Deployment and evaluation of a low-cost sensor system for atmospheric CO2 monitoring on a sea-air interface buoy

Abstract

Direct in-situ observation of marine CO2 concentrations is crucial for estimating air-sea CO2 fluxes, yet such observations remain scarce. Drawn on experiences from urban CO2 monitoring and buoy-based measurements, this study deployed a sea-air interface buoy platform in the northern South China Sea, near Maoming, Guangdong Province, China. This platform was equipped with three low-cost SenseAir K30 sensors to enable continuous atmospheric CO2 measurement. This paper presents the first detailed account of the methodology, encompassing hardware design, environmental corrections, land-based validation tests, offshore deployment procedures, and initial observational results. These findings thus provide valuable insights for advancing marine CO2 observations practices. To mitigate the impacts of temperature, humidity, and pressure on sensor readings - while simultaneously compensating for zero-drift - an environmental correction method was implemented. This approach significantly improved data accuracy: after 1 min temporal averaging of raw data and a subsequent 1 h moving average, the root mean square error was reduced from 8.03 to 3.64 ppm in land tests and from 24.26 to 1.59 ppm in marine observations. Importantly, this level of precision meets the requirements for resolving sea surface CO2 dynamics (∼ 420-480 ppm). Observed concentrations were consistent with HYSPLIT-simulated long-range atmospheric transport, revealing the stable and homogeneous nature of the marine atmospheric boundary layer, with diurnal variations of approximately 3 ppm, and capturing localized or short-term fluctuations due to terrestrial carbon sources. These results demonstrate the effectiveness of the method, offering a low-cost, high-density solution for marine atmospheric CO2 monitoring and providing key inputs for inversely estimating ocean carbon sink.