Development of a cavity-enhanced absorption spectrometer for airborne measurements of CH4 and CO2

[摘要] High-resolution CH₄ and CO₂ measurements were made on board theFAAM BAe-146 UK (Facility for Airborne Atmospheric Measurements, British Aerospace-146) atmospheric research aircraft during a number of fieldcampaigns. The system was based on an infrared spectrometer using the cavity-enhancedabsorption spectroscopy technique. Correction functions to convertthe mole fractions retrieved from the spectroscopy to dry-air mole fractionswere derived using laboratory experiments and over a 3 month period showedgood stability. Long-term performance of the system was monitored using WMO (World Meteorological Office) traceable calibration gases. During the first year of operation (29 flights)analysis of the system's in-flight calibrations suggest that itsmeasurements are accurate to 1.28 ppb (1σ repeatability at 1 Hz = 2.48 ppb)for CH₄ and 0.17 ppm (1σ repeatability at 1 Hz = 0.66 ppm)for CO₂. The system was found to be robust, no major motionor altitude dependency could be detected in the measurements. Aninter-comparison between whole-air samples that were analysed post-flightfor CH₄ and CO₂ by cavity ring-down spectroscopy showed a meandifference between the two techniques of −2.4 ppb (1σ = 2.3 ppb)for CH₄ and −0.22 ppm (1σ = 0.45 ppm) for CO₂. InSeptember 2012, the system was used to sample biomass-burning plumes inBrazil as part of the SAMBBA project (South AMerican Biomass BurningAnalysis). From these and simultaneous CO measurements, emission factors forsavannah fires were calculated. These were found to be 2.2 ± 0.2 g(kg dry matter)⁻¹ for CH₄ and 1710 ± 171 g (kg drymatter)⁻¹ for CO₂, which are in excellent agreement withprevious estimates in the literature.

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