аЯрЁБс>ўџ 68ўџџџ5џџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџџьЅС7 №ПVbjbjUU "87|7|Vџџџџџџl(2 (_іJJJJJJJJорррррр$U u „JJJJJЄJJЄЄЄJJJоЄJоЄ.ЄввJ> €Ќ„С(і`:вв /0_вљ š љ вЄ((йCALIBRATION HEADER FOR HYDROFLUOROCARBONS (HFCs) and HYDROCHLOROFLUOROCARBONS (HCFCs) Primary multi-component calibration standards are obtained commercially in proprietary-treated high-pressure (15 MPa) aluminum cylinders from Linde Gases (UK) Ltd and maintained at the University of Bristol (UB). These standards are prepared gravimetrically with a target accuracy of Б1%, and contain individual HFCs and HCFCs with mole fractions ranging between 18-1290 ppm, using N2O as the bulk diluent gas. In addition, each primary standard contains the two chlorofluorocarbons CCl2F2 (CFC-12) and CCl3F (CFC-11) at concentrations of 5040 and 2360 ppm, respectively. Known volumes of these primary standards are transferred into ~35-liter internally electropolished stainless steel cylinders, and diluted in a single-stage with halocarbon-free "zero air" at UB to obtain secondary standards of near atmospheric concentrations for the CFCs and hydrohalocarbons, using an ultraclean stainless steel dilution apparatus based on a system designed at Scripps Institution of Oceanography (SIO). The “zero air” was 99.9995% (Air Products (UK) Ltd) and further purified by passage through a filter of charcoal and 13x molecular sieve maintained in a dry ice-ethanol slurry. The purity of this “zero air” was verified by measuring “blanks” by gas chromatography-mass spectrometry (GC-MS). To obtain a more accurate calibration of the individual species concentrations in a secondary standard, it is subsequently intercalibrated at Mace Head, Ireland, with an in-situ AGAGE GC-ECD whole-air standard calibrated by SIO. This technique uses CCl2F2 in the secondary mixture as a "bootstrap" internal standard whose concentration can be assigned to the SIO98 scale through this intercomparison. The bootstrap method assumes no changes in ratios of gases to one another during the above single-stage secondary dilution. This allows the assignment of concentrations for individual hydrohalocarbons in the secondary standard to be linked to the original primary standard scale relative to CCl2F2 (as well as relative to CCl3F, CHCl3, and N2O if necessary) for which SIO maintains absolute calibration standards. Absolute calibration standards for relatively few of the hydrohalocarbons have so far been developed by us or by others. To develop a traceable calibration scale for these compounds several “Gold” standards have been prepared at SIO using ~ 35-liter electropolished cylinders filled with clean ambient air. Absolute calibrations for CCl3F, CCl2F2, CCl2FCClF2 (CFC-113), CH3CCl3, CCl4, CHCl3, N2O, CHClF2 (HCFC-22), CH3Cl and CH3Br have been assigned to these standards at SIO in the SIO98 calibration scale. Interim calibration values for all of the HFCs and HCFCs have been assigned based on the Linde/CCl2F2 bootstrap calibration method described above. The “Gold” standards are regularly inter-compared to test for possible long-term relative drifts in their assigned standard values. Secondary field standards for routine use at the Mace Head, and Cape Grim, Tasmania, stations are then propagated from these “Gold” standards, which are maintained at SIO, UB and CSIRO. The synthetic secondary standards were used in the field at Mace Head until May 2000. After that time secondary field standards were prepared by cryo-trapping ~1000 liters of clean background whole-air samples into the 35-liter cylinders. From an intercomparison between a synthetic and whole-air secondary standard relative to a “Gold” standard it was demonstrated that differences were <2% for the majority of the HFCs and HCFCs. Larger differences were observed for CHF2CF3, (-5.5%), CHCl2CF3, (6.6%), and CH3Br (-5.8%), which may indicate subtle differences between secondary standards prepared with synthetic zero air compared with the whole-air. It is also possible that small non-linearity errors could also be introduced, when comparing air with synthetic secondary standards containing halocarbon concentrations that differ significantly from air. At Cape Grim, the secondary field standards have all been prepared by cryo-trapping ~1400 liters of background whole-air samples into 35-liter stainless steel cylinders (4000 Kpa). After approximately 3 months, when the secondary field standard has dropped in pressure to 300 Kpa, it is changed for a new secondary field standard, which has been propagated, from the “Gold” standards. In addition, the incoming and outgoing secondary standards are compared to each other. Prior to the introduction of “Gold” standards at Cape Grim, several secondary field standards were set aside (interim “Gold” standards) and compared at Cape Grim on a regular basis to each other, to working secondary field standards and occasionally to the Linde primary standards at UB. These interim “Gold” standards have been compared to SIO long-term standards maintained at Cape Grim. As a further precaution a sub-set of the GC-MS secondary field standards are routinely sent to SIO for intercomparison with their absolute standards. The mean difference between the SIO CFC-12 and HCFC-22 calibration measurements and the Bristol assigned CFC-12 and HCFC-22 concentrations were 0.2 Б 0.42% (SIO higher) and 0.85 Б 0.61% (SIO higher) respectively, based on an intercomparison of subsets of six different field standards. The NOAA/CMDL laboratories as part of an on-going series of intercalibration exercises have also analyzed subsets of the secondary field standards for several HFCs and HCFCs. The mean difference between the two laboratories was 2.05 Б 0.048% for CH2FCF3, HFC-134a, (CMDL higher); 0.3 Б 2.36% for CH3CCl2F, HCFC-141b (CMDL lower); 0.73 Б 1.1% for CH3CClF2, HCFC-142b (CMDL lower); and 0.16 Б 0.81% for CHCl2F, HCFC-22 (CMDL higher). In practice, this provides an independent test of the principal assumption made during the single-stage dilution; namely that all components in the mixture will have been diluted in proportion to their original Linde primary concentrations. As new absolute calibration standards are developed, concentrations will be assigned to individual HFCs and HCFCs in the “Gold” standards and these values will then be propagated across the entire group of secondary field standards. 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