Global Ocean Spectrophotometric pH Assessment: Consistent Inconsistencies

Area of Science:

• Marine chemistry and oceanography
• Climate change science
• Environmental monitoring

Background:

• Ocean acidification (OA), driven by anthropogenic CO2 absorption, poses a significant threat to marine ecosystems.
• Accurate monitoring of seawater pH is essential for understanding and addressing OA impacts.
• Current methods for quantifying pH, spectrophotometric measurement and calculation from total alkalinity (TA) and dissolved inorganic carbon (DIC), exhibit persistent inconsistencies.

Purpose of the Study:

• To evaluate the agreement between spectrophotometric and calculated seawater pH measurements over time.
• To identify the primary sources of discrepancies in pH measurements.
• To highlight the need for improved metrology in ocean acidification research.

Main Methods:

• Comparison of spectrophotometric pH measurements with pH calculated from TA and DIC data.
• Analysis of pH data from the 2000s and 2010s to identify trends in measurement agreement.
• Assessment of potential variations in spectrophotometric pH standard operating procedures (SOPs).

Main Results:

• Agreement between measured and calculated pH improved from 0.02 pH units in the 2000s to 0.01 pH units in the 2010s for pH > 7.6.
• Discrepancies exceeding 0.01 pH units persist in waters with pH < 7.6.
• Variations in spectrophotometric pH SOPs are identified as a likely cause of persistent inconsistencies.

Conclusions:

• Despite improvements, significant inconsistencies remain in seawater pH measurements, particularly in more acidic waters.
• Standard operating procedures for spectrophotometric pH determination require urgent reassessment and standardization.
• Advancements in metrology are critical for reliable ocean acidification monitoring and carbon cycle research.