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Precision Gas System (PGS) General Purpose

This precision gas system collects high accuracy, high precision measurements of CO2 concentration at 2, 4, 25, and 60 m above the ground. Based at the 60-m tower at the central facility, Southern Great Plains.

Primary Quantities Measured with System

CO2 concentration in dry air.

Overall Uncertainties for Primary Quantities Measured

Samples are bracketed by certified standards.

Detailed Description

List of Components

IRGA (Infra-Red Gas Analyzer), LI-6252 (see http://env.licor.com/)

  • Drying of sample is accomplished with a cooling unit and a nafion membrane drier.
  • Flow is regulated with mass flow controller, pumps, and valves.
  • A 1 liter buffer volume is used to even out very short-term variation in CO2 concentration.
  • System is directed by a datalogger (Campbell 23x) that downloads to a computer every day.

Data collection system

  • 266 - 600 MHz Intel Pentium III PC clone; operating system Windows NT or 2000
  • Data collection software: 1/1/2001 - 12/18/2001 Gillsonic (courtesy Dennis Baldocchi, UC Berkeley) 1/19/2001 - present: WinfluxWMP (courtesy Joe Verfaillie, CSU San Diego)

Description of System Configuration and Measurement Methods

We measure one sample for 3 minutes at a time; the reported value is the average of the last 30 seconds.

The IRGA (Infra-Red Gas Analyzer) is run in differential mode, with an estimated 360 ppm reference gas through the reference cell at all times. Every 15 minutes we analyze the reference gas through the sample cell as well, to correct for gain (zero drift) in the system. Every 4 hours we run four calibration standards (NOAA-CMDL certified standards at approximately 330, 360, 390, and 420 ppm). Sample CO2 concentration is calculated with the standards run before and after it.

We have followed protocols developed by the NOAA climate monitoring diagnostics laboratory (e.g. Bakwin et al.) for their high precision, continuous CO2 monitoring instruments.

Current Status and Locations

The system operates continuously, except for occasional power outages. It is located at the central facility, and the base of the 60 m tower.

We are in the process of getting data-stream into the ARM archive.

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Data Quality

Not available yet.

Current Health and Status

The system operates continuously, except for occasional power outages. We have ordered better uninterruptible power supply systems for the instruments.

Data User Notes

None

Automated Quality Control/Flagging Contained within NETCDF Files

None

Instrument Mentor Quality Control Checks

ARM staff do preventive maintenance checks weekly, and then post preventive maintenance reports on the web. ARM Carbon scientists at LBNL check these reports each week. We evaluate the zero drift (change in reference gas value) and specifications of all instrument components every week.

Examples of instrument specifications that we check out:

  • Temperature of IRGA,
  • Pressure of gas in IRGA
  • Pressure in each sample inlet system after pumping,
  • Temperature of the cooling (drying) unit
  • Flow rate of sample gas through the IRGA

Value Added Procedures

None at present

Quality Measurement Experiments

None at present

Examples of Data

Data are reported as ppm (V) CO2 in dry air.

Data Quicklooks/Near Realtime

Data stream 1: Quick view of the Precision Gas System data for yesterday - http://co2anal.lbl.gov/worldview/pgs-yest.html

Data stream 2: Quick view of the Precision Gas System data for the past week - http://co2anal.lbl.gov/worldview/pgs-week.html

Calibration and Maintenance

Calibration Theory

Every four hours the instrument measures four calibration gases which are absolute standards.

We also run samples into the inlet from a tank (using an open split) to make sure that the measured value is consistent and without trend.

Calibration History

We did a blind comparison of a tank of CO2 in air with the NOAA-CMDL standards lab. Our measurement agreed with theirs to 0.05 ppm.

Maintenance Procedures

ARM staff do preventive maintenance checks weekly, and then post preventive maintenance reports on the web. ARM Carbon scientists at LBNL check these reports each week. We evaluate the zero drift (change in reference gas value) and specifications of all instrument components every week.

Examples of instrument specifications that we check out:

  • Temperature of IRGA,
  • Pressure of gas in IRGA
  • Pressure in each sample inlet system after pumping,
  • Temperature of the cooling (drying) unit
  • Flow rate of sample gas through the IRGA

We change a chemical dessicant as indicated by visual inspection.

Online Maintenance Documentation

None except the preventive maintenance report.

Supplemental Assessment of Instrument Calibration and Maintenance Procedures

None.

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Frequestly Asked Questions-FAQs

Where do I get more information?

Contact the instrument mentor.

Software Documentation for this Instrument

None.

Contacts

Instrument Mentor

Margaret Torn, Staff Scientist

Earth Sciences Division Mail Stop 90-1116 E.O. Lawrence Berkeley National Laboratory 1 Cyclotron Rd. Berkeley, CA 94720

Tel 510-495-2223 • FAX 510-486-7070 email mstorn@lbl.gov

URL • http://www-esd.lbl.gov/ESD_staff/torn/index.html

Vendor/Instrument Developer

LBNL staff built the system.

Li-Cor is the vendor for the IRGA, model LI-6252 CO2 Analyzer (see http://env.licor.com/products/gasanalyzers/li6252/6252.htm for product description).

Citable References

None at this time.

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