Is there an ozone depletion in volcanic plumes?

Volcanic plumes are known to contain reactive halogen species, especially bromine oxide. Therefore, local ozone (O₃) depletion (OD) is expected inside volcanic plumes. This OD has been measured in several field studies and is also found in several modelling studies. Recently, in order to quantify O₃ mixing ratios in volcanic plumes, mainly UV absorption monitors have been used as these have become the standard technique for ambient O₃ monitoring. However, these instruments show a large positive interference with sulphur dioxide (SO₂). In fact, these instruments are approximately only 100 times more sensitive to O₃ than to SO₂. This poses a significant problem for volcanic measurements since SO₂ mixing ratios can exceed O₃ mixing ratios by factors of 1000 or more. Thus, laborious SO₂ filtering introducing further problems, as e.g. humidity dependence, needed to be employed.

In this work simultaneous O₃ measurements inside a fumarole were conducted with a compact and mobile (backpack-size, ~10kg) chemiluminescence (CL) ozone monitor and a conventional UV absorption monitor at the summit of Mt Etna volcano, Italy. In parallel, SO₂ and CO₂ measurements were carried out with a MultiGAS-instrument. The CL monitor was used since no interference from trace gases contained in volcanic plumes is expected. Indeed, in this first field study inside a fumarole, we observed no significant interference with volcanic SO₂ concentrations for the CL monitor. Under field conditions the CL monitor’s detection limit was determined to be ~1 ppb (1σ) at an integration time of 1 second.

Additionally, a rough calculation to estimate the expected OD in volcanic plumes was made. Contrary to popular belief, this calculation suggests for typical bromine oxide concentrations no significant (i.e. <1%) reactive halogen catalysed O₃-loss in volcanic plumes.