A new reference scale for atmospheric observations of CO is described here. The new scale, WMO CO X2014, is based on primary reference gases prepared in 2000, 2006 and 2011 by a gravimetric method (Novelli et al., 1991; Hall et al., 2007). This scale, while still considered preliminary, supersedes previous versions (WMO CO X2000 and WMO CO X2004) from 2000 to present. Its development is based on measurements of the three sets of primary standards. Three analytical methods were used: 1) gas chromatography with hot mercuric oxide reduction (GC-HgO, from 2000-2005), 2) resonance fluorescence in the vacuum ultraviolet (VUV, 2004-2010), and 3) off-axis Integrated Cavity Output Spectroscopy (ICOS, 2010 to present). The calibration of samples measured on either GC or ICOS used response curves derived from multiple standards. The VUV instrument which had a linear response used a single point calibration with reference gases between 300 and 400 nmol mol-1. The reference gases used on GC-HgO were revised to the new scale and the response curves re-determined. The new scale was evaluated for its consistency using calibration results for eight surveillance standards measured between 2001 and 2104 (Table 1). WMO CO X2014 extends the range of calibration to 500 nmol mol-1.

Samples measured on the GC with mole fractions > 200 nmol mol-1 have not yet been revised, nor have all measurements on VUV with CO > 350 nmol mol-1. Otherwise a preliminary revision to mole fractions determined between 2000 to present are available at www.esrl.noaa.gov/gmd/ccl/refgas.html. The uncertainty of the scale, estimated from calibrations of the surveillance cylinders, is approximately 1.2 nmol mol-1 or 0.6% of the mole fraction (k=2), whichever is greater. Measurement results prior to 2000 will also be revised to WMO CO X2014.

Table 1: Preliminary CO mole fractions assigned 8 surveillance cylinders on the X2014 scale. Mean CO mole fractions (nmol mol-1) with n=4 and uncertainty (k=2) of multiple measurements.

Tank ID GC-HgO*
2000-2004
VUV**
2004-2005
2006 VUV#
2006-2007
ICOS%
01-03 2012
MEAN
1 58.0 (0.9) 57.8 (0.8) 57.8 (0.2) 58.6 (0.2) 58.1 (0.8)
2 77.0 (1.3) 77.5 (0.8) 77.3 (0.4) 77.8 (0.3) 77.3 (0.8)
3 107.9 (0.8) 107.2 (0.5) 107.4 (0.2) 107.4 (0.4) 107.4 (0.8)
4 135.2 (0.7) 133.9 (0.3) 134.2 (0.2) 134.4 (0.2) 134.3 (0.8)
5 155.0 (1.4) 153.3 (0.3) 153.6 (0.2) 153.7 (0.3) 153.9 (1.1)
6 175.6 (1.2) 175.3 (0.3) 175.5 (0.8) 174.9 (0.2) 175.3 (0.9)
7 202.4 (1.5) 202.5 (0.7) 202.3 (0.3) 202.3 (0.3) 202.4 (1.0)
8 NA 301.5 (1.2) 302.4 (0.3) 302.9 (0.6) 302.3 (1.4)^

For more information or to be added to the mailing list for users of the WMO CO scale please contact Paul Novelli at paul.c.novelli@noaa.gov

See: