Direct analysis of dissolved CO₂ in coastal waters: development and validation of a simple method

The partial pressure of CO₂ (p(CO₂)) dissolved in seawater is often indirectly determined through a mathematical calculation involving two variables of the carbonate system, which requires the use of expensive equipment, materials, and reagents, and is usually demanding in terms of human resources. In this study, we present a simple, low-cost method for directly measuring dissolved CO₂ (p(CO₂)) in coastal waters using a small gas equilibrator coupled with a non-dispersive infrared (NDIR) detector in a closed-loop system. The cost of this methodology can be further reduced by utilizing a low-cost NDIR detector and inexpensive materials. Its analytical performance was validated against the conventional indirect approach based on pH and dissolved inorganic carbon (DIC), showing high accuracy (91 %), excellent linearity (r² = 0.998), and a quantification limit (∼21 μatm) approximately 20 times lower than the average surface seawater p(CO₂). The method proved robust to typical variations in salinity (15-45 PSU) and temperature (20-25 °C), though a new calibration is recommended for salinities below 15 PSU. Application to samples from a tropical coastal lagoon and adjacent oceanic area showed statistically comparable results with both methodologies. This approach could enable direct measurement in the field, thereby reducing labor, equipment, and material costs. This low-cost approach enhances in situ p(CO₂) monitoring, offering significant potential for long-term ocean acidification studies in resource-limited settings.