Global ocean spectrophotometric pH assessment: consistent inconsistencies

• Published in: Environmental Science & Technology
• Main Authors: Álvarez, Marta, Fajar, Noelia, Carter, Brendan R., Fernández-Guallart, E., Pérez, Fiz F., Woosley, Ryan J., Murata, Akihiko
• Other Authors: Ministerio de Economía y Competitividad (España)
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Chemical Society 2020
• Subjects: Cierva; Ocean acidification
• Online Access: http://hdl.handle.net/10261/221793; https://doi.org/10.1021/acs.est.9b06932; https://doi.org/10.13039/501100003329

12 pages, 5 figures Ocean acidification (OA)—or the decrease in seawater pH resulting from ocean uptake of CO2 released by human activities—stresses ocean ecosystems and is recognized as a Climate and Sustainable Development Goal Indicator that needs to be evaluated and monitored. Monitoring OA-related pH changes requires a high level of precision and accuracy. The two most common ways to quantify seawater pH are to measure it spectrophotometrically or to calculate it from total alkalinity (TA) and dissolved inorganic carbon (DIC). However, despite decades of research, small but important inconsistencies remain between measured and calculated pH. To date, this issue has been circumvented by examining changes only in consistently measured properties. Currently, the oceanographic community is defining new observational strategies for OA and other key aspects of the ocean carbon cycle based on novel sensors and technologies that rely on validation against data records and/or synthesis products. Comparison of measured spectrophotometric pH to calculated pH from TA and DIC measured during the 2000s and 2010s eras reveals that (1) there is an evolution toward a better agreement between measured and calculated pH over time from 0.02 pH units in the 2000s to 0.01 pH units in the 2010s at pH > 7.6; (2) a disagreement greater than 0.01 pH units persists in waters with pH < 7.6, and (3) inconsistencies likely stem from variations in the spectrophotometric pH standard operating procedure (SOP). A reassessment of pH measurement and calculation SOPs and metrology is urgently needed M.A. was supported by IEO RADIALES and RADPROF program. N.M.F was supported by a Juan de la Cierva postdoctoral contract (FJCI2015-24394), E.F.G. by a Personal Técnico de Apoyo contract (PTA2016-12441-I) both from the Spanish Ministry of Science, Innovation and Universities. F.F.P. was supported by the project ARIOS (CTM2016-76146-C3-1-R) from the Spanish Ministry of Science, Innovation and Universities, co-funded by the FEDER 2014-2020. ...