Seawater pH is frequently measured at 25°C (pH25), and can be converted thermodynamically to pH at the in situ temperature (T), (pHinsitu) using an additional carbonate chemistry parameter, which is the total alkalinity (TA), dissolved inorganic carbon (DIC), or the partial pressure of CO2 (pCO2) of seawater. Although rates of temporal change of pHinsitu ( math formula) and pH25 ( math formula) are both extensively used in studies of ocean acidification, the difference between math formula and math formula has not yet been quantified. This study deducts from 816 sets of data of the surface oceans over wide ranges of T (1–31°C) from six time series to reveal that the difference between calculated pHinsitu and pH25 is a1 (T math formula 25°C), where a1 is a nearly constant of −0.0151 pH unit °C−1. We illustrate that math formula equals ( math formula + a1 math formula), where math formula is the rate of temporal change of T. We further show that uneven distributions of sampling points significantly widen the difference between math formula and math formula, making the degree of ocean acidification unclear. Distributions of a1 values are modeled for the surfaces of the global oceans at various pCO2 levels, and they closely match the observations from the studied time series. Without the use of an additional carbonate chemistry parameter, the pHinsitu and pH25, as well as math formula and math formula can now be converted into each other using only T, facilitating the study of the changing carbonate chemistry of seawater under the influences of increasing atmospheric CO2 concentration.
Lui H.-K. & Chen C.-T. A., in press. Reconciliation of pH25 and pHinsitu acidification rates of the surface oceans: A simple conversion using only in situ temperature. Limnology and Oceanography: Methods. Article.