Revelle factor

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The Revelle factor (buffer factor) is the ratio of instantaneous change in carbon dioxide (

carbon budget
analysis and anthropogenic climate change studies.


Revelle factor = Δ[CO2]/ [CO2] / Δ[DIC]/[DIC] where Δ[CO2] / [CO2] is the instantaneous change in pCO2 and Δ[DIC] / [DIC] is the instantaneous change in dissolved inorganic carbon at the oceans surface.

Thermodynamics

In order to enter the ocean, carbon dioxide gas has to partition into one of the components of carbonic acid: carbonate ion, bicarbonate ion, or protonated carbonic acid, and the product of these many chemical dissociation constants factors into a "back-pressure" that limits how fast the carbon dioxide can enter the surface ocean.

DIC

The species of DIC present in ocean waters are dependent on the pH of the system, and are illustrated by the Bjerrum plot below (Figure 1). Carbonate is dominant in higher pH (basic) environments, whereas carbon dioxide is dominant in lower pH (acidic) environments. Bicarbonate ions are abundant in relatively mid-pH waters. As the pH decreases, most of the DIC will be present as CO2 and hence increases its partial pressure (pCO2), and the buffer factor will increase.[2] An increased buffer factor results in a decreased buffering effect, which could lead to the uptake of more CO2 from the atmosphere, and decreasing the pH even more.[2]

Figure 1: Curves illustrating the molar fraction of carbonate species present in seawater across pH, with salinity set at 5,000ppm, and temperature set to 25 degrees Celsius. Note temperature and salinity affect carbonate species present, and vary with location and season.

DIC and alkalinity govern carbonate and acid-base chemistry in the world's oceans, and their effects on the Revelle factor is no exception. The ratio of DIC to total alkalinity, and the changes in pCO2 are the main cause of Revelle Factor variability. Higher levels of DIC result in a lower Revelle factor, and consequently a larger buffering effect.[3] Higher levels of pCO2 results in a higher Revelle factor, a positive feedback loop, and consequently a smaller buffering effect. Typically, the buffer factor ranges between 8 and 13.

Anthropogenic CO2

The capacity of the ocean waters to take up surplus (

North Pacific
has higher Revelle factors, and has lower anthropogenic CO2. This is due to the fact that the alkalinity values in the North Pacific are as much as 100μmol/kg lower than those in the North Atlantic.

The Revelle effect

The Revelle effect describes how only a small fraction of pCO2 is present in ocean water when much larger amounts are added to the atmosphere. Depending on the alkalinity of the water, DIC is either present as CO3, HCO3, or CO2. When the pH is high (basic) the Revelle factor is greatest, causing much of the DIC to exist as HCO3 or CO3, and not CO2. So, the greater the buffering effect (low Revelle Factor) the more DIC occurs as CO3 or HCO3, effectively lowering the pCO2 levels in both the atmosphere and ocean.[3]

See also

References

  1. ^ McCracken, Mark. "Definition of Buffer Factor (Revelle Factor)" (website). Science. Retrieved 2015-11-24.
  2. ^ a b "7.3.4.2 Carbon Cycle Feedbacks to Changes in Atmospheric Carbon Dioxide" (.pdf). 2010. Retrieved 2019-02-15.
  3. ^
    S2CID 93920859. Archived from the original
    (PDF) on 2015-04-13.
  4. S2CID 5607281
    .
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