GHOST - A novel airborne gas chromatograph for in situ measurements of long-lived tracers in the lower stratosphere: method and applications
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 mega-bore 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 N2O and CF2Cl2 (CFC-12) or CFC-12 and CFCl3 (CFC-d11) have been measured simultaneously, with a time resolution of 2 min for both modes. Under flight conditions the instrument precision for theses species is better than 0.9 %, and the accuracy is better than 2.0 % of the tropospheric values of all measured compounds. The detection limits are below 28 ppb for N2O, 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 N2O 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 N2O lifetime is re-assessed. When referenced to the WMO reference CFC-11 lifetime of 45 +/- 7 years and N2O lifetime of 91 +/- 15 years is derived, a value substantially smaller than the WMO reference lifetime of 120 years. Moreover, this value implies a stratospheric N2O sink strength of 16.3 +/- 2.7 Tg (N) yr-1 which is 30 % larger than previous estimates.