<p>Volcanic plumes contain traces of bromine monoxide, BrO, which catalyze destruction of ozone, O<sub>3</sub>, mixed into the plume. Therefore, local depletion of O<sub>3 </sub>in the plume could be possible. However, calculations comparing mixing with the rate of O<sub>3 </sub>destruction suggest that no significant decline in the O<sub>3</sub> concentration should be expected. On the other hand several studies at different volcanoes have found varying degrees of O<sub>3</sub> depletion inside the plume. So far, ozone and its concentration distribution in volcanic plumes have only been insufficiently determined. Reliable ozone measurements would make a decisive contribution to the understanding of volcanic plume chemistry.</p>
<p>The standard technique for ambient O<sub>3</sub> monitoring is the short-path ultraviolet (UV) absorption instrument. But in volcanic plumes this technique suffers from strong interference of the overlapping SO<sub>2</sub> absorption features in the UV. SO<sub>2</sub> is one of the major compounds in volcanic plumes.</p>
<p>We want to overcome this problem by relying on the chemiluminescence (CL) reaction between ozone and ethene, a standard technique for O<sub>3</sub> measurement in the 1970s and 1980s, which we found to have no interference from trace gases abundant in volcanic plumes. The key component of a CL O<sub>3</sub>-instrument is a reaction chamber, where ethene is mixed into the ambient air and a photomultiplier tube detects the resulting photons.</p>
<p>Field measurements with existing CL O<sub>3</sub>-monitors are complicated, because they are usually heavy and bulky. Therefore we designed a more compact and lightweight version (10 kg backpack size CL instrument), which was used in a field study at Mount Etna. However, the campaign was restricted to plumes that are pushed down to ground in areas accessible by foot.</p>
<p>Here we report on a further improved version of the instrument weighing around 1 kg, which we can mount onto a drone to carry it into the plume. In particular, we describe the design advances making the reduction in weight and size possible.</p>