To make matters worse, the reported output of any individual volcano is itself an estimate based on limited rather than complete measurement.
One may reasonably assume that in each case, such estimates are based on a representative and statistically significant quantity of empirical measurements.
Another class of problem for monitoring stations plagues "Christmas Island", which is actually Kiribati Island (02º00'N, 157º20'W) where the Clipperton Fracture Zone (Taylor, 2006) crosses the Christmas Ridge and is nowhere near Christmas Island (10º29'S, 105º38'E; located on the other side of Australia, 10,000 km due west of Kiribati).
Christmas Ridge is formed in a concentration of Pacific Seamounts.
Observatories located within active volcanic provinces are not the only problem.
There is also the problem of pressure systems carrying volcanic plumes several hundred kilometers to station locations.
Volatiles, such as CO is the second most abundantly emitted volcanic gas next to steam.