Sulfur hazes are believed to have been the primary cause of the global cooling that occurred after the Pinatubo and Tambora eruptions. For many months, a satellite tracked the sulfur cloud produced by Pinatubo. The image shows the cloud about three months after the eruption. It is already a continuous band of haze encircling the entire globe. You can learn more about the cooling effects of sulfur hazes by clicking here .
Volcanoes also release large amounts of water and carbon dioxide.
This causes the air below to get warmer. Therefore, you might think that a major eruption would cause a temporary warming of the atmosphere rather than a cooling. However, there are very large amounts of water and carbon dioxide in the atmosphere already, and even a large eruption doesn't change the global amounts very much. In addition, the water generally condenses out of the atmosphere as rain in a few hours to a few days, and the carbon dioxide quickly dissolves in the ocean or is absorbed by plants. Consequently, the sulfur compounds have a greater short-term effect, and cooling dominates.