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

Area of Science:

• Environmental Science
• Analytical Chemistry
• Spectroscopy

Background:

• Portable greenhouse gas analyzers (PGAs) utilizing cavity ring-down spectroscopy (CRDS) are increasingly used for measuring methane (CH4) and carbon dioxide (CO2).
• These instruments offer advantages in speed and field operability for small sample volumes (< 1 L) compared to conventional methods.

Purpose of the Study:

• To systematically assess uncertainties, problems, and issues in achieving reliable and repeatable measurements with CRDS-based PGAs in a closed-loop configuration.
• To evaluate the adaptation, measurement range, calibration, maintenance, accuracy, and operational efficiency of an example instrument.

Main Methods:

• The study involved adapting a CRDS-based PGA for closed-loop measurements of CH4 and CO2.
• Systematic assessment of factors including calibration, loop volume estimation, leak detection, and injection practices.
• Evaluation of measurement ranges, uncertainties, and repeatability under various operational conditions.

Main Results:

• Accurate results depend on regular open-loop calibration, precise loop volume estimation, leak-free systems, and meticulous injection practices.
• For 100 μl injections, uncertainties were ±5.9% for CH4 and ±3.0% for CO2.
• Individual operator repeatability was 1.9% (CH4) and 2.48% (CO2), with inter-operator injection error around 3% for both gases. Slow syringe plunger operation (>1s) improved sample delivery.

Conclusions:

• Reliable and repeatable measurements with CRDS-based PGAs require stringent adherence to calibration and operational protocols.
• Slow injection and automated data retrieval are recommended to enhance accuracy, efficiency, and reduce operator error.
• The technique is viable for field measurements of CH4 and CO2 concentrations with proper implementation.