Revelle factor

The Revelle factor (buffer factor) is the ratio of instantaneous change in carbon dioxide (CO₂) to the change in total dissolved inorganic carbon (DIC), and is a measure of the resistance to atmospheric CO₂ being absorbed by the ocean surface layer.^[1] The buffer factor is used to examine the distribution of CO₂ between the atmosphere and the ocean, and measures the amount of CO₂ that can be dissolved in the mixed surface layer. It is named after the oceanographer Roger Revelle, who was one of the first scientists to study global warming.

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

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 is dependent on the systems alkalinity, and is illustrated by the Bjerrum plot below (Figure 1). Carbonate is dominant in higher pH environments (basic), whereas carbon dioxide is dominant in lower pH environments (acidic). Bicarbinate ions are abundant in relatively mid-pH waters. As the pH decreases, most of the DIC will be 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 pCO₂ from the atmosphere, and decreasing the pH even more.^[2]

Figure 1: Figure 1 illustrates 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 effect 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 (anthropogenic) CO₂ is inversely proportional to the value of the Revelle factor. Hence, in modern-day oceans, it is possible to see the concentrations of anthropogenic CO₂ by measuring the Revelle factor; the lower the Revelle factor, the greater the amount of anthropogenic CO₂.^[4] Low Revelle factors are typically found in the warmer tropical to subtropical waters, whereas higher Revelle factors are found in the colder high latitude waters of the North Atlantic. The 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⁻¹ 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 CO₂ or HCO₃, effectively lowering the pCO₂ levels in both the atmosphere and ocean.^[3]

See also

• Alkalinity
• Solubility pump

References