An evaluation of calibration techniques for in situ carbon dioxide measurements using a programmable portable trace-gas measuring system

The construction and deployment of a portable trace-gas measurement system (TGaMS) is described. The air-collection system (dubbed HYDRA) collects air samples from 18 different locations and was connected to either one or two LI-COR LI-7000 gas analyzers to measure CO₂. An in situ "field calibration" method, that uses four calibration gases with an uncertainty on the order of ±0.1 μmol mol^-1 relative to the WMO CO₂ mole fraction scale, revealed CO₂ output from the LI-7000 had a slightly nonlinear relationship relative to the CO₂ concentration of the calibration gases. The sensitivity of the field-calibrated CO₂ to different forms of the field-calibration equation is investigated. To evaluate TGaMS performance, CO₂ from collocated inlets, portable gas cylinders, and nearby independent CO₂ instruments are compared. Results are as follows: 1) CO₂ measurements from HYDRA multiple inlets are feasible with a reproducibility of ±0.4 μmol mol^-1 (based on the standard deviation of the CO₂ difference between collocated inlets when HYDRA was operating with two LI-7000s); 2) CO₂ differences among the various field-calibration equations were on the order of ±0.3 μmol mol^-1; and 3) comparison of midday hourly CO₂ measurements at 30 m AGL between TGaMS and an independent high-accuracy CO₂ measurement system (within 300 m of TGaMS) had a median difference and standard deviation of 0.04 ± 0.81 μmol mol^-1 over two months.