements are typically dominated by random noise from the measurement instruments. However, the data processing software is designed to identify other sources of interference that affect the measurements. The most frequent source of interference is airborne material (e.g. rain) that briefly obscures the sound or light path of the sensors. See description of processing algorithms given below. In normal
operation instrument noise limits measurements as follows: Detailed DescriptionList of components for eddy covariance calculations3-D Sonic Anemometer, Gill Solent Windmaster Pro
Infrared Gas Analyzer, Licor Inc. LI-7500 (see http://env.licor.com/)
Data collection system
List of components for Meteorological and Soil Measurements Included in 4 m Data Sets
Description of System Configuration and Measurement MethodsPairs of anemometers and IRGAs are located on and near the Central Facility 60m tower. Data from the anemometer is transmitted to a personal computer (PC) in an instrument shed at the base of the tower. The PC collects and stores the serial binary data stream from the sonic anemometer and IRGA (for more details, see data collection system notes below). The raw data is transferred to LBNL, processed into the ARM archive format, and inspected for problems on a daily basis. Processed files are sent to ARM Archive using the Site Transfer Suite on a weekly basis. Theory of OperationsTurbulent fluxes are calculated using standard methods in biometeorology. See references for discussions. The 3-D sonic anemometer uses three pairs of orthogonal ultrasonic transmit/receive transducers to measure the transit time of sound signals traveling between the transducer pairs. The wind speed along each transducer axis is determined from the difference in transit times. The sonic temperature is computed from the speed of sound which is determined from the average transit time along the vertical axis. A pair of measurements are made along each axis 100 times per second. Ten measurements are averaged to produce 10 wind measurements along each axis and 10 temperatures each second. The infrared gas analyzer measures CO2 and H2O densities by detecting the absorption of infrared radiation by water vapor in the light path. Details of the IRGA operation and performance can be obtained from Licor Env. Inc. (http://env.licor.com/PDF%20Files/LI7500.pdf). Data collection on a standard personal computer. Data is collected in 1/2 hour intervals, using the computer clock start as a time reference. Each 1/2 hour data file has a time stamp reflecting the start time of the file. The computer clock is updated on a regular basis using time server software. The daily collection of 48 raw data files are downloaded from the data collection computer to a processing computer at the Lawrence Berkeley National Laboratory on a daily basis and reduced to produce eddy covariance estimates of turbulent fluxes. A set of data processing algorithms are used to create files suitable for inspection and ingest into the ARM data archive. Current Status and LocationsThe number and location of the sensors on the SGP 60m tower has changed over time as follows: September, 2000. Initial installation of one system on the south east boom at 60 m on the Central Facility (CF) tower. December, 2002. First system removed from South-East boom. Systems installed on the 25, and 60 m West booms on the CF tower. Combined eddy covariance system at 4.5 m and supporting 4m meteorological measurement tower installed near base of 60 m tower. Data Processing AlgorithmsThe first program processes the raw (a0) data to produce intermediate (a1) data files. The averaging time for calculations can be varied to produce EC average values for each 1/2 hour (other averaging times can be requested of 1, 2, and 4 hr). The calculation is performed as follows: aotoa1:
Identify and remove spikes from data using 100 second running mean filter. Spikes are identified as data points with values more than a set number of standard deviations away from running mean. Spike data are given value of running mean and are not used to update mean. Spikes are counted and the mean value of the spikes is calculated. A QC flag is raised if more than 100 data points in a given interval are flagged as spikes.
a1tob1: processes intermediate (a1) files to produce estimates of turbulent fluxes with initial QC flags as follows:
met.b1: for the 4 m systems, merge meteorological data with b1 data file. Data QualityData quality for the flux data is judged by inspecting QC flags and variables in processed data. No data quality checks or flags are included for the meteorological data at this time. Current Health and StatusFully operational. Data User NotesEddy Covariance Calculations The algorithm that computes the turbulent fluxes (a1tob1) for the data collected at 4m uses the air temperature, pressure, and relative humidity from meteorological sensors to calculate the density and specific heat. However, at present, the 25 and 60m systems use the virtual temperature measured by the anemometer and the H20 density measured by the IRGA to estimate air density and specific heat, assuming a constant pressure of 98 kPa. This will cause a small errors in cases where pressure, or temperature are slightly different from the measured values. No corrections are made for loss of spectral energy due to sensor separation. Using the work of Moore (1986), we have estimated these corrections to be in the range of 3-7% for most conditions at 4m above the crops but is unlikely to be significant for the measurements at 25 and 60m. The fluxes only reflect turbulent fluxes and do not include corrections for storage of CO2, H2O, or heat in the air between the sensor and the land surface. Although this is unlikely to be an important correction for the 4m system, this correction is often significant for the 60m system. We are working to incorporate data from a precision gas system to include a storage correction for the 25 and 60m heights. Soil Temperature and Moisture The depths at which the soil temperature sensors were installed is as follows: T1, T4 = 25cm; T2, T5= 15 cm; T3, T6 = 5 cm The depths at which the soil moisture sensors were installed varied with time as follows: July, 2001: M1, M3, M5, M7 = 15 cm; M2, M4, M6, M8 = 5 cm December, 2002- present: M1, M3, M5, M7 = 25 cm; M2, M4, M6, M8 = 5 cm It has been observed that the soil moisture sensors exhibit a large temperature sensitivity. This is evident in sensors located in shallow soil where temperature variations are large. Correction for this artifact has not been corrected by processing algorithm to date. This correction will be included in future files. People interested in performing their own corrections may want to consider using the diurnal soil temperature variations to diurnal variations in moisture signals. Automated Quality Control/Flagging Contained within data filesOutput files include QC flags as described below. Raw data QA/QC Spike count for u,v,w,T, q, c This is a summary of the qc flags in a1 and b1 files. ******Flags present in _a1_ files:
Processed Data Checks******Flags present in _b1_ files:
VALID DATA RANGES are provided in data format file Instrument Mentor Quality Control ChecksVisual QC frequency: daily to weekly QC delay: typically 1-2 days QC type: - Instrument mentor Marc Fischer and data processing assistant Igor Pesenson routinely view graphical displays produced at LBNL. The displays include graphs of CO2, H20, sensible fluxes, mean and variance of CO2 concentration (not corrected for barometric pressure) and wind speed. Value Added ProceduresNone at present. A gap filled data file is being developed. Quality Measurement ExperimentsNone at present. Examples of Data (from yesterday)See data quick look: Data QuicklooksSee above URLS. Calibration and MaintenanceCalibration TheoryThe sonic anemometer does not require maintenance or calibration. The IRGA offset and gain need to be calibrated on a periodic basis. The IRGA is calibrated by introducing gas of know concentration into a calibration hood that surrounds the light path over which infrared absorption is measured. The offset is typically calibrated using dry N2 from a gas bottle. The gain of the CO2 and H2O channels are calibrated using a bottle with a known concentration of CO2 and flow from a H2O vapor generator (e.g. Licor Inc. LI-610 Dew Point Generator). Calibration HistoryThe system in longest continuous operation is the system at 60m on the 60m tower. The calibration interval for that system is: October,
18, 2000 December, 20, 2002 - replaced The portable flux systems are calibrated before each portable deployment period. Maintenance ProceduresThe sonic anemometer does not require maintenance or calibration. The IRGA offset and gain are calibrated on a periodic basis following the manufacturers recommended procedure. Online Maintenance DocumentationIRGA calibration procedure is available from instrument mentor. Supplemental Assessment of Instrument Calibration and Maintenance ProceduresNone. Frequestly Asked Questions-FAQsWhere do I get more information?
Software Documentation for this InstrumentGeneral description of the data product formats can be found in: ContactsInstrument MentorMarc L. Fischer, Staff Scientist Atmospheric
Sciences Department Tel 510-486-5539
• FAX 510-486-5298 web page http://eetd.lbl.gov/env/mlf
Instrument Developer and Co-Mentor David P. Billesbach, Research Professor 206 L.W. Chase Hall Tel 402-472-7961
VendorsGill Solent, UK (US dist. Texas Electronics, 800-424-5651) http://www.gill.co.uk/ Licor Environmental (Lincoln, NE, 800-447-3576) http://env.licor.com/ Campbell Scientific (Logan, UT, 435-753-2342) http://www.campbellsci.com/ Citable References
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