High-accuracy continuous airborne measurements of greenhouse gases (CO2 and CH4) using the cavity ring-down spectroscopy (CRDS) technique

摘要

High-accuracy continuous measurements of greenhouse gases (CO andCH) during the BARCA (Balanço Atmosférico Regional de Carbonona Amazônia) 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 CO and CH 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 CO 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, CO 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 CO 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 CO and CH.