Technical note: Assessing gas equilibration systems for continuous pCO₂ measurements in inland waters

High-frequency continuous measurements of the partial pressure of CO₂ (pCO₂) are crucial for constraining the spatiotemporal dynamics of CO₂ emissions from inland water systems. However, direct measurements of pCO₂ are scarce, and no systematic comparisons have been conducted on the suitability of the widely used measurement systems for continuous underway or long-term deployment in various field conditions. We compared spray-and marble-type equilibrators and a membrane-enclosed CO₂ sensor to assess their suitability for continuous long-term or underway pCO₂ measurements in an urbanized river system in Korea. Both equilibrators had a shorter response time compared with the membrane-enclosed sensor, and could capture large spatial variations of pCO₂ during a transect study along a highly urbanized river reach. The membrane-enclosed sensor based on passive equilibration provided comparable underway measurements along the river sections where pCO₂ varied within the sensor detection range. When deployed in a eutrophic river site, the membrane-enclosed sensor was able to detect large diel variations in pCO₂. However, biofouling on the membrane could reduce the accuracy of the measurement during long deployments exceeding several days. The overall results suggest that the fast response of the equilibrator systems facilitates capturing large spatial variations in pCO₂ during short underway measurements. However, the attendant technical challenges of these systems, such as clogging and desiccant maintenance, have to be addressed carefully to enable their long-term deployment. The membraneenclosed sensor would be suitable as an alternative tool for long-term continuous measurements if membrane biofouling could be overcome by appropriate antifouling measures such as copper mesh coverings.