Comparison of two closed-path cavity-based spectrometers for measuring air–water CO₂ and CH₄ fluxes by eddy covariance

Abstract. In recent years several commercialised closed-path cavity-based spectroscopic instruments designed for eddy covariance flux measurements of carbon dioxide (CO₂), methane (CH₄), and water vapour (H₂O) have become available. Here we compare the performance of two leading models – the Picarro G2311-f and the Los Gatos Research (LGR) Fast Greenhouse Gas Analyzer (FGGA) at a coastal site. Both instruments can compute dry mixing ratios of CO₂ and CH₄ based on concurrently measured H₂O, temperature, and pressure. Additionally, we used a high throughput Nafion dryer to physically remove H₂O from the Picarro airstream. Observed air–sea CO₂ and CH₄ fluxes from these two analysers, averaging about 12 and 0.12 mmol m⁻² day⁻¹ respectively, agree within the measurement uncertainties. For the purpose of quantifying dry CO₂ and CH₄ fluxes downstream of a long inlet, the numerical H₂O corrections appear to be reasonably effective and lead to results that are comparable to physical removal of H₂O with a Nafion dryer in the mean. We estimate the high-frequency attenuation of fluxes in our closed-path set-up, which was relatively small ( ≤  10 %) for CO₂ and CH₄ but very large for the more polar H₂O. The Picarro showed significantly lower noise and flux detection limits than the LGR. The hourly flux detection limit for the Picarro was about 2 mmol m⁻² day⁻¹ for CO₂ and 0.02 mmol m⁻² day⁻¹ for CH₄. For the LGR these detection limits were about 8 and 0.05 mmol m⁻² day⁻¹. Using global maps of monthly mean air–sea CO₂ flux as reference, we estimate that the Picarro and LGR can resolve hourly CO₂ fluxes from roughly 40 and 4 % of the world's oceans respectively. Averaging over longer timescales would be required in regions with smaller fluxes. Hourly flux detection limits of CH₄ from both instruments are generally higher than the expected emissions from the open ocean, though the signal to noise of this measurement may improve closer to the coast.