Laboratory and field assessment of mid-infrared absorption (MIRA) instrument performance for methane and ethane dry mole fractions

Abstract. Concurrent measurements of methane (CH₄) and ethane (C₂H₆) can be used to identify and separate methane sources, as ethane is present in thermogenic sources (e.g., oil and natural gas) but not in biogenic sources (e.g., agriculture). In this study, we evaluated the performance of multiple Aeris MIRA Ultra instruments (Versions 1 and 2) through controlled laboratory tests and tower-based deployments under field conditions. The systems were modified with an external pump, flow control, a Nafion dryer, and a custom-built auxiliary box to automate the system and transmit near real-time data. We determined the best calibration approach for our application, given practical limitations, to be a full calibration cycle (with ambient and high calibration cylinders) about once per day and an ambient calibration cylinder sampled hourly. Measurement uncertainty was assessed, including the uncertainty due to instrument noise as a function of calibration frequency, uncertainty in the water vapor correction, and cylinder assignment uncertainty. Instrument noise was the dominant source of uncertainty for C₂H₆, while the water vapor correction dominated the CH₄ uncertainty. For Version 2 systems with hourly calibrations and a Nafion dryer with counterflow, the mean total uncertainty, including both systematic errors and noise, of hourly averages was 0.8–3.0 ppb CH₄ and 0.35–0.37 ppb C₂H₆. Laboratory intercomparisons showed network compatibility within 1.2 ppb CH₄ and 0.23 ppb C₂H₆, and a collocated deployment with a NOAA Picarro system agreed within 1.8 ppb CH₄. Instrument noise varied substantially amongst the instruments, with errors reaching up to 11 ppb CH₄ and 2 ppb C₂H₆ for hourly means, with similar variability indicated in a 50-h cylinder test. With appropriate engineering and calibration, the Aeris MIRAUltra shows the capability to measure ethane and methane with sufficient stability to distinguish regional methane emission sources in many field settings.