Sulfate Aerosol Overview   

Approximately 100 million tons of sulfur are injected into the earth's atmosphere annually.  The sulfur gets converted into sulfate aerosols, and gets deposited back to the earth.    Understanding this sulfur cycle is important due to the impacts sulfur has on ecology:  reduced visibility, acid rain, lung damage, and climate change.

The Sulfur Cycle

Emissions
Approximately 100 Tg (teragrams) of sulfur are added to the atmosphere annually (roughly 100 million tons).   75% of these emissions are anthropogenic, while 25% occur "naturally".   The biggest anthropogenic source is from burning fossil fuels in power plants, while the biggest natural source is from marine plankton in the oceans.

Conversion
Sulfur compounds (predominantly sulfur dioxide from humans, and dimethylsulfide from marine plankton) are converted into the sulfate aerosol via many different reactions.  The two primary reactions are via the hydroxy radical in the gas phase and hydrogen peroxide in the aqueous phase.

Growth
Sulfate aerosols grow by absorbing water vapor from the atmosphere.  Additionally, in high humidity cloud environments, growth also occurs via coagulation (the combining of two drops into one larger drop). 

Deposition
Sulfur compounds are removed from the atmosphere via dry deposition and wet deposition. Dry deposition is the settling of sulfur to a surface by gravity and winds.  Wet deposition is the removal of sulfur during rain events.

The Effects of Sulfur

Visibility
Sulfates (as well as other types of aerosols) scatter light and reduce visibility in the atmosphere. 

Acid Rain
Sulfates dissolve in water to form sulfuric acid.  This damages the local plant and animal ecology.

Lung Damage
Sulfates not only damage lungs due to their acidity, but also tend to be shifted to a very small size distribution (with a high fraction in the PM2.5 region) which can penetrate deep into the lungs and cause long term damage.

Climate Change
Sulfates have a direct and indirect effect on climate.   The direct effect is due to sulfates scattering light, changing the earth's albedo and the amount of radiation absorbed at the top of the atmosphere as well as the surface.  The indirect effect is due to sulfates modifying cloud properties by increasing cloud lifetime, and shifting cloud drops to smaller, more efficient light-scattering size distributions. (See the next section!)

Size Matters!!

The light-scattering efficiency of aerosols is highly-dependent upon the particle size as shown in the diagram below.

Particle sizes between 0.2 and 2 um are most efficient at scattering light, with 0.8 um size being the most efficient.  Particles 0.8 um in diameter are approximately 5 times as efficient at scattering light than those of 0.08 um or 8 um.

It is clear that knowledge of particle size is extremely important to understand the effects of aerosols on climate.  The integration of the computer model CARMA with CAM should provide more accurate simulations of particle size. 

background courtesy of Bureau of Meteorology, Commonwealth of Australia

this page updated on July 4, 2007