Friday, 24 July 2020


The main direct effect of sulfates on the climate involves the scattering of light, effectively increasing the Earth's albedo. The term albedo was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria.

Albedo is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that reflects all incident radiation. The average albedo of the Earth from the upper atmosphere, its planetary albedo, is 30–35% because of cloud cover, but widely varies locally across the surface because of different geological and environmental features.

This effect is moderately well understood and leads to cooling from the negative radiative forcing of about 0.4 W/m2 relative to pre-industrial values, partially offsetting the larger (about 2.4 W/m2) warming effect of greenhouse gases. The effect is strongly spatially non-uniform, being largest downstream of large industrial areas.

% of Diffusely Reflected Sunlight
The first indirect effect is also known as the Twomey effect. Sulfate aerosols can act as cloud condensation nuclei and this leads to greater numbers of smaller droplets of water. Many smaller droplets can diffuse light more efficiently than a few larger droplets. 

The second indirect effect is the further knock-on effects of having more cloud condensation nuclei. It is proposed that these include the suppression of drizzle, increased cloud height, to facilitate cloud formation at low humidities and longer cloud lifetime. Sulfate may also result in changes in the particle size distribution, which can affect the clouds radiative properties in ways that are not fully understood. 

Chemical effects such as the dissolution of soluble gases and slightly soluble substances, surface tension depression by organic substances and accommodation coefficient changes are also included in the second indirect effect.

The indirect effects probably have a cooling effect, perhaps up to 2 W/m2, although the uncertainty is very large. Sulfates are therefore implicated in global dimming. Sulfate is also the major contributor to a stratospheric aerosol formed by oxidation of sulfur dioxide injected into the stratosphere by impulsive volcanoes such as the 1991 eruption of Mount Pinatubo in the Philippines. This aerosol exerts a cooling effect on climate during its 1-2 year lifetime in the stratosphere

Diagram: The percentage of diffusely reflected sunlight relative to various surface conditions. By CC BY-SA 2.5,

  • Lewis, Gilbert N. (1916). "The Atom and the Molecule". J. Am. Chem. Soc. 38: 762–785. doi:10.1021/ja02261a002. (See page 778.)
  • Pauling, Linus (1948). "The modern theory of valency". J. Chem. Soc.: 1461–1467. doi:10.1039/JR9480001461.
  • Coulson, C. A. (1969). "d Electrons and Molecular Bonding". Nature. 221: 1106. Bibcode:1969Natur.221.1106C. doi:10.1038/2211106a0.
  • Mitchell, K. A. R. (1969). "Use of outer d orbitals in bonding". Chem. Rev. 69: 157. doi:10.1021/cr60258a001.