The following factors of a volcanic eruption that can cause a decrease in the Earth's average atmospheric temperature is the ash and gases taht are being released by volcanoes. Although volcanoes are a big emitter of CO2 gases, volcanoes also emits a lot of sulfur dioxide which when released into the stratosphere, they would then be converted to sulfuric acid which condenses rapidly to form sulfur aerosols. This sulfur aerosols then reflects more sunlight back to space, which could have a potential cooling effect on the entire planet if released in massive amounts.
