High-accuracy continuous airborne measurements of greenhouse gases (COsub2/sub and CHsub4/sub) using the cavity ring-down spectroscopy (CRDS) technique

摘要

High-accuracy continuous measurements of greenhouse gases (CO₂ andCH₄) during the BARCA (Balan?o Atmosférico Regional de Carbonona Amaz?nia) phase B campaign in Brazil in May 2009 were accomplishedusing a newly available analyzer based on the cavity ring-down spectroscopy(CRDS) technique. This analyzer was flown without a drying system or anyin-flight calibration gases. Water vapor corrections associated withdilution and pressure-broadening effects for CO₂ and CH₄ werederived from laboratory experiments employing measurements of water vapor bythe CRDS analyzer. Before the campaign, the stability of the analyzer wasassessed by laboratory tests under simulated flight conditions. During thecampaign, a comparison of CO₂ measurements between the CRDS analyzerand a nondispersive infrared (NDIR) analyzer on board the same aircraftshowed a mean difference of 0.22±0.09 ppm for all flights over theAmazon rain forest. At the end of the campaign, CO₂ concentrations ofthe synthetic calibration gases used by the NDIR analyzer were determined bythe CRDS analyzer. After correcting for the isotope and thepressure-broadening effects that resulted from changes of the composition ofsynthetic vs. ambient air, and applying those concentrations as calibratedvalues of the calibration gases to reprocess the CO₂ measurements madeby the NDIR, the mean difference between the CRDS and the NDIR during BARCAwas reduced to 0.05±0.09 ppm, with the mean standard deviation of0.23±0.05 ppm. The results clearly show that the CRDS is sufficientlystable to be used in flight without drying the air or calibrating in flightand the water corrections are fully adequate for high-accuracy continuousairborne measurements of CO₂ and CH₄.