Design and optimization of wireless in‐situ sensor coupled with gas–water equilibrators for continuous pCO₂ measurement in aquatic environments

Abstract

Affordable, highly precise, and accurate CO2 sensor is a prerequisite for more extensive CO2 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 CO2 concentration (mole fraction), appropriate for inland freshwater and eutrophic coastal environments. We describe the construction of a CO2 sensing device (SIPCO2‐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 CO2 monitoring to be used with SIPCO2‐II. The performance of the SIPCO2‐II is evaluated in detail, providing information on the sensor sensitivity, detection limit and stability, and comparing these to a commercially available CO2 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 SIPCO2‐II against CO2 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 CO2 monitoring regimes without compromising the accuracy and precision of the measurements.

Lotic‐SIPCO2: Adaptation of an open‐source CO2 sensor system and examination of associated emission uncertainties across a range of stream sizes and land uses

“…The performance of the original SIPCO2 (Hunt et al 2017) has been favorably compared to other aquatic CO 2 sensors, suggesting the accuracy of measurement using the design presented in this study is similar (Lee et al 2022). Indeed, the lotic‐SIPCO2 measurements compared relatively well to grab samples across the streams in this study (NRMSE < 20%; Fig.…”

“…The design adaptations we describe are necessary for deployment of the lotic‐SIPCO2 in streams and rivers, and address some issues potentially present in other aquatic ecosystems (i.e., biofouling). Additional variations of the SIPCO2 method were described recently which integrate wireless communication capability and alternative gas–water equilibrators (Lee et al 2022). Combined, these adaptations offer unique and relatively affordable means of increasing CO 2 measurements across lotic ecosystems without conceding accuracy.…”

“…We adapted a relatively inexpensive (~ 500 USD) and user‐built headspace equilibrium‐based p CO 2 sensor for deployment in lotic ecosystems, the lotic‐SIPCO2. The original SIPCO2 design (Hunt et al 2017) and published adaptations (Lee et al 2022) are specified for marine and estuarine deployments, thus moving into lotic ecosystems necessitated adaptation for variable flow and stream stage. Using these sensors, the concentration of dissolved CO 2 was measured at 15‐min intervals and validated against grab samples in 10 streams and rivers ranging in watershed size from 0.4 to 476.7 km 2 over the course of 1–6 yr (2015–2020) during ice‐free months (approximately May to October).…”

“…The use of pre‐defined and generalized quantities, such as the manufacturer's specifications, provide a more impartial approach to assess general patterns across sites. This could be viewed as a conservative estimate of uncertainty, given the SIPCO2 design has been shown to be more accurate than manufacturer's specifications (Hunt et al 2017; Lee et al 2022). The resulting sensitivities provide a framework to potentially differentiate which parameters contribute to uncertainty and under which conditions, such as in smaller and larger streams or under high or low CO 2 saturation conditions.…”

“…We first describe the adaptation of the SIPCO2 to lotic environments, and then analyze the uncertainty in estimating CO 2 emissions using these sensors. Because numerous studies have explored the accuracy of similarly designed aquatic p CO 2 sensors in detail (Yoon et al 2016; Hunt et al 2017; Blackstock et al 2019; Lee et al 2022), we instead explore the effectiveness of this headspace sensor systems in estimating CO 2 emissions from several lotic systems spanning a range of streamflow and land use conditions, biofouling potential, and other environmental factors. While some high‐frequency dissolved CO 2 datasets have been published from lotic ecosystems, no study that we are aware of has analyzed the utility and uncertainty of using sensors to estimate CO 2 emissions from streams and rivers over longer time scales.…”