Comparison of spectrophotometric and electrochemical <scp>pH</scp> measurements for calculating freshwater <scp><i>p</i>CO<sub>2</sub></scp>
Fischer L. Young, Qipei Shangguan, Cory M. Beatty, Makenzy D. Gilsdorf, Michael D. DeGrandpre
Abstract
Abstract Inland waters have an important role in the global carbon cycle, contributing significantly to terrestrial carbon fluxes through downstream export and exchange of CO 2 with the atmosphere. However, large uncertainties in freshwater inorganic carbon fluxes remain. One contributing factor is uncertainty in carbonate system calculations for estimating the partial pressure of CO 2 ( p CO 2 ) from pH and alkalinity in freshwater systems. The uncertainty stems largely from inaccurate pH values caused by glass pH electrode measurements in low ionic strength systems. This study compares indicator‐based spectrophotometric and electrochemical pH measurements and their application for calculating freshwater p CO 2 . Our study found that, compared to a p CO 2 reference method, pH electrode‐based estimates of p CO 2 were overestimated by 230 ± 200 μ atm ( n = 54) where indicator‐based spectrophotometric pH estimates of p CO 2 were 58 ± 33 μ atm ( n = 34) over the range of 100–1600 μ atm. Furthermore, we found that when ionic strength was assumed to be zero, calculated p CO 2 error was ~ 20% of the reference p CO 2 . A 19‐d field study using autonomous spectrophotometric pH and p CO 2 sensors found an average error in calculated p CO 2 of −70 ± 57 μ atm ( n = 1685). Although, our focus is on riverine CO 2 , these findings and subsequent conclusions apply to all freshwater systems. Spectrophotometric pH measurements will improve future freshwater p CO 2 calculations and better quantify inland waters' role in the global carbon budget.