Fugacity of CO2 (fCO2)

The fugacity of CO2 (fCO2) is the effective partial pressure of carbon dioxide corrected for the non-ideal behavior of the gas, the quantity that actually sets the chemical-potential difference driving air-sea CO2 exchange. It is the partial pressure pCO2 multiplied by a fugacity coefficient slightly below 1, about 0.997 near surface conditions, so fCO2 runs a few tenths of a percent under pCO2. Surface-ocean fCO2 near 400 microatmospheres is compared with the atmospheric value to sign the air-sea flux. The global SOCAT database compiles surface fCO2 to map ocean carbon uptake.