Alert Barrow Cape Grim Mauna Loa

Niwot Ridge Samoa (U.S.) South Pole

Period of Record

1977-92 (varies with site)

Methods

Measurements of CFC-11 (CCl₃F), CFC-12 (CCl₂F₂), HCFC-22 (CHClF₂), H-1301 (CBrF₃), and H-1211 (CbrClF₂) were made from flask samples collected for the National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring and Diagnostics Laboratory (CMDL) at climate monitoring stations (Barrow, Alaska, U.S.A.; Mauna Loa, Hawaii, U.S.A.; Cape Matatula, American Samoa; and the South Pole) and cooperative sampling sites (Niwot Ridge, Colorado, U.S.A.; Alert, Northwest Territories, Canada; and Cape Grim Baseline Air Pollution Station, Tasmania, Australia).

CFC-11 and CFC-12

Starting in 1977, paired samples were collected each week in 300-mL or 850-mL electropolished, stainless steel flasks at the four NOAA/CMDL stations and Niwot Ridge. After 1980, flask pairs at the South Pole were collected once a week only during the austral summer, when the station was open. Collection of flask pairs began at Alert in 1988 and at Cape Grim in 1991. All flasks were shipped back to Boulder, Colorado, for analysis by electron capture-gas chromatography (EC-GC) (Thompson et al. 1985; Elkins et al. 1988). The same GC was used throughout the entire sampling; the only major modifications were automating the EC-GC, adding a precolumn, and replacing a gas sampling valve in 1987 to permit backflushing and more rapid sampling.

During the period of record, calibration of the flask measurements involved only three secondary gas standards, filled with air from Niwot Ridge. The first secondary standard was compared yearly against the calibration scale of R.A. Rasmussen (Rasmussen and Khalil 1986) until it was depleted in 1985. After this time, the other two secondary standards were compared periodically to a new set of gravimetric standards (Novelli et al. 1991) and agreed with older sets to better than 1% (Montzka et al. 1992). Analysis of long-term drift of the three secondary standards revealed that the first standard (used from 1977 to 1984) had a statistically significant drift rate and that the other two standards (used after 1984) did not drift significantly (Montzka et al. 1992).

During data selection, outliers were removed if they were greater than 3 sigma away from the fits to fourth order polynomial and simple harmonic series (Elkins et al. 1993).

HCFC-22

Measurements of atmospheric HCFC-22 were made from air samples collected in electropolished, stainless steel flasks every 1-2 months from November 1991 through December 1992 at three of the four NOAA/CMDL flask network sites: Barrow, Mauna Loa, and Samoa. The fourth station, located at the South Pole, was sampled only during the austral summer. Sample collection for HCFC-22 also took place at three cooperative sampling sites: Alert, Niwot Ridge, and Cape Grim.

Subsamples of 150-250-mL were cryotrapped onto uncoated, fused-silica tubing and analyzed for HCFC-22 by gas chromatography-mass spectrometry (GC-MS) as described in Montzka et al. (1993). All mixing ratios are traceable to gravimetrically prepared standards, made up in the laboratory at NOAA/CMDL. This procedure for preparing standards is similar to that used by the National Institute for Standards and Technology (Zielinski et al. 1986). The absolute calibration is believed to be accurate within 5%. The coefficient of variation (standard deviation : mean) for repeat measurement or analytical precision of these samples was ~1.5%. The average difference (range) between the two simultaneously collected flasks was 1.4 parts per trillion by mole (ppt). Because of occasional problems with contamination of the flasks and with analytical instrumentation, obvious outliers have been removed by visual inspection.

H-1301 and H-1211

Paired samples were collected every 1-2 months in electropolished, stainless steel flasks from mid-1989 through early 1992 at three of the four NOAA/CMDL flask network sites (Barrow, Mauna Loa, and Samoa) and at Niwot Ridge. The fourth station, located at the South Pole, was sampled only during the austral summers of two years; hence the record is not contiguous. In 1991, sample collection for halons was begun at two cooperative sampling sites, Alert and Cape Grim. Approximately 100-mL subsamples from the flasks were cryotrapped onto Al₂O₃-lined, fused-silica tubing and analyzed by temperature- programed, electron capture-gas chromatography (EC-GC) as described in Butler et al. (1989). In 1992, 250-mL subsamples were also cryotrapped onto uncoated, fused-silica tubing and analyzed for H-1211 by gas chromatography-mass spectrometry (GC-MS). Measurements between the two systems agreed within 0.1 ppt (1sigma). All mixing ratios are traceable to gravimetrically prepared standards made up in the laboratory at NOAA/CMDL. This procedure for preparing standards is similar to that used by the National Institute for Standards and Technology (Zielinski et al. 1986) and typically is accurate within 1% (1 sigma). The coefficient of variation (standard deviation : mean) for repeat measurements of the samples was ~5%. Because H-1301 and H-1211 are present at extremely low mixing ratios (1-3 ppt), problems occasionally result from contamination of the flasks and analytical instrumentation. Thus, data selection is sometimes required: outliers >3 sigma are removed from the fitted curve.

NOAA/CMDL
Halocarbon flask
monitoring sites