Volcanoes in themselves are an amazing part of the natural world. The surrounding region is suddenly rocked by earthquakes, lava flow, and explosions of gas and ash.
It is the emissions that alter the atmosphere. In fact, emissions from volcanoes have been partially credited with creating our atmosphere in the first place.The earths atmosphere developed with volcanic emissions full of carbon dioxide, water vapor, nitrogen, methane, sulfur, and chlorine.
In 1784, Benjamin Franklin was one of the first to publish the correlation between weather and volcanoes. Following the Laki eruption, (located in Iceland) Franklin noted the link of the volcanic eruption to the increased levels of fog and cold weather in Europe.
There is an immediate change to the local weather that is caused by the massive plumes of ash. Ash collects water droplets and is linked to the likelihood of the subsequent thunder and lightning.
What is of interest to us all is how the worlds climate responds to the massive plumes. The volcano pumps a significant amount of ash and sulfur dioxide. Together they form a veil. The size and height of the veil determines the change in worlds climate.
Large particles quickly dissipate from the veil as they fall to the ground. Small particles in the lower atmosphere have an effect, but a low lying veil will only last days. It is the gases and small particles in the upper atmosphere that can change the global weather. A large eruption will form a veil in the upper atmosphere that can last for weeks, months, and even years.
In the stratosphere the sulfur dioxide becomes sulfuric acid. The sulfuric acid becomes sulfate aerosols and sulfate aerosols are excellent at reflecting the suns radiation. The result is a slight decrease in global temperature (.5°F). However, following the eruption of Mount Pinatubo (June 15, 1991), the earths surface was cooled for 3 years by as much as 1.3°F.
On the other hand, it could be argued that the release of water vapor and carbon dioxide could increase the global temperature. This may be true if there were several eruptions, but a single eruption typically does not emit high enough levels of water and carbon dioxide to have a significant effect.