Applications of in situ pH measurements for inorganic carbon calculations

This study examines the utility of combining pH measurements with other inorganic carbon parameters for autonomous mooring-based carbon cycle research. Determination of the full suite of inorganic carbon species in the oceans has previously been restricted to ship-based studies. Now with the availability of autonomous sensors for pH and the partial pressure of CO 2 ( p CO 2 ), it is possible to characterize the inorganic carbon system on moorings and other unmanned platforms. The indicator-based pH instrument, SAMI-pH, was deployed with an autonomous equilibrator-infrared p CO 2 system in Monterey Bay, California USA from June to August 2007. The two-month time-series show a high degree of short-term variability, with pH and p CO 2 changing by as much as 0.32 pH units and 240 μatm, respectively, during upwelling periods. The pH and salinity-derived alkalinity (A Tsalin ) were used to calculate the other inorganic carbon parameters, including p CO 2 , total dissolved inorganic carbon (DIC) and CaCO 3 saturation states. The calculated p CO 2 was within 2 μatm of the measured p CO 2 during the first day of the deployment and within 8 μatm over the first month. The DIC calculated from pH–A Tsalin and p CO 2 –A Tsalin were within 5 μmol kg −1 of each other during the first month. However, DIC calculated from pH– p CO 2 differed by ~ 50 μmol kg −1 from the other estimates over the same period, reflecting the sensitivity of the pH– p CO 2 calculation to measurement error. The data continued to diverge during the final month and this difference was likely driven by extensive biofouling. Because of the relative insensitivity of CO 3 2− concentration to these errors, aragonite saturation calculated from the pH– p CO 2 pair was within 0.15 of the pH–A Tsalin values over the entire deployment. These results show that in situ pH, when combined with other CO 2 parameters, can provide valuable insights into both data quality and inorganic carbon cycling.