Advanced Global Atmospheric Gases Experiment (AGAGE)

Continuous high-frequency gas chromatographic measurements of two biogenic/anthropogenic gases (CH₄ and N₂O) and five anthropogenic gases (CFCl₃, CF₂Cl₂, CH₃CCl₃, CF₂ClCFCl₂, and CCl₄) are being carried out at globally-distributed sites to quantitatively determine the source and sink strengths and circulation of these chemically and radiatively important long-lived gases. Hydrogen, carbon monoxide, and a wide range of hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and other halocarbons are also measured at certain stations. The station locations are Cape Grim, Tasmania (41°S, 145°E), Cape Matatula, American Samoa (SMO) (14°S, 171°E), Ragged Point, Barbados (13°N, 59°W), Trinidad Head, California (41°N, 124°W), and Mace Head, Ireland (53°N, 10°W). The program, which began in 1978, is divided into three parts associated with three changes in instrumentation: the Atmospheric Lifetime Experiment (ALE) which utilized Hewlett-Packard HP5840 gas chromatographs, the Global Atmospheric Gases Experiment (GAGE) which utilized HP5880 gas chromatographs, and the Advanced (AGAGE) phase now underway which uses a new fully-automated system from the Scripps Institution of Oceanography (SIO) containing a custom-designed sample module and HP5890 and Carle Instruments gas chromatographic components. Also, as part of AGAGE, a new Finnigan gas chromato-graph/mass spectrometer system for measuring HCFCs and HFCs is now operating at Mace Head and a second such instrument will be installed at Cape Grim in late 1996.

The data for the seven long-lived gases measured in GAGE and AGAGE during 1994-1995 continue to be of very good quality with significant improvements in the frequencies, precisions, and accuracies of the measurements resulting from the transition from GAGE to AGAGE. Some significant conclusions have been reached over the past year as a result of analyzing the data.

First, Prinn et al. [1995] showed that the atmospheric concentration of the volatile anthropogenic chemical CH₃CCl₃, which was steadily increasing at 4.5 0.1% per year until mid-1990, has subsequently decreased at a rate of 2.2 0.4% per year (all stated uncertainties are 1). This recent rapid decrease is consistent with its short lifetime and recent industrial emission reductions. The observed decreases began in early 1991 in the northern hemisphere and in mid-1992 in the southern hemisphere, reflecting the predominantly northern hemispheric emissions of this industrial chemical and the approximately 1-year interhemispheric exchange time. The measurements, combined with industrial emissions, were used in an inverse method to deduce a globally averaged CH₃CCl₃ lower atmospheric lifetime (200 to 1000 mbar) of 4.6 ± 0.3 years and a total atmospheric lifetime (0 to 1000 mbar) of 4.8 ± 0.3 years. Assuming a lifetime for loss of CH₃CCl₃ to the oceans of 85 years, we deduced a global weighted-average lower atmospheric OH concentration of (9.7 ± 0.6) 105 radicals cm^-3. The rate of change of this OH concentration is 0.0 ± 0.2% per year, implying that the oxidation capability of the lower atmosphere has not changed significantly from 1978 to 1994. Our conclusions concerning the OH concentrations and trend depend on the accuracy of the industrial emission estimates and our new absolute calibration. For methane (CH₄), these OH concentrations imply lower atmospheric and total atmospheric lifetimes of 8.0 ± 0.5 and 8.9 ± 0.6 years, respectively. The deduced lifetimes for CH₃CCl₃ and CH₄ are substantially less and the deduced OH concentrations substantially more than previous estimates. This substantially lowers the potential of these two gases (and most other hydrogen-containing gases) for affecting the ozone layer and climate compared to previous estimates.

Second, observations at the ALE/GAGE stations of CCl₂FCClF₂ have been reported by Fraser et al. [1996]. The observations from Cape Grim have been extended back to 1978 using archived air samples. The global atmospheric abundance of CCl₂FCClF₂ is indicated to have been growing exponentially between 1978 and 1987 with an e-folding time of approximately 7.6 years; it has been growing less rapidly since that time. On January 1, 1994, the mean inferred northern hemispheric mixing ratio in the lower troposphere was 84.4 ± 0.4 ppt (parts-per-trillion dry air mole fraction) and the southern hemispheric value was 80.6 ± 0.4 ppt; the global growth rate in 1991-1993 is estimated to have averaged approximately 3.1 ± 0.1 ppt per year. The differences between the northern and southern hemispheric concentrations are calculated to be consistent with almost the entire northern hemispheric release of this gas. The annual release estimates of CCl₂FCClF₂ by industry, which include estimates of eastern European emissions, consistently exceeds those deduced from the measurements by approximately 10% from 1980 to 1993. The uncertainties in each estimate is approximately 5%. This difference suggests that up to 10% of past production might not yet have been released. The measurements indicate atmospheric releases of CCl₂FCClF₂ have been decreasing rapidly since 1989 and in 1993 amounted to 78 ± 27 106 kg or 42 15% of the 1985-1987 emissions.

Third, using GAGE/AGAGE observations Cunnold et al. [1996] showed that global concentrations of CCl₃F reached a maximum in 1993 and decayed slightly in 1994, while CCl₂F₂ concentrations increased approximately 7 ppt in 1993 and 1994. These changes suggest that world emissions decreased faster in these 2 years than industry production figures would suggest and faster than expected under the Montreal Protocol and its amendments. An analysis of regional pollution events at the Mace Head site suggest that industry may be underestimating the decline of emissions in Europe. It is argued, however, that the decline in European emissions is not biasing the background Mace Head measurements (or the GAGE global averages). Combining the chlorofluorocarbon measurements, including CCl₂FCClF₂, with GAGE/AGAGE measured global decreases in CH₃CCl₃ and CCl₄ after 1992 and with Cape Grim archived air measurements of CHClF₂, the measurements suggest total anthropogenic atmospheric chlorine loading from these six gases maximized in 1992 at 2.95 ± 0.04 ppb (parts-per-billion dry air mole fraction) and that it had decreased by 0.02 ± 0.01 ppb by the beginning of 1995.

During 1996, operations at the American Samoa Observatory (SMO) will make the transition from the GAGE instrument to the new AGAGE system; it is to be installed in the new laboratory building. During 1994-1995 the GAGE HP5880 continued to be operated by the CMDL station personnel in collaboration with the Scripps Institution of Oceanography (SIO) group. Operations were quite smooth and uneventful during this period with most problems being of a routine maintenance and repair nature.

The new instrument systems for AGAGE represent a significant technological advance. All operations and data acquisition are by a Sun Microsystems workstation using custom software for signal processing integration and storage of the data and chromatograms. The instrument measures its own nonlinearity for all the AGAGE gases on a regular basis using a pressure-programmed constant-volume injection system and a single gas standard. All channels of the instrument are fitted with precolumns to avoid column contamination by late-eluting gases and, as a result the frequency of measurement, has been increased three-fold versus GAGE. Precision is also greatly improved over the GAGE instruments. The system works interactively with its uninterruptible power supply (UPS). With the new installation at SMO completed, the new AGAGE instruments will be operational at all five AGAGE stations.

Another major component of the AGAGE program is the development of new absolute calibration scales. This work is being done using an extension of the "bootstrap" calibration method used earlier at SIO. In the AGAGE work, gravimetrically-determined aliquots of pure CFCs are mixed with about 12 L of gravimetrically-determined pure N₂O. A small aliquot (~0.4 cm3) of this mixture is then introduced into a 35-L electropolished stainless steel canister to which about 10 torr of water vapor has been added to reduce wall reactions and adsorption. The canister is then filled with about 40 atmospheres of repurified "zero air" to bring the N₂O mole fraction to a near-ambient value. The resulting N₂O concentration is then calibrated gas chromatographically against existing SIO standards, and the CFC concentrations are determined by multiplying the measured N₂O mole fraction by the gravimetric CFC/ N₂O ratios of the original mixture. In this way we have been able to obtain improved accuracy for all AGAGE gases, but especially for the lower vapor pressure and more adsorptive gases. AGAGE and CMDL are now engaged in an active intercalibration program (through the IGAC/NOHALICE activity) for all of the AGAGE gases as well as for methyl halides and some HCFCs.

The ALE/GAGE/AGAGE data are archived at the Department of Engery Carbon Dioxide Information Analysis Center, Oak Ridge, Tennessee, and are available to interested scientists. Potential users of the data may contact CDIAC on the Internet (CDP@ORNL.GOV).

Acknowledgments. The AGAGE is supported by NASA Grants NAGW-732 and NAGW-2034, NOAA Contract NA85-RAC05103, CSIRO Australia, Australian Bureau of Meteorology, the U.K. Department of Environment, and the Alternative Fluorocarbons Environmental Acceptability Study. We thank Mark Winey and the other NOAA staff at SMO for their continued excellent local support for our instrumentation there.

Cunnold, D.M., R. Weiss, R.G. Prinn, D. Hartley, P.G. Simmonds, P.J. Fraser, B. Miller, F.N. Alyea, and L. Porter, GAGE/AGAGE measurements indicating reductions in global emissions of CCl₃F and CCl₂F₂ in 1992-1994, J. Geophys. Res., submitted, 1996.

Fraser, P., D. Cunnold, F. Alyea, R. Weiss, R. Prinn, P. Simmonds, B. Miller, and R. Langenfelds, Lifetime and emission estimates of 1,1,2-trichlorotrifluorethane (CFC-113) from daily global background observations June 1982-June 1994, J. Geophys. Res., 101, 12,585-12,599, 1996.

Prinn, R.G., R.F. Weiss, B.R. Miller, J. Huang, F.N. Alyea, D.M. Cunnold, P.J. Fraser, D.E. Hartley, and P.G. Simmonds, Atmospheric trends and lifetime of CH₃CCl₃ and global OH concentrations, Science, 269, 187-192, 1995.

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