5.1.4. Gravimetric Standards
The NOAH standards project was expanded in 1996 with the addition of one full-time research assistant. A new gas chromatograph, similar to the CATS GC, was tested and calibrated. The new GC will provide measurement of CH3Br, CH3Cl, and HCFC-22 in the standards laboratory and eliminate the need for three separate GCs for measuring the other seven gases. Thirty gravimetric standards were prepared during 1996-1997. A total of 132 secondary and primary gravimetric standards were analyzed in the standards laboratory. Thirty-three standards were made for outside organizations and other NOAA laboratories. The leader of the standards project left for a new position at NIST at the end of 1997.
One of the major goals of the standards program was to provide a uniform standardization between the major networks responsible for monitoring N2O and many halocompounds in the atmosphere. A workshop was held during May 1997 in Boulder to begin this process. A series of round-robin tanks are being distributed for intercomparison. A number of tests involving cylinder stability are underway.
Calibrations for CFC-12, methyl chloroform, and carbon tetrachloride were revised since the last report [Elkins et al., 1996b]. All atmospheric CFC-12 data reported after 1993 were about 2.3% higher than actual. The reason for the error was the reliance on a set of 1993 standards that deviated from the rest of the standards which were made in 1991 and 1997 (Figure 5.14). Removal of that set resulted in better agreement (within 1%) for atmospheric measurements of CFC-12 between the CMDL and AGAGE networks. New gravimetric standards were prepared for CH3CCl3 and CCl4 in 1996. After comparing standards for all gases, the same 1993 set was found to have problems with these gases too (Figure 5.15). The values of the 1993 CCl4 standards are on average 4% lower near ambient levels (102 ppt) than those made in 1991 and 1997. The net effect to atmospheric CCl4 values (RITS GCs) reported in the last summary report is small, because a calibration tracking error in the opposite direction was found. The 1993 CH3CCl3 standards exhibited greater imprecision than the combined 1991 and 1996 sets but no significant offset. The atmospheric values are relatively unchanged from the last summary report. The new scales for CH3CCl3 and CCl4 give atmospheric values that are about 8 ppt and 3 ppt higher in 1997 than those measured by AGAGE. An exchange of weighed pure solvents in sealed microtubes is planned to test methods used by both networks.
Fig. 5.14. The relative difference in percent (observed-gravimetric value) for CFC-12 standards versus the gravimetric value for those standards made in 1993 and those prepared in 1991 and 1997 for the complete range of ambient ppt mixing ratios.
Fig. 5.15. The relative difference (%) in the observed-gravimetric value for (a) CCl4 and (b) CH3CCl3 standards versus the gravimetric value for those standards made in 1993 and those prepared in 1991 and 1997 for the complete range of ambient ppt mixing ratios.