Comparison of two closed-path cavity based spectrometers for measuring air-water CO<sub>2</sub> and CH<sub>4</sub> fluxes by eddy covariance
Abstract. In recent years several commercialized closed-path cavity based spectroscopic instruments designed for eddy covariance flux measurements of carbon dioxide (CO2), methane (CH4), and water vapor (H2O) have become available. Here we compare the performance of two state-of-the-art models – the Picarro G2311-f and the Los Gatos Research (LGR) FGGA at a coastal site. Both instruments can compute dry mixing ratios of CO2 and CH4 based on the concurrently measured H2O. Additionally, we used a high throughput Nafion dryer to physically remove H2O from the Picarro air stream. Observed air-sea CO2 and CH4 fluxes from these two analyzers, averaging about 12 mmol m−2 d−1 and 0.12 mmol m−2 d−1 respectively, agree within the measurement uncertainties. For the purpose of quantifying dry CO2 and CH4 fluxes, the numerical H2O corrections appear to be effective and lead to results that are comparable to physical removal of H2O with a Nafion dryer. We estimate the high frequency attenuation of fluxes in our closed-path setup, which was relatively small (≤ 10 %) for CO2 and CH4 but very large for the much stickier 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 d−1 for CO2 and 0.02 mmol m−2 d−1 for CH4. For the LGR these detection limits were about 8 mmol m−2 d−1 and 0.05 mmol m−2 d−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.