Carbon Dioxide Concentration

Mole fraction of carbon dioxide (xCO₂) in sea water near the ocean surface in units of micromoles per mole (µmol/mol). For instance, an xCO₂ of 400 µmol/mol indicates that a mole of sea water contains 400 micromoles of CO₂, or 400 millionths of a mole, or 0.0004 mole.

info:

• http://www.ioos.noaa.gov/ocean_acidification/
• http://www.pmel.noaa.gov/co2/story/Ocean+Acidification