Development of the Channelized Optical System II for In Situ, High-Frequency Measurements of Dissolved Inorganic Carbon in Seawater

This study describes the development of the CHANnelized Optical System II (CHANOS II), an autonomous, in situ sensor capable of measuring seawater dissolved inorganic carbon (DIC) at high frequency (up to ∼1 Hz). In this sensor, CO2 from acidified seawater is dynamically equilibrated with a pH-sensitive indicator dye encapsulated in gas-permeable Teflon AF 2400 tubing. The pH in the CO2 equilibrated indicator is measured spectrophotometrically and can be quantitatively correlated to the sample DIC. Ground-truthed field data demonstrate the sensor’s capabilities in both time-series measurements and surface mapping in two coastal sites across tidal cycles. CHANOS II achieved an accuracy and precision of ±5.9 and ±5.5 μmol kg–1. The mean difference between traditional bottle and sensor measurements was −3.7 ± 10.0 (1σ) μmol kg–1. The sensor can perform calibration in situ using Certified Reference Materials (CRMs) to ensure measurement quality. The coastal time-series measurements highlight high-frequency variability and episodic biogeochemical shifts that are difficult to capture by traditional methods. Surface DIC mapping shows multiple endmembers in an estuary and highlights fine-scale spatial variabilities of DIC. The development of CHANOS II demonstrates a significant technological advance in seawater CO2 system sensing, which enables high-resolution, subsurface time-series, and profiling deployments.