A compact pCO2 measurement system using a gas-permeable membrane for underway and moored observations in coastal waters.

Accurate measurement of the partial pressure of CO₂ (pCO2) is essential for evaluating CO₂ fluxes in aquatic coastal environments. However, compact and low-power instruments suitable for both underway and moored deployments remain limited. We developed a battery-powered pCO2 system incorporating a gas-permeable membrane equilibrator. The equilibrator employs a modified commercial membrane module with its outer housing removed, allowing direct contact between the membrane surface and ambient water while carrier gas flows through the interior. This configuration minimizes clogging risk and enhances gas exchange efficiency. In laboratory tests, the system achieved a response time of 7.1 ± 2.8 min, comparable to or faster than conventional devices. Underway observations in Tokyo Bay recorded pCO2 values ranging from 200 to 1200 µatm, showing good agreement with a commercial analyzer. In brackish areas, where pCO₂ varied by approximately 1000 µatm over 4 km, slower ship speeds are recommended to achieve sufficient spatial resolution. Moored observations in a seagrass meadow revealed pCO2 values ranging from 74 to 206 µatm, with nighttime values approximately 1.3 times higher than daytime values. The system supports both active (pumped) and passive (moored) modes and can operate continuously for approximately 60 h using six D-size batteries, enabling reliable capture of diel pCO2 variations. These features make it suitable for high-frequency estimation of air-water CO₂ exchange in dynamic aquatic environments. Its compact design and cost-effectiveness enhance the accessibility of autonomous pCO₂ monitoring, contributing to improved assessment of coastal carbon dynamics and a better understanding of local CO₂ fluxes for environmental management.