What other sources of volcanic gas emissions can there be on earth?
Volcanic gases are emitted by a number of ways besides eruptions:
Dissolved gases in heated groundwater.
Using a geothermal complex like Yellowstone National Park along Wyoming-Montana border in USA, volcanic gases are emitted through groundwater and water vapor from heated geothermal springs. One of the most impressive aspects of Yellowstone is that steam rises from everywhere: in parking lot storm drains, potholes, not just from the geysers or springs.
Gas venting from volcanic crater when the volcano is not active.
Gases will vent from volcanic craters between eruption cycles. The emitted gases can be used to determine the level of activity beneath the surface by monitoring volume and chemical composition. Changes in volume and composition can indicate a change in activity up or down.
Often the gases emissions are not visibly noticeable.
Gases can be emitted from a underlying magma chamber through cracks and fissures.
Mount Nyiragongo in Congo has localized carbon dioxide toxicity, known locally as 'mazuku' which are where the gas seeps from the ground in relatively high levels, without the dispersing effects of wind. This can occur miles from the volcano's summit.
The Great African Rift complex has numerous lakes where CO2 build is a serious concern. Strongly stratified lakes can trap large amounts of CO2 deep within the lake. This is evidence that volcanic gases can be emitted across a wide area over a volcanic complex not limited to surface boundaries of a volcano.