Theories of the EARLY atmosphere
Scientists are particulary interested in volcanoes because they release LARGE amount of gases. Volcanoes these days mainly release Carbon Dioxide and Water Vapour.
So scientists believe these gases were in the earth's EARLY atmosphere.
1)The atmosphere of Titan is 98% nitrogen , which some believe that most of the atmosphere was caused by volcanoes. They think that this could have been similar to the world's early atmosphere.