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Mauna Loa
-CH4-
| Observation |
| Category |
: |
Air sampling observation |
| Situation |
: |
ongoing |
| Time zone |
: |
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| Sampling |
| Sampling height |
: |
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| Description |
: |
continuous |
| Sampling and analysis frequency |
: |
Continuous |
| Sampling environment |
: |
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| Description for sampling analysis |
: |
Sampling interval is 60 to 96 times per day.
Silica gel pre-column back flushed to vent (8 feet x 1/8 inch outer diameter). Molecular sieve analytical column (3.5 feet x 1/8 inch outer diameter). Oven, FID temperature about 80°C. Working standard are dry, natural air. See Dlugokencky et al. [1995] for the experimental details.
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| Instrument and Analysis |
| Measurement method |
: |
Gas Chromatography (FID) |
| Current status and history of instrument |
: |
GC-FID |
| Description of instrument |
: |
EG&G, Tulsa, Oklahoma
CarleSeeries 400 |
| Calibration |
| Current scale employed in the measurement |
: |
NOAA04 scale |
| Measurement calibration |
: |
Working standard gases are calibrated against primary standards in Boulder. Each sample aliquot is bracketed by standard gas. |
| Scale and calibration(treasability) |
: |
NOAA04 scale (a gravimetrically-prepared CH4 standard scale (see Dlugokencky et al., 2005)). This scale results in CH4 mole fractions that are a factor of 1.0124 greater than previous NOAA methane measurement scale. All CH4 measurements, from 1986 through 2004, are reported on this scale. |
| Data Processing |
| Measurement unit |
: |
ppb |
| Data processing |
: |
Data are first edited using a rule-based editing algorithm (Masarie et al., 1991) to exclude measurements obtained when the analytical instrument was not working optimally. Hours for which there are no valid measurements have been assigned a default mole fraction (-9999999) and flag (*..). See Dlugokencky et al. (1995) for the etails.
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| Processing for averaging |
: |
Processing for Hourly Data:
Each hourly average is computed from individual measurements (typically 4 per hour).
Valid hourly averaged data are selected to distinguish samples of regionally representative air (background) from samples influenced by local sources and sinks (non-background). Background hourly data are identified with a "..." selection flag. A ".C." flag is assigned to data identified as non-background.
The criteria for determining background conditions are site specific. The background criteria for Mauna Loa are during predominatly "downslope" meteorological conditions as indicated by local time of day, 0000-0659. See Dlugokencky et al. (1995) for details.
Processing for Daily Data:
Daily means are calculated from background (QC Flag is "...") hourly averages.
Days for which only one hourly value is used to determine the average are assigned a default (-99999) standard deviation.
Processing for Monthly Data:
Monthly means are calculated using values extracted at one-day timesteps from a smooth curve (Thoning et al., 1989) fitted to the daily averages. |
| Data flag |
: |
... : no flag applied.
*.. : no acceptable CH4 mole fractions were obtained during this hour.
.C. : data are non-background.
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| Data remarks |
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| Other Information |
| Scientific aim |
: |
In situ measurements are used to understand the methane budget over local, regional, and lager spatial scales by studying correlations with other observed species, trajectory clusters, or meteorological data. The quasi-continuous measurements are also a valuable quality control tool for the discrete sampling program. |
| Reference |
: |
Dlugokencky, E.J., R.C. Myers, P.M. Lang, K.A. Masarie, A.M. Crotwell, K.W. Thoning, B.D. Hall, J.W. Elkins, and L.P. Steele, Conversion of NOAA CMDL atmospheric dry air methane mole fractions to a gravimetrically-prepared standard scale, J. Geophys. Res., in press, 2005.
Dlugokencky, E.J., L.P. Steele, P.M. Lang, and K.A. Masarie, Atmospheric methane at Mauna Loa and Barrow observatories: presentation and analysis of in situ measurements, J. Geophys. Res., 100, 23,103-23,113, 1995.
Masarie, K.A., L.P. Steele, and P.M. Lang, A rule-based expert system for evaluating the quality of long-term, in situ, gas chromatographic measurements of atmospheric methane, NOAA Tech. Memo. ERL CMDL-3, NOAA Environ. Res. Lab., Boulder, Colorado, 1991.
Thoning, K.W., P.P. Tans, and W.D. Komhyr, Atmospheric carbon dioxide at Mauna Loa Observatory 2. Analysis of the NOAA GMCC data, 1974-1985, J. Geophys. Res., 94, 8,549- 8,565, 1989. |
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| submitted by NOAA/ESRL Global Monitoring Division ( the former CMDL ) |
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