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Mace Head
-CO-
| Observation |
| Category |
: |
Air sampling observation |
| Situation |
: |
ongoing |
| Time zone |
: |
UTC |
| Sampling |
| Sampling height |
: |
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| Description |
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flask |
| Sampling and analysis frequency |
: |
Weekly |
| Sampling environment |
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| Description for sampling analysis |
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Data are provided through December 2009.
Through most of the period 1988-1991, one flask of a sample pair was analyzed for CO, and when there was suitable pressure remaining in the flask, two or more aliquots were analyzed. We have used the difference in CO mixing ratios between the two aliquots as an indication of the precision of the measurement. Details of the analytical procedures through 1990 are described in Novelli et al., 1992. From the beginning of CO measurements in flask samples (mid 1987) to December 1990 we used a single point calibration sequence, as the response characteristics of the CO instrument had been shown to be linear over a range of 0 to 1000 ppb CO (Novelli et al., 1991).
In January 1991, we changed instruments used for the analysis of CO in flask air samples. The new instrument exhibited a non-linear response over the range of 0 to 250 ppb CO. Therefore it was necessary to change from a single-point calibration routine to a multiple standard calibration scheme (a multi-point calibration procedure based upon that used for calibration of standards, as described in Novelli et al. (1994, 1998) was also used for flask analysis).
In October 1991 we began analyzing a single aliquot from both members of a flask pair (rather than 2 or more aliqouts from a single flask). The principle reasons for the change were to simplify flask handling procedures (the CMDL carbon dioxide and methane projects also measure both flasks of the sample pair) and to have flask pair agreement (the difference in mixing ratio between the two flasks collected simultaneously) as an additional diagnostic to use in evaluating the quality of the data.
In 2002 we applied a correction to the CO data. Based upon several sets of gravimetric standards a time dependant correction to was applied to all air samples analyzed since 1991. Smallest changes are found in the earlier data. A full description of the revision is avaliable in Novelli et al. (2003).
Measurements made using the GC-HgO reduction method require a multi-point calibration to correct for detector non-linearity. Our calibrations cover the range of the background troposphere (between 50 and 200 ppb). Mixing ratios above and below the calibrated span contain greater uncertainty relative to those within the calibration. Estimated errors relative to the calibrated span for between 35-50ppb and 200-230 ppb are small, less than +/- 1%, however mixing ratios below 25 ppb and above 230 ppb should be used with caution as errors can be substantially larger and therefore not suitable for quantitative analysis. CO results considered out of range are flagged with a "C" in the 3rd column.
Ambient and standard air samples are injected into the gas chromatograph (GC). Carbon monoxide (CO) and molecular hydrogen (H2) are separated from other sample constituents using dual columns. CO and H2 are reacted with hot HgO bed to produce mercury (Hg). Hg is then determined photometrically. The non-linear detector requires a multipoint calibration (we use 6 standards in the atmospheric range). This process is highly automated for field and laboratory operations.
Measurements are reported in units of nanomol/mol (10^-9 mol CO per mol of dry air (nmol/mol) or parts per billion (ppb)) relative to the WMO CO scale (Novelli et al., 1991, Novelli et al., 1994, Novelli et al., 1998). Reproducibility of our measurements, based on repeated analysis of air from a high-pressure cylinder, is 1 nmol/mol at 50 nmol/mol and 2 nmol/mol at 200 nmol/mol over the period of our measurements. The absolute accuracy of our CO scale is unknown. |
| Instrument and Analysis |
| Measurement method |
: |
GC-HgO |
| Current status and history of instrument |
: |
GC-HgO |
| Description of instrument |
: |
Trace Analytical lnc. |
| Calibration |
| Current scale employed in the measurement |
: |
WMO CO scale |
| Measurement calibration |
: |
|
| Scale and calibration(treasability) |
: |
WMO CO scale |
| Data Processing |
| Measurement unit |
: |
ppb |
| Data processing |
: |
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| Processing for averaging |
: |
Processing for Monthly Data:
Monthly means are produced for each site by first averaging all valid measurement results in the event file with a unique sample date and time. Values are then extracted at weekly intervals from a smooth curve (Thoning et al., 1989) fitted to the averaged data and these weekly values are averaged for each month to give the monthly means recorded in the files. Flagged data are excluded from the curve fitting process. Some sites are excluded from the monthly mean directory because sparse data or a short record does not allow a reasonable curve fit. Also, if there are 3 or more consecutive months without data, monthly means are not calculated for these months. |
| Data flag |
: |
NOAA ESRL uses a 3-column quality control flag where each column is defined as follows:
[column 1]
REJECTION flag. An alphanumeric other than a period (.) in the FIRST column indicates a sample with obvious problems during collection or analysis. This measurement should not be interpreted.
[column 2]
SELECTION flag. An alphanumeric other than a period (.) in the SECOND column indicates a sample that is likely valid but does not meet selection criteria determined by the goals of a particular investigation.
[column 3]
COMMENT flag. An alphanumeric other than a period (.) in the THIRD column provides additional information about the collection or analysis of the sample.
WARNING: A "P" in the 3rd column of the QC flag indicates the measurement result is preliminary and has not yet been carefully examined by the PI. The "P" flag is removed once the quality of the measurement has been determined.
If both the first and second column contain a period (e.g., "..." and "..I"), the sample is RETAINED.
Flag : Description
[RETAINED]
... : Accepted as background air sample
..I : sample has also been measured by another lab as part of an intercomparison experiment
..C : mixing ratio falls outside of calibration range (see Section ).
..< : first reference sample flagged
..> : second reference sample flagged
[SELECTED]
.X. : determined using a statistical method to be an outlier
.N. : flagged manually as an outlier
NOTE: Some sites known to be heavily impacted by regional pollution are not SELECTED for background conditions. Among these are BAL, BSC, HUN, OBN and TAP.
[REJECTED]
*.. : off-scale or broken flask
W.. : flask type unacceptable for measurement of CO
Z.. : one or more sample aliquots diff by 3 ppb or more
P.. : flasks in pair differ by 3 ppb or more
A.. : rejected due to error in analysis
E.. : sample collected as part of CCG experiment
S.. : sample collected through observatory CO2 analyzer
LLL : Indicates that for the specified site, measurements of this and more recent samples have not yet been evaluated by the PI. |
| Data remarks |
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| Other Information |
| Scientific aim |
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| Reference |
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Lang, P.M., L.P. Steele, R.C. Martin, and K.A. Masarie, Atmospheric methane data for the period 1983-1985 from the NOAA/GMCC global cooperative flask sampling network, NOAA Technical Memorandum ERL CMDL-1, 1990a.
Lang, P.M., L.P. Steele, and R.C. Martin, Atmospheric methane data for the period 1986-1988 from the NOAA/CMDL global cooperative flask sampling network, NOAA Technical Memorandum ERL CMDL-2, 1990b.
Novelli, P.C., J.E. Elkins, and L.P. Steele, The development and evaluation of a gravimetric reference scale for measurements of atmospheric carbon monoxide, J. Geophys. Res., 96, 13,109-13,121, 1991.
Novelli, P.C., L.P. Steele, and P.P. Tans, Mixing ratios of carbon monoxide in the troposphere, J. Geophys. Res., 97, 20,731-20,750, 1992.
Novelli, P.C., J.E. Collins, Jr, R.C. Myers, G.W. Sachse, and H.E. Scheel, Reevaluation of the NOAA/CMDL carbon monoxide reference scale and comparisons to CO reference gases at NASA-Langley and the Fraunhofer Institute, 99, 12,833- 12,839, 1994.
Novelli, P.C., K.A. Masarie, and P.M. Lang, Distributions and recent changes in carbon monoxide in the lower troposphere, J. Geophys. Res., 103, 19,1015- 19,033, 1998.
Novelli, P.C., K.A. Masarie, P.M. Lang, B.D. Hall, R.C. Myers, and J.W. Elkins, Re-analysis of tropospheric CO trends: Effects of the 1997-1998 wild fires, J. Geophys. Res., 108, D15 : 4464, doi:10.1029/2002JD003031, 2003. |
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| submitted by Earth System Research Laboratory, NOAA |
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