GLOBALVIEW uses discrete and quasi-continuous measurements from fixed surface and tower sites and moving ship and aircraft sites. Discrete samples are collected in situ at weekly to monthly intervals and returned to the collaborating measurement laboratory for analysis. Quasi-continuous samples are measured in situ using systems located at the sampling location.
Each measurement record used to derive GLOBALVIEW-CO2 has been carefully edited and selected by the organization or institution contributing the observations. Observations used to derive the data product have been selected for baseline conditions to exclude samples influenced by local source and sinks . Baseline selection is site-specific. In most instances, selection is done by the PIs before submission to GLOBALVIEW and based on their knowledge of local conditions. The measurement records are accumulated at NOAA ESRL along with documentation and references. Wherever possible, NOAA ESRL attempts to reproduce the selected data set based on descriptions in the literature. Users are encouraged to review the literature for further details on baseline selection strategies.
Quasi-continuous data used to derive GLOBALVIEW have been preprocessed to produce a single value per day. Often this averaging process is performed by the collaborating laboratories using well-established methods that have been published in the literature. In some instances, the averaging is done at NOAA and in cooperation with the contributing laboratories. Table 1 summarizes the different averaging strategies. Users are encouraged to review the literature and contact the measurement labs directly for details about and access to the actual observations.
1<site> is a placeholder for site code.
2<ht> is a placeholder for tower intake height (magl).
3Daily values computed by NOAA unless otherwise specified.
4In this context "Valid Data" means the observation is thought to be free of sampling and analytical problems and has not been locally influenced.
A primary challenge for the GLOBALVIEW project and for the atmospheric trace gas measurement community is to ensure that measurements made using independent techniques can be integrated into larger cooperative data sets without introducing significant biases. This is a difficult challenge (see Data Integration). Several strategies exist to make this assessment including ongoing comparisons of 1) measurements of air from the same high-pressure cylinders; 2) measurements from glass flasks filled from the same high-pressure cylinders; 3) measurements from low-pressure cylinders decanted from high-pressure cylinders; 4) measurements of air from the same ambient samples; and 5) measurements from the same location (co-located) using different methodologies. Ongoing direct comparison of co-located atmospheric measurements is one of the more effective strategies [See, for example, Masarie et al., 2001]. Where ongoing comparison experiments of atmospheric measurements do not exist, we must rely on other less direct methods. In all instances, selected measurements are compared to other measurement records that are nearby in latitude as an additional assessment of potential calibration or sampling problems. The first step in assessing measurement comparability between independent records is to ensure data are reported on a consistent 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 (see Table 2). 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.
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. 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 2. Traceability to the WMO scale based on most recent calibration by the CCL1.
LAB [lab #] LAST CALIBRATION EVENT1 # of CYLINDERS3 REPORTED SCALE NOAA  2013-08 17 (recal) WMO CSIRO  2012 8 (cal) WMO NCAR  2011-04 6 (recal) WMO SIO  -- -- 2008A SIO6 EC  2012-01 8 (recal) WMO NIPR  -- -- Tohoku Univ. 20106 LSCE  2008-01 1 (cal) WMO4 IAFMS  1998-10 5 (cal) WMO4 JMA  2012-10 14 (recal) WMO NIES  2006-09 5 (recal) NIES096 UHEI-IUP  2010-07 4 (cal) WMO SNU  2004-09 4 (cal) 2008A SIO6 IPEN  2009-11 4 (cal) WMO AEMET  2008-09 2 (cal) WMO4 FMI  2012-01 4 (cal) WMO CMA  2009-04 6 (cal) WMO4 HMS  2012-09 4 (recal) and 2 (cal) WMO SAWS  2012-06 6 (recal) WMO NIES and MRI  2006-09 5 (recal) NIES096 RUG  2012-05 5 (cal) WMO KUP  2012-12 5 (cal) WMO LBNL  2012-06 4 (cal) WMO
1Only the most recent calibration event is shown. These postings are based on NOAA records and may not be completely accurate.
2Calibration made at ESRL relative to the ESRL secondary standards.
3Initial (cal)ibration or (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.
6Lab 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 results from the recent (5th) RR experiment (courtesy of Dr. Lingxi Zhou, WMO RR Referee), the majority of participating laboratories agreed to within 0.1 µmol mol-1. Please note: These results only indicate how well each participant's laboratory standard scale is linked to the WMO scale at a particular moment in time. The level of compatibility based on ongoing and direct comparison of atmospheric measurements will likely be considerably different and vary in time.