Reconciliation of pH 25 and pH insitu acidification rates of the surface oceans: A simple conversion using only in situ temperature

• Published in: Limnology and Oceanography: Methods
• Main Authors: Lui, Hon‐Kit, Chen, Chen‐Tung Arthur
• Other Authors: Ministry of Science and Technology, Taiwan
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2017
• Subjects: Ocean acidification
• Online Access: http://dx.doi.org/10.1002/lom3.10170; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Flom3.10170; https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.1002/lom3.10170

Abstract Seawater pH is frequently measured at 25°C (pH 25 ), and can be converted thermodynamically to pH at the in situ temperature ( T ), (pH insitu ) using an additional carbonate chemistry parameter, which is the total alkalinity (TA), dissolved inorganic carbon (DIC), or the partial pressure of CO 2 (pCO 2 ) of seawater. Although rates of temporal change of pH insitu ( ) and pH 25 ( ) are both extensively used in studies of ocean acidification, the difference between and 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 pH insitu and pH 25 is a 1 ( T 25°C), where a 1 is a nearly constant of −0.0151 pH unit °C −1 . We illustrate that equals ( + a 1 ), where is the rate of temporal change of T . We further show that uneven distributions of sampling points significantly widen the difference between and , making the degree of ocean acidification unclear. Distributions of a 1 values are modeled for the surfaces of the global oceans at various pCO 2 levels, and they closely match the observations from the studied time series. Without the use of an additional carbonate chemistry parameter, the pH insitu and pH 25 , as well as and 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 CO 2 concentration.