Feasibility study of using a "travelling" CO2 and CH4 instrument to validate continuous in situ measurement stations

[摘要] In the course of the ICOS (Integrated Carbon Observation System)Demonstration Experiment a feasibility study on the usefulness of atravelling comparison instrument (TCI) was conducted in order to evaluatecontinuous atmospheric CO₂ and CH₄ measurements at two Europeanstations. The aim of the TCI is to independently measure ambient air inparallel to the standard station instrumentation, thus providing acomprehensive comparison that includes the sample intake system, theinstrument itself as well as its calibration and data evaluation. Observeddifferences between the TCI and a gas chromatographic system, which acted asa reference for the TCI, were −0.02 ± 0.08 μmol mol⁻¹for CO₂ and −0.3 ± 2.3 nmol mol⁻¹ for CH₄. Overa period of two weeks each, the continuous CO₂ and CH₄ measurementsat two ICOS field stations, Cabauw (CBW), the Netherlands and Houdelaincourt(Observatoire Pérenne de l'Environnement, OPE), France, were compared toco-located TCI measurements. At Cabauw mean differences of0.21 ± 0.06 μmol mol⁻¹ for CO₂ and0.41 ± 0.50 nmol mol⁻¹ for CH₄ were found. For OPE themean differences were 0.13 ± 0.07 μmol mol⁻¹ forCO₂ and 0.44 ± 0.36 nmol mol⁻¹ for CH₄. Offsetsarising from differences in the working standard calibrations orleakages/contaminations in the drying systems are too small to explain theobserved differences. Hence the most likely causes of these observeddifferences are leakages or contaminations in the intake lines and/or theirflushing pumps. For the Cabauw instrument an additional error contributionoriginates from insufficient flushing of standard gases. Although the TCI isan extensive quality control approach it cannot replace other quality controlsystems. Thus, a comprehensive quality management strategy for atmosphericmonitoring networks is proposed as well.

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