The most abundant gas is water vapor (H2O), followed by carbon dioxide (CO2), and sulfur dioxide (SO2). Secondary gases are also commonly emitted from volcanoes and include hydrogen sulfide (H2S), hydrogen (H), carbon monoxide (CO), hydrogen chloride (HCl), hydrogen fluoride (HF), and helium (He).owever, volcanic gases reach the stratosphere, a layer in the atmosphere from about 10 km to about 50 km in altitude, during large eruptions. This relationship is complicated by the fact that the elevation between the volcano summit and the distance to the troposphere/stratosphere decreases with latitude.