Measuring CO2 and CH4 with a portable gas analyzer: Closed-loop operation, optimization and assessment.

The use of cavity ring-down spectrometer (CRDS) based portable greenhouse gas analyzers (PGAs) in closed-loop configuration to measure small sample volumes (< 1 l) for CH4 and CO2 concentrations is increasing and offers certain advantages over conventional measurement methods in terms of speed as well as the ability to measure directly in field locations. This first systematic assessment of the uncertainties, problems and issues associated with achieving reliable and repeatable measurement with this technique presents the adaptation, measurement range, calibration and maintenance, accuracy and issues of efficient operation, for one example instrument. Regular open-loop calibration, a precise loop volume estimate, leak free system, and a high standard of injection practices are necessary for accurate results. For 100 μl injections, measured values ranging from 4.5 to 9 x104 ppm (CH4), and 1000 ppm to 1 x106 ppm (CO2) are possible with uncertainties ±5.9% and ±3.0%, respectively, beyond 100 ppm CH4 correction may be necessary. Uncertainty arising from variations water vapour content and atmospheric pressure are small (0.24% and -0.9% to +0.5%, respectively). With good practice, individual operator repeatability of 1.9% (CH4) and 2.48% (CO2) can be achieved. Between operator injection error was around 3% for both gases for four operators. Slow syringe plunger operation (> 1s) is recommended; generally delivered more (ca. 3-4%) sample into the closed instrument loop than did rapid operation. Automated value retrieval is recommended; we achieved a 3 to 5-fold time reduction for each injection cycle (ca. <2 min), and operator reading, recording, and digitization errors are eliminated.