Reliable Ozone Measurements in Volcanic Plumes: A Way to Resolve the Volcanic Ozone Enigma

In addition to CO2 and sulphur dioxide (SO2), volcanic plumes also contain reactive halogen species. Bromine monoxide (BrO) can reliably be quantified by remote sensing measurements and is known to catalyse ozone (O3) destruction. Therefore, local O3 depletion is commonly assumed inside volcanic plumes.Contrary to popular belief, a calculation comparing atmospheric turbulent mixing with the rate of O3 destruction inside the (young) plume suggests no significant halogen catalysed O3 loss (1% or less) in the plume. So far, however, O3 and its concentration distribution in volcanic plumes have only been insufficiently determined since commonly used short-path ultraviolet (UV) absorption O3 monitors show a severe, positive interference with SO2, an abundant volcanic gas.This interference problem can be overcome by using a chemiluminescence (CL) O3 monitor, a standard technique for O3 measurements in the 1970s (and still the standard instrument for air pollution monitoring), which shows no interference with trace gases in volcanic plumes and therefore allows reliable O3 measurements in volcanic plumes.However, field measurements with existing CL O3 monitors are challenging, since they are usually heavy and bulky. We therefore designed an improved and lightweight version of the CL O3 instrument (1kg, shoebox size), which can be easily carried or mounted onto a drone, thus opening up completely new measurement possibilities. After test measurements in Heidelberg, including ground-based as well as drone-based measurements, during which we determined vertical O3 profiles, we performed drone-based O3 measurements in the plume of Etna volcano. The latter data show an anti-correlation between O3 and simultaneously determined SO2, suggesting an O3 depletion of up to ~60% in the plume of Etna. This raises the question which – probably unknown - process leads to this observed O3 depletion.