REEVALUATION OF THE NOAA-CMDL CARBON-MONOXIDE REFERENCE SCALE AND COMPARISONS WITH CO REFERENCE GASES AT NASA-LANGLEY AND THE FRAUNHOFER-INSTITUT

The carbon monoxide (CO) reference scale created by the National Ocean ic and Atmospheric Administration/Climate Monitoring and Diagnostics L aboratory (NOAA/CMDL) is used to quantify measurements of CO in the at mosphere, calibrate standards of other laboratories and to otherwise p rovide reference gases to the community measuring atmospheric CO. This reference scale was created based upon a set of primary standards pre pared by gravimetric methods at CMDL and has been propagated to a set of working standards. In this paper we compare CO mixing ratios assign ed to the working standards by three approaches: (1) calibration again st the original gravimetric standards, (2) calibration using only work ing standards as the reference gas, and (3) calibration against three new gravimetric standards prepared at CMDL. The agreement between thes e values was typically better than 1%. The calibration histories of CM DL working standards are reviewed with respect to expected rates of CO change in the atmosphere. Using a Monte Carlo approach to simulate th e effect of drifting standards on calculated mixing ratios, we conclud e that the error solely associated with the maintenance of standards w ill limit the ability to detect small CO changes in the atmosphere. We also report results of intercalibration experiments conducted between CMDL and the Diode Laser Sensor Group (DACOM) at the NASA Langley Res earch Center (Hampton, Virginia), and CMDL and the Fraunhofer-Institut (Garmisch-Partenkirchen, Germany). Each laboratory calibrated several working standards for CO using their reference gases, and these resul ts were compared to calibrations conducted by CMDL. The intercompariso n of eight standards (CO concentrations between approximately 100 and approximately 165 ppb) by CMDL and NASA agreed to better than +/-2%. T he calibration of six standards (CO concentrations between approximate ly 50 and approximately 210 ppb) by CMDL and the Fraunhofer-Institut a greed to within +/- 2% for four standards, and to within +/- 5% for al l six standards.