Presentation Title

Decreasing error associated with calculations of freshwater pCO2 using more accurate pH measurements

Authors' Names

Fischer Young

Presentation Type

Oral Presentation

Abstract/Artist Statement

As atmospheric CO2 continues to rise, understanding how CO2 cycles through the environment is of continuing importance. Although terrestrial and oceanic CO2 dynamics have been studied for decades, freshwater CO2 dynamics have not been studied as intensively. Increasing CO2 partial pressure (pCO2) in freshwaters indicates that increasing atmospheric CO2 is impacting freshwaters as well. However, calculating freshwater pCO2 accurately is not well established. Thus, it is necessary to establish an accurate method for calculating freshwater pCO2 to understand how increasing atmospheric CO2 is impacting freshwater systems.

Due to error in the pH measurement, large overestimations in pCO2 calculations have been demonstrated by previous studies. This study will examine methods to minimize the error from pH and total alkalinity and thus provide more accurate and precise calculated pCO2 values. A laboratory controlled mixing tank with a pCO2 infrared analyzer will act as the true value used for comparisons of this study. pH will be determined spectrophotometrically using purified meta-cresol purple as the indicator. This method is extremely popular for ocean water analysis and has been adapted for freshwater analysis for this study. In addition, glass pH electrode measurements were also taken in order to compare the error associated with each method when calculating pCO2. An edited MATLAB version of CO2sys, a carbonate system modelling tool, that includes a correction for freshwater ionic strength will be utilized for the indirect calculations. Preliminary findings indicate that using the ionic strength correction for freshwater and spectrophotometric pH instead of a glass pH electrode resulted in more accurate and precise pCO2 calculations. The spectrophotometric pH pCO2 values gave a reproducibility of ±10 μatm whereas the glass pH electrode pCO2 values gave a reproducibility of ±100 μatm for multiple samples. It is increasingly important to be able to obtain accurate and precise freshwater pCO2 values in order to designate a freshwater system as either a system that takes up CO2 (sink) or expels CO2 (source) from or to the atmosphere. Due to poor pH accuracy and the overestimation of calculated pCO2, it is likely that some freshwater systems have been incorrectly designated as sources of pCO2 to the atmosphere. Freshwater pCO2 is a small but important key to providing accurate global carbon budgets. It is the goal of this study to improve the accuracy and precision of calculating pCO2 of freshwater systems using pH data and to determine the importance freshwater systems have in global carbon budgets.

Mentor Name

Michael DeGrandpre

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Feb 22nd, 10:00 AM Feb 22nd, 10:15 AM

Decreasing error associated with calculations of freshwater pCO2 using more accurate pH measurements

UC 333

As atmospheric CO2 continues to rise, understanding how CO2 cycles through the environment is of continuing importance. Although terrestrial and oceanic CO2 dynamics have been studied for decades, freshwater CO2 dynamics have not been studied as intensively. Increasing CO2 partial pressure (pCO2) in freshwaters indicates that increasing atmospheric CO2 is impacting freshwaters as well. However, calculating freshwater pCO2 accurately is not well established. Thus, it is necessary to establish an accurate method for calculating freshwater pCO2 to understand how increasing atmospheric CO2 is impacting freshwater systems.

Due to error in the pH measurement, large overestimations in pCO2 calculations have been demonstrated by previous studies. This study will examine methods to minimize the error from pH and total alkalinity and thus provide more accurate and precise calculated pCO2 values. A laboratory controlled mixing tank with a pCO2 infrared analyzer will act as the true value used for comparisons of this study. pH will be determined spectrophotometrically using purified meta-cresol purple as the indicator. This method is extremely popular for ocean water analysis and has been adapted for freshwater analysis for this study. In addition, glass pH electrode measurements were also taken in order to compare the error associated with each method when calculating pCO2. An edited MATLAB version of CO2sys, a carbonate system modelling tool, that includes a correction for freshwater ionic strength will be utilized for the indirect calculations. Preliminary findings indicate that using the ionic strength correction for freshwater and spectrophotometric pH instead of a glass pH electrode resulted in more accurate and precise pCO2 calculations. The spectrophotometric pH pCO2 values gave a reproducibility of ±10 μatm whereas the glass pH electrode pCO2 values gave a reproducibility of ±100 μatm for multiple samples. It is increasingly important to be able to obtain accurate and precise freshwater pCO2 values in order to designate a freshwater system as either a system that takes up CO2 (sink) or expels CO2 (source) from or to the atmosphere. Due to poor pH accuracy and the overestimation of calculated pCO2, it is likely that some freshwater systems have been incorrectly designated as sources of pCO2 to the atmosphere. Freshwater pCO2 is a small but important key to providing accurate global carbon budgets. It is the goal of this study to improve the accuracy and precision of calculating pCO2 of freshwater systems using pH data and to determine the importance freshwater systems have in global carbon budgets.