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.