A new reference scale for atmospheric observations of CO is described here. The scale, WMO X2014, is based on primary reference gases prepared in 2000, 2006 and 2011 by a gravimetric method (Novelli et al., 1992; Hall et al., 2007). This scale supersedes previous versions (X2000 and X2004, WMO 2010) from 2000 to present. Its development and evaluation are based on measurements of the primary standards using three analytical methods: gas chromatography with hot mercuric oxide reduction (GC-HgO, from 2000-2005), resonance fluorescence in the vacuum ultraviolet (VUV, 2004-2010), and off-axis Integrated Cavity Output Spectroscopy (ICOS, 2010 to present). The different instruments used several approaches for calibration, including response curves derived from multiple standards, two standards with linear regression and single point calibration. The scale is evaluated using calibration results for eight surveillance standards measured between 2001 and 2104 (Table 1). Its uncertainty is estimated as 1.2 nmol mol-1 or 0.6%, whichever is greater. Revised mole fractions for standards determined since 2000 are available at www.esrl.noaa/gmd/ccl/refgas.html. Measurement results determined prior to 2000 will be revised to X2014.

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

Tank ID GC-HgO*
2000-2004
VUV**
2004-2005
2006 VUV#
2006-2007
ICOS%
01-03 2012
MEAN
ND15749 58.0 (0.9) 57.8 (0.8) 57.8 (0.2) 58.6 (0.2) 58.1 (0.8)
ND15807 77.0 (1.3) 77.5 (0.8) 77.3 (0.4) 77.8 (0.3) 77.3 (0.8)
ND17445 107.9 (0.8) 107.2 (0.5) 107.4 (0.2) 107.4 (0.4) 107.4 (0.8)
ND16443 135.2 (0.7) 133.9 (0.3) 134.2 (0.2) 134.4 (0.2) 134.3 (0.8)
ND17435 155.0 (1.4) 153.3 (0.3) 153.6 (0.2) 153.7 (0.3) 153.9 (1.1)
ND16439 175.6 (1.2) 175.3 (0.3) 175.5 (0.8) 174.9 (0.2) 175.3 (0.9)
ND17431 202.4 (1.5) 202.5 (0.7) 202.3 (0.3) 202.3 (0.3) 202.4 (1.0)
ND16416 NA 301.5 (1.2) 302.4 (0.3) 302.9 (0.6) 302.3 (1.4)^
* GC-HgO response curves were based on measurements of the 2000 primary standards. The calibration range covered 55 to 200 nmol mol-1; expanded to 330 nmol mol-1 for samples above 200
** VUV measurements were referenced to two 2000 primary standards with 330 and 398 nmol mol-1
# Response curves were based on 2006 primary standards (47 to 497 nmol mol-1)
% ICOS used response curves were based on reference gases secondary to the 2011 primary standards (20 to 500 nmol mol-1)
^ Revised results above 200 nmol mol-1 are not yet available

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