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How Do Sulfates Cool the Earth?

Question: 
How does sulfur from volcanoes actually cool the Earth?
Answer: 

Volcanoes have two ways of cooling the Earth.  Both are classified as "aerosol dimming". The most obvious is the volcanic ash blown into the atmosphere by violent explosions. The ash, typically black to brown (and possibly reddish) simply blocks sunlight, cooling the Earth. The idea that the ash is just dirt and does no environmental damage when it falls to earth is not quite true.  In addition to smothering plants and possibly animals, the ash may also contain lead, arsenic, and mercury.

The second way of cooling the Earth is through sulfur compounds. The primary effect is to reflect sunlight back into space. However, there is a secondary effect that is much more important. Sulfur compounds are particularly good at attaching themselves to small particles (of dust, etc.) in the atmosphere. These result in nuclei for many more small water droplets than there would be otherwise and so in more, higher, and longer lasting clouds. These clouds also reflect sunlight (about five times as well as the sulfates themselves).

This is particularly effective in the stratosphere (10-50 km or 30,000-160,000' high) and the sulfates stay aloft longer at high altitudes. Unfortunately, sulfur compounds also result in acid rain and they destroy ozone in the stratosphere (which protects us from ultraviolet light). Although it is really high, large volcanic explosions routinely hurl vast amounts of dust and sulfur compounds into the stratosphere. In 1991, Mount Pinatubo in the Philippines sent ash up to 34 km (110,000') into the air!