Sulfur Dioxide

Sulfur dioxide (SO2) is an important component of the atmosphere, present as the result of both natural and human activity. Although it is a primary pollutant, causing respiratory irritation and damage to plants, it is the secondary pollutants produced from SO2 that are particularly important in connection with global climate change.

Sulfur dioxide is notorious as the cause of acid rain, but it is also a precursor to the formation of clouds. Hence, its release to the atmosphere is a major contributor to global dimming, a process that is thought to offset some of the effects of global warming.

There are, therefore, important implications of SO2 release for the global climate change agenda. The reduction in SO2 pollution in recent decades, stimulated by health concerns and by the effects of acid rain, is removing an unexpected and previously unidentified protection against increasing global temperatures. This illustrates the complexity of climate science that compounds the social and political responses to the threat of climate change.

Formation of Clouds

Once in the atmosphere, SO2 is rapidly oxidized, ultimately producing sulfuric acid. Although this transformation is well known in the formation of acid rain, it also has broader climatological significance. The liquid sulfuric acid forms as an aerosol (tiny droplets suspended in the air), and this sulfuric acid aerosol attracts water vapor, which dissolves in the acid. In this way, the gas-to-liquid

conversion of SO2 to sulfuric acid brings about the nucleation of clouds: sulfuric acid aerosol is a cloud condensation nucleus (CCN).

Clouds play important roles in the atmosphere and in the climate, principally acting to transport water (and energy) between regions and affecting the Earth’s radiation balance. Clouds have a very strong tendency to reflect sunlight (they have a high albedo) and absorb energy from the sun, so that the amount of cloud present in the atmosphere affects the amount of sunlight reaching the surface: more clouds result in a dimmer planet. This dimming effect of clouds is well documented.

Sulfur Dioxide and Climate Change

Furthermore, the influence of SO2 and subsequent aerosol formation has been observed directly during volcanic eruptions. For example, the 1991 eruption of Mount Pinatubo in the Philippines released an estimated 20 megatons (20 million tons) of SO2 into the atmosphere. The force of the explosion injected a large fraction of this material, along with dust particles, directly into the stratosphere, from which removal via rainout is very slow. The aerosol and clouds that formed as a consequence of this lasted for many years, with measurable effects on global temperatures.

In the year following the eruption, the global average temperature reduced by 0.9 degree F (0.5 degree C), and even in 1993, the temperature was depressed by as much as 0.45 degree F (0.25 degree C). In the lower atmosphere, the rate of SO2 gasto-liquid conversion is increased in the presence of other materials, notably particles such as soot, and this has an important effect on cloud condensation. 

The typically hydrophobic, waterrepelling surfaces of soot particles catalyze the chemical reactions that convert SO2 to sulfuric acid, so that the soot ends up coated with a water-loving, hydrophilic layer. The simultaneous emission of SO2 and soot (e.g., from burning coal or diesel fuel) therefore increases the concentration of cloud condensation nuclei in the air, affecting both the amount and nature of cloud formation.

Because there are many more cloud condensation nuclei under these conditions than in the clean atmosphere, clouds form with smaller, more numerous droplets. More numerous particles means that the clouds reflect more light, and smaller droplets take longer to form raindrops. Hence, the clouds formed on sulphate/soot aerosol CCN are longer lived and have a higher albedo than ordinary clouds. In this way, SO2 emissions in combination with soot increase the amount of incoming sunlight reflected away from the Earth, effectively dimming the planet’s surface.

Sulfur dioxide pollution–related global dimming is thought to explain the slight global cooling trend in the period from 1950 to the late 1970s. With the recent legislation-driven decrease in emissions of SO2 and soot particles from industry and transport in industrialized nations, there has been a steady rise in the amount of sunlight reaching the Earth. It is suspected that reducing this form of pollution is removing an effect that has been offsetting the full force of anthropogenic global climate change.