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GLOBALVIEW-CO2 : Methodology
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Methodology
Each measurement record used to derive GLOBALVIEW-CO2 has been carefully edited and selected by the organization or institution contributing the observations. The measurement records are accumulated atNOAA ESRL along with documentation and references. Wherever possible, NOAA ESRL attempts to reproduce the selected data set based on descriptions in the literature. Details of methodology and standard scale can often be obtained from the documentation and literature. Selected measurements are then compared to other measurement records that are nearby in latitude as an additional assessment of potential calibration or sampling problems. Occasionally, two or more organizations make observations at the same location, providing an opportunity to directly compare independent measurement programs [Masarie et al., 2001].
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Standard Scale
The majority of laboratories contributing to the GLOBALVIEW-CO2 data product are members of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) network. Data from the GAW network are reported relative to the WMO CO2 Mole Fraction Scale, which is maintained and propagated by the Central CO2 Laboratory (CCL). GAW laboratories are required to maintain direct traceability of their internal calibration scale to the CCL. A few laboratories contributing to the data product are not part of the WMO GAW program and thus provide data referenced to some other scale. This section describes ongoing efforts to assess the comparability of calibration scales and atmospheric observations.
- The WMO CO2 Mole Fraction Scale
The WMO Scale is based on regular determinations of the mole fraction of
CO2 in dry air from a set of 15 primary standards using a high precision manometric system with NIST-traceable measurements to temperature, pressure and volume [Tans et al., 2003; Zhao et al., 1997].
Uncertainty of the WMO Scale is estimated to be ~0.06 µmol mol-1 (one sigma).
Reproducibility of the determinations is about 0.03 µmol mol-1 (one sigma), based on repeated manometric analyses [Zhao and Tans, 2006]. The scale as defined by the primary standards (projected 30 year average lifetime) is subsequently propagated to a set of 9 secondary (transfer) standards (3-4 year average lifetime) using relative nondispersive infrared (NDIR) measurement techniques. NDIR measurement reproducibilityp is ~0.01 µmol mol-1. Propagation of the WMO Scale from the Primary cylinders to working standards, via intermediate Secondary standards maintained by the CCL, has a reproducibility of ~0.02 µmol mol-1 (one sigma). Cylinders are calibrated for other laboratories against the transfer standards using the NDIR methodology. The use of a calibration hierarchy enables the CCL to occasionally re-assign, when justification for such a change is strong, the value of a primary or secondary standard and propagate the change, in a straightforward manner, to all dependent calibrations.
Recent History
In 1995, the WMO designated NOAA ESRL as the Central CO2 Laboratory (CCL) responsible for the maintenance of the absolute WMO Mole Fraction Scale for carbon dioxide. Before that time, the scale had been maintained by the Scripps Institution of Oceanography (SIO).
In 1990, ESRL prepared 15 CO2-in-air reference gas mixtures in large aluminum high pressure cylinders for use as primary standards, ranging in CO2 mole fraction from approximately 250 to 520 µmol mol-1. These cylinders were calibrated four times at SIO by the NDIR method from mid-1991 to 1999. In 1996, ESRL began making absolute manometric determinations of its 15 "primary" standards. Between 1996 and 2001, values assigned to the 15 primaries were based on both SIO NDIR measurements and ESRL manometric determinations. Starting in 2002, the values assigned to the primaries have been based on manometric determinations by ESRL alone.
Revisions to calibrations provided before 2005 by the CCL have been made. This is mainly due to revisions of the calibrations performed by Scripps between 1991 and 1999. Until 1996 the assigned values of the primary standards were based entirely on the infrared calibrations by Scripps. The average of all assigned values to the primaries increased by 0.16 µmol mol-1 from 1993 to 2002. Since then the average of all assigned values of the primaries has decreased by 0.01 µmol mol-1.
The CO2 Mole Fraction Scale is defined by a polynomial curve fit to the Primary Standards. This is done to smooth out the uncertainty of assigned values to individual Primary Standards caused by the imprecision of the absolute calibrations, which is 0.03 ppm (see above). In September 2005, the WMO scale was revised, and a quadratic curve fit was used. After another set of calibrations of the Primaries in 2006-2007, the individual Primaries were revised by only minor amounts, up to 0.01 ppm. However, in defining the revised scale for 2007, we chose to use a cubic polynomial for the curve fit, which led to mole fraction-dependent differences between WMO-X2007 and WMO-X2005 between -0.03 and 0.03 ppm in the range of ambient air. All laboratories who have had cylinders calibrated by the CO2 CCL should have received revised calibration assignments based on the new scale. If you would like to receive revised values based on the new scale please contact Duane Kitzis (NOAA/ESRL).
- Traceability to the WMO Scale
Not all data contributed to the Cooperative Atmospheric Data Integration Project for CO2 are directly traceable to the WMO Mole Fraction Scale. A few laboratories have never had their standard gases calibrated by the CCL and report CO2 measurements relative to some other scale (see Table 1). Measurements from these laboratories are not directly traceable to the WMO Mole Fraction Scale. Several other laboratories have, at one time, had their standards calibrated by the CCL but have not maintained a routine recalibration schedule. Because the mole fraction of CO2 contained in high-pressure cylinders can potentially change with time due to CO2 adsorption or production within the cylinder or regulator, or through other effects, a laboratory's internal scale may potentially change with time relative to the WMO scale, which itself is anchored through absolute manometric determinations. Without routine recalibration by the CCL to reestablish direct traceability to the WMO-X2007 scale, laboratories contribute CO2 data that are no longer directly traceable to the WMO scale. Please note that recent calibration with the CCL does not necessarily imply measurements are on the most current WMO Mole Fraction Scale.
Table 1. Traceability to the WMO scale based on calibration by the CCL.
| LABORATORY [lab #] |
RECENT CAL. DATES1 |
# of CYLINDERS3 |
REPORTED SCALE |
| ESRL [01] |
2007 11 / 2007 02 |
16 (recal) / 16 (recal) |
WMO |
| CSIRO [02] |
2004 08 / 2004 05 |
3 (cal) / 8 (recal) |
WMO4 |
| NCAR [03] |
2006 04 |
6 (cal) |
WMO |
| SIO [04] |
-- |
-- |
Scripps 08A |
| EC [06] |
2009 06 / 2008 09 |
2 (cal) / 2 (recal) |
WMO |
| LSCE [11] |
2008 01 / 2007 07 |
1 (cal) / 7 (recal) |
WMO |
| NIWA [15] |
2008 10 / 2004 12 |
2 (cal) / 1 (cal) |
WMO |
| IMS [17] |
1998 10 |
5 (cal) |
WMO4 |
| JMA [19] |
2008 10 / 2006 12 |
14 (recal) / 14 (cal) |
WMO |
| NIES [20] |
2006 09 / 2002 12 |
5 (recal) / 2 (cal) |
NIES09 |
| UBA/UHEI-IUP [23] |
2006 04 / 2003 11 |
6 (recal) / 6 (recal) |
WMO |
| SNU [24] |
2004 09 / 2003 06 |
4 (cal) / 5 (cal) |
WMO4 |
| AEMET [27] |
2006 03 / 2002 03 |
8 (cal) / 9 (cal) |
WMO |
| ENEA [28] |
2008 12 / 2008 02 |
4 (recal) / 4 (cal) |
WMO |
| PNRA/DNA [29] |
2001 07 / 2000 12 |
3 (cal) / 3 (cal) |
WMO4 |
| FMI [30] |
2007 05 / 2007 02 |
2 (cal) / 6 (cal) |
WMO |
| CAMS [33] |
2008 10 / 2007 10 |
6 (cal) / 6 (cal) |
WMO |
| HMS [35] |
2007 11 / 2006 09 |
6 (cal) / 6 (cal) |
WMO |
| SAWS [36] |
2009 04 / 2007 05 |
2 (cal) / 2 (recal) |
WMO |
| MPI-BGC [45] |
2009 04 / 2009 02 |
1 (cal) / 2 (cal) |
WMO |
1Only the two most recent calibration events are shown.
2Calibration made at ESRL relative to the ESRL secondary standards.
3Initial (cal)ibration and subsequent (recal)ibration by the CCL (NOAA) are specified.
4Traceability to the WMO Mole Fraction Scale has lapsed. A recalibration schedule of every 3 years is thought to be the minimum frequency for maintaining traceability to the WMO scale.
5Insufficient number of cylinders calibrated to properly link laboratory internal scale to WMO Mole Fraction Scale. The minimum number of standards required to establish traceability to the WMO Mole Fraction Scale is three.
6Internal scale is indirectly linked to WMO Mole Fraction Scale.
- Comparisons of Standard Scales
In an attempt to assess differences in standard scales among organizations making CO2 measurements, laboratories contributing to GLOBALVIEW-CO2 have participated in recent interlaboratory intercomparison or round robin (RR) experiments endorsed by the WMO and IAEA. Based on preliminary results from the 2002-2006 intercomparison of standard gases, the majority of participating laboratories agreed to within 0.1 µmol mol-1 [see Table 2]. The preliminary results were kindly provided by Dr. Lingxi Zhou (CAMS) [Zhou et al., in preparation]. Final results were not available at the time of this update. Results from previous RR experiments are also available [Peterson et al., 1999].
Table 2. 2002-2006 Round Robin Results [per comm. Lingxi Zhou, 2008-08-20]: Differences from NOAA ESRL (Lab minus ESRL).
| LABORATORY [lab #] |
MEASUREMENT DATE |
DIFFERENCE from NOAA (µmol mol-1) |
| Nominal Tank Concentration |
355 |
365 |
385 |
| CSIRO [02] |
2005 10 |
-0.01 |
-0.03 |
-0.08 |
| NCAR [03] |
2006 06 |
0.07 |
-0.04 |
-0.04 |
| SIO [04] |
2005 06 |
0.10 |
0.02 |
0.02 |
| EC [06] |
2005 05 |
0.06 |
-0.05 |
-0.06 |
| MRI [10] |
2003 07 |
-0.16 |
-0.16 |
-0.08 |
| LSCE [11] |
2002 12 |
0.10 |
0.03 |
0.05 |
| NIWA [15] |
2005 05 |
-0.08 |
-0.08 |
-0.09 |
| IMS [17] |
2002 10 |
0.08 |
0.02 |
-0.03 |
| JMA [19] |
2004 01 |
0.13 |
0.00 |
-0.02 |
| NIES [20] |
2003 04 |
-0.10 |
-0.15 |
-0.14 |
| UBA/UHEI-IUP [23] |
2002 10 |
-0.01 |
-0.06 |
-0.06 |
| SNU1 [24] |
-- |
-- |
-- |
-- |
| AEMET1 [27] |
-- |
-- |
-- |
-- |
| ENEA [28] |
2002 11 |
0.05 |
-0.15 |
-0.26 |
| PNRA/DNA1 [28] |
-- |
-- |
-- |
-- |
| FMI [30] |
2003 01 |
-0.02 |
-0.04 |
-0.14 |
| CAMS [33] |
2004 08 |
-0.03 |
-0.20 |
0.02 |
| HMS [35] |
2003 02 |
0.06 |
-0.21 |
-0.06 |
| SAWS [36] |
2005 12 |
-0.02 |
-0.09 |
-0.18 |
| MPI-BGC [45] |
2003 12 |
0.04 |
0.02 |
-0.02 |
1Did not participate in the 2002-2006 RR experiment.
Every attempt has been made to ensure that the data sets used to derive the GLOBALVIEW-CO2 data product are comparable to within 0.2 µmol mol-1. At present, the Cooperative Atmospheric Data Integration Project for Carbon Dioxide has made no standard scale adjustments to any of the measurement records integrated into GLOBALVIEW-CO2. Records that appear to be affected by a serious scale discrepancy have been omitted at this time.
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