GHOSTAA novel airborne gas chromatograph for in situ measurements of long-lived tracers in the lower stratosphere: method and applications

• Published in: Journal of atmospheric chemistry Vol. 39; no. 1; pp. 37 - 64
• Main Authors: Bujok, Oliver, Tan, Viceith, Klein, Erich, Nopper, Ralf, Bauer, Reimar, Engel, Andreas, Gerhards, Marie-Theres, Afchine, Armin, McKenna, Daniel S, Schmidt, Ulrich, Wienhold, Frank G, Fischer, Horst
• Format: Journal Article
• Language: English
• Published: 01.01.2001
• ISSN: 0167-7764

A novel fully-automated airborne gas chromatograph for in situ measurements of long-lived stratospheric tracers has been developed, combining the high selectivity of a megabore PLOT capillary column with recently developed sampling and separation techniques. The Gas cHromatograph for the Observation of Stratospheric Tracers (GHOST) has been successfully operated during three STREAM campaigns (Stratosphere TRoposphere Experiment by Airborne Measurement) onboard a Cessna Citation II aircraft in two different modes: Either N sub(2) O and CF sub(2) Cl sub(2) (CFC-12) or CFC-12 and CFCl sub(3) (CFC-11) have been measured simultaneously, with a time resolution of 2 min for both modes. Under flight conditions the instrument precision (1 sigma ) for these species is better than 0.9%, and the accuracy (1 sigma ) is better than 2.0% of the tropospheric values of all measured compounds. The detection limits (3 sigma ) are below 28 ppb for N sub(2) O, 14 ppt for CFC-12, and 8 ppt for CFC-11, respectively, i.e., well below 10% of the tropospheric values of all measured compounds. Post-mission optimization of the chromatographic separation showed a possible enhancement of the time resolution by up to a factor of 2, associated with a comparable increase in precision and detection limit. As test of actual performance of GHOST results from an in-flight N sub(2) O intercomparison with a tunable diode laser absorption spectrometer (TDLAS) are presented. They yield an excellent agreement between both instruments. Furthermore, on the basis of the hitherto most extensive set of upper tropospheric and lower stratospheric data, the relative stratospheric N sub(2) O lifetime is re-assessed. When referenced to the WMO reference CFC-11 lifetime of 45 plus or minus 7 years an N sub(2) O lifetime of 91 plus or minus 15 years is derived, a value substantially smaller than the WMO reference lifetime of 120 years. Moreover, this value implies a stratospheric N sub(2) O sink strength of 16.3 plus or minus 2.7 Tg (N) yr super(-) super(1) which is 30% larger than previous estimates.