, Volume 39, Issue 1, pp 37-64

GHOST – A Novel Airborne Gas Chromatograph for In Situ Measurements of Long-Lived Tracers in the Lower Stratosphere: Method and Applications

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Abstract

A novel fully-automated airborne gas chromatograph for in situmeasurements of long-lived stratospheric tracers hasbeen developed, combining the high selectivity of a megabore PLOTcapillary column with recently developed sampling and separationtechniques. The Gas cHromatograph for theObservation of Stratospheric Tracers (GHOST)has been successfully operated during three STREAM campaigns(Stratosphere TRoposphere Experiment byAirborne Measurement) onboard a Cessna Citation IIaircraft in two different modes: Either N2O andCF2Cl2(CFC-12) or CFC-12 and CFCl3 (CFC-11) have been measuredsimultaneously, with a time resolution of 2 min for both modes.Under flight conditions the instrument precision (1σ) forthese species is better than 0.9%, and the accuracy(1σ) is better than 2.0% of the tropospheric values ofall measured compounds. The detection limits (3σ) arebelow 28 ppb for N2O, 14 ppt for CFC-12, and 8 ppt forCFC-11, respectively, i.e., well below 10 % of the troposphericvalues of all measured compounds. Post-mission optimization of thechromatographic separation showed a possible enhancement of thetime resolution by up to a factor of 2, associated with acomparable increase in precision and detection limit. As test ofactual performance of GHOST results from an in-flight N2Ointercomparison with a tunable diode laser absorptionspectrometer (TDLAS) are presented. They yield an excellentagreement between both instruments. Furthermore, on the basis ofthe hitherto most extensive set of upper tropospheric and lowerstratospheric data, the relative stratospheric N22O lifetime isre-assessed. When referenced to the WMO reference CFC-11 lifetimeof 45 ± 7 years an N2O lifetime of 91 ± 15 yearsis derived, a value substantially smaller than the WMO referencelifetime of 120 years. Moreover, this value implies astratospheric N2O sink strength of 16.3 ± 2.7 Tg (N)yr−1 which is 30% larger than previous estimates.