Design and optimization of wireless in‐situ sensor coupled with gas–water equilibrators for continuous pCO 2 measurement in aquatic environments

• Published in: Limnology and oceanography, methods methods, 2022-08, Vol.20 (8), p.500-513
• Main Authors: Lee, Daryl Jia Jun, Kek, Kah Teik, Wong, Wei Wen, Mohd Nadzir, Mohd Shahrul, Yan, Jinpei, Zhan, Liyang, Poh, Seng‐Chee
• Format: Article
• Language: English
• ISSN: 1541-5856; 1541-5856
• DOI: 10.1002/lom3.10500

Affordable, highly precise, and accurate CO 2 sensor is a prerequisite for more extensive CO 2 measurements in various environmental settings, and thus more accurate local and global carbon budgets. Here, we propose the use of an autonomous system for the detection of CO 2 concentration (mole fraction), appropriate for inland freshwater and eutrophic coastal environments. We describe the construction of a CO 2 sensing device (SIPCO 2 ‐II) using off‐the‐shelf commercial products that integrate wireless communication capability. We also report the proof of concept of two newly designed gas–water equilibrators (GWEs) for in‐situ CO 2 monitoring to be used with SIPCO 2 ‐II. The performance of the SIPCO 2 ‐II is evaluated in detail, providing information on the sensor sensitivity, detection limit and stability, and comparing these to a commercially available CO 2 analyzer (LGR 915‐0011; Los Gatos Research). Comparison results demonstrate good agreement between the two devices with the root mean square error of 5%. The accuracy and precision of SIPCO 2 ‐II against CO 2 reference gases are evaluated and values are within 99–104% recovery and 0.9–3 ppm, respectively. The GWEs which comprised either a semi‐passive floater (free‐drifting float equilibrator; FDFE) or a hybrid showerhead‐marble equilibrator (SME) are tested in the laboratory and in the environment. Overall, the SME outperforms FDFE in terms of response time (4 vs. 16 min). The FDFE can cover larger survey areas with lower cost, due to its semi‐passive feature that allows it to drift with water current. The outcome of our study provides an alternative to budget‐constrained CO 2 monitoring regimes without compromising the accuracy and precision of the measurements.