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Amsterdam Island - NOAA/ESRL

Parameter Inventory Parameter Metadata Station Contributor
CH4 ** *
CO2 * *
Observation Processing/Calibration Contact Person Reference Data/Quick Plot
Category Air sampling observation
Sampling Type flask
Sampling Height/Depth
Sampling and Analysis Frequency Weekly
Sampling Environment
Measurement Method Gas Chromatography (FID)
Current status and history of Instruments GC-FID
Description of Instruments
Time Zone UTC
Data Period 1983-01-01 - 1990-12-07
Data Type event, monthly
Parameter detail
Other Descriptions for Sampling and Analyses All samples were analyzed for methane at NOAA ESRL in Boulder, Colorado by gas chromatography with flame ionization detection, and each sample aliquot was referenced to the NOAA 2004A methane standard scale (see www.esrl.noaa.gov/gmd/ccl/ch4_scale.html; Dlugokencky et al., 2005). 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. We have used the difference in methane mole fraction between the first and second aliquot to establish the repeatability of the measurement. Over the full period of the record described here, the average repeatability has been approximately 2 ppb. In October 1991, our analysis procedure was altered; we began analyzing a single aliquot from both members of the flask pair. The principle reasons for the change were to simplify flask handling procedures (the carbon dioxide and carbon monoxide projects also measure both flasks of the sample pair) and to have flask pair agreement (the difference in methane mole fraction 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 now assessed by two approaches: approximately monthly measurements of a target tank, 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.)

Uncertainties are reported for each measurement based on analytical repeatability and our ability to propagate the WMO CH4 mole fraction standard scale. Analytical repeatability is based on the average absolute value of pair agreement between pairs collected nearly simultaneously. It varies with analytical instrument from 0.8 to 2.3 ppb. Propagation of the scale is based on a comparison of flask and continuous measurements at MLO and BRW observatories. It has a fixed value of 0.7 ppb based on the mean difference between discrete samples and the nearest continuous hourly average for the entire period of overlap. The two terms are added in quadrature (square root of the sum of the squares) to estimate the measurement uncertainty and is considered equivalent to a 68% confidence interval.

Ambient and standard air samples are injected into a 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 NOAA 2004A CH4 standard scale. Repeatability of our measurements, based on repeated analysis of air from a high-pressure cylinder, has ranged from 1 to 3 nmol/mol over the period of our measurements. The absolute accuracy of our 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) Centre des Faibles Radioactivities/TAAF [CFR/TAAF]
Last update 2017-08-14
Situation 1990/10/15

Category Parameter Country/Territory Contributor
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