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Kaashidhoo - NOAA/GMD
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| Category |
Air sampling observation |
| Sampling Type |
flask |
| Sampling Height/Depth |
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| Sampling and Analysis Frequency |
Weekly |
| Sampling Environment |
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| Measurement Method |
Gas Chromatography (FID) |
| Current status and history of Instruments |
GC-FID |
| Description of Instruments |
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| Time Zone |
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| Data Period |
1998-03-02 - 1999-07-15 |
| Data Type |
event |
| Parameter detail |
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| Other Descriptions for Sampling and Analyses |
Through most of the period 1983-1991, one flask of a sample pair was analyzed for methane, and, whenever the overpressure was sufficient, at least two aliquots were analyzed. Difference in methane mixing ratio between first and second aliquot is used to establish the precision of the measurement. Over the full period of the record described here, the average precision has been approximately 3 ppb.
In October 1991,analysis procedure was altered; analysis of a single aliquot from both members of the flask pair begun. The principle reasons for the change were to simplify flask handling procedures and to have flask pair agreement (the difference in methane mixing ratio between the two flasks collected simultaneously) as an additional diagnostic to use in evaluating the quality of the data. The precision of the analytical instrument is necessary by two approaches: approximately monthly intercomparisons of methane reference gases, and assessing the relative stability
of the calibration gas samples during each day of flask measurements. (See Steele et al., 1987, Lang et al., 1990a,b, Dlugokencky et al., 1994b, and Dlugokencky et al., 2005 for details of the sampling network, equipment, standards, and procedures.)
The air samples are collected by two general methods: flushing and then pressureizing glass flasks with a pump, or opening a sopcock on an evacuated glass flask. During each sampling event, a pair of flasks is filled.
All samples were analyzed for methane at NOAA/GMD in Boulder, Colorado by gas chromatography with flame ionization detection, and sample aliquot was referenced to the NOAA04 methane standard scale (see Dlugokencky et al., 2005). Ambient and standard air samples are injected into the gas chromatograph (GC) with a sampling valve. Methane (CH4) is separated from other sample constituents using packed columns and detected using flame ionization (FID). This process is highly automated for field and laboratory operations.
Instrument response of the sample must be compared to a standard of known CH4 content.
Measurements are reported in units of nanomol/mol (10^-9 mol CH4 per mol of dry air (nmol/mol) or parts per billion (ppb)) relative to the NOAA04 CH4 standard scale.
Reproducibility of the measurements, based on repeated analysis of air from a high- pressure cylinder, has ranged from 1 to 3 nmol/mol over the period of the measurements.
The absolute accuracy of the scale is estimated at 0.2%, or about 3 nmol/mol. |
| Scientific Aim |
Obtain observational constraints on the global methane budget, including global burden, long-term trend, interannual variations in trend, seasonal cycles, and spatial variations (e.g., latitude gradient). Use the observations to quantify methane sources and sinks. |
| Supporting Contributor(s) |
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| Last update |
2009-07-23 |
| Situation |
1999 07 |
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This site is maintained by the Japan Meteorological Agency
in cooperation with the World Meteorological Organization
(Created : 2001/07/02 Modified : 2010/07/31)
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WMO World Data Centre for Greenhouse Gases
c/o Japan Meteorological Agency
1-3-4, Otemachi, Chiyoda-ku
Tokyo 100-8122, Japan
| Tel: |
+81-3-3287-3439 |
| Fax: |
+81-3-3211-4640 |
| E-mail: |
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WDCGG Data Submission and Dissemination Guide (PDF 1.2Mbyte)
WDCGG leaflet (PDF 2.7MByte, July 2008)