AbstractEfforts to estimate air‐water carbon dioxide (CO2) exchange on regional or global scales are constrained by a lack of direct, continuous surface water CO2 observations. Sensor technology for the in situ measurement of the partial pressure of carbon dioxide (pCO2) has progressed, but still poses limitations including expense and biofouling concerns. We describe a simple, inexpensive, in situ pCO2 method (SIPCO2) in which a non‐dispersive infrared (NDIR) detector is paired with an air pump in an enclosed housing to produce air‐sea equilibration. We first evaluated this approach in a laboratory setting, then in an estuarine‐coastal ocean laboratory for several months to continuously monitor aquatic pCO2. An accepted, accurate NDIR‐based CO2 measurement technique was employed alongside SIPCO2 to provide an assessment of sensor performance. SIPCO2 allows for low‐cost, relatively accurate measurements of pCO2 (mean difference of −5 ± 5 μatm from validation system after laboratory calibration) without reagents or membranes, and can be assembled and operated with a minimal amount of technical skill. While not suitable for some exacting applications, this SIPCO2 approach could rapidly and effectively increase the number of quality CO2 observations in a range of aquatic environments. We also provide detailed instructions for the assembly of SIPCO2 from commercially available components.
