Towards Correction-Free Open-Path Eddy Covariance Methane Flux Measurements

Ecologically critical regions such as wetlands, coastal beaches, and high-latitude ecosystems play a indispensable role in the global methane (CH₄) budget. However, limited power availability in these environments constrains long-term CH₄ flux observations. As a result, methane eddy covariance (EC) measurements increasingly rely on low-power consumption, highly integrated open-path analyzers. Unlike closed-path systems that measure dry mixing ratios, open-path sensors measure gas density, making EC flux calculations susceptible to spectroscopic effects and density perturbations induced by fluctuations in air temperature and humidity. These effects necessitate complex post-processing corrections and substantially complicate uncertainty quantification.

Here we present a novel open-path CH₄/H₂O analyzer (HT8600P, HealthyPhoton Co., Ltd.) together with a minimally corrective flux calculation framework. Through an innovative instrument design, we establish a pseudo dry mixing ratio formulation that enables point-by-point conversion from density to mixing ratio without relying on spatially separated temperature or water vapor measurements. This allows EC fluxes to be calculated in a manner analogous to closed-path systems, while preserving the logistical advantages of open-path deployment.

Dedicated field experiments, including a zero-flux test, demonstrate that the proposed approach yields near-zero methane fluxes with a random error of 0.057 mg m⁻² h⁻¹. The magnitude of required corrections is an order of magnitude smaller than that of a co-located commercial open-path analyzer. We further identify a “phantom” random error inherent to conventional density-based EC methods, whereby temperature- and humidity-driven fluctuations are misinterpreted as turbulent variance, leading to substantial overestimation of random uncertainty. By removing these artifacts at the signal level, the pseudo dry mixing ratio method reduces apparent random errors by 60–70%, producing uncertainty estimates consistent with the empirically determined noise floor.

Together, the HT8600P analyzer and the optimized pseudo dry mixing ratio EC framework provide a correction-light, noise-resilient solution for expanding long-term CH₄ flux observations in remote regions critical to the global methane budget.