<p>Imaging of trace gases by optical remote sensing provides insight in the dynamics of physical and chemical processes within the atmosphere. Among the various sources for atmospheric trace gases, volcanoes pose additional challenges, as their highly variable emissions necessitate a high spatio-temporal resolution and their sometimes remote and inaccessible locations call for a robust and also portable measuring device.</p><p>We applied Fabry-Perot interferometer (FPI) correlation spectroscopy (IFPICS) that fulfils all of the above criteria. The periodic transmission of an FPI is matched to the periodicity of the vibronic narrowband absorption structure of the target trace gas absorption. The apparent absorptivity is then calculated from the difference of optical densities in two measurement settings whereas the FPI transmission coincides with the maxima of trace gas absorption in one setting and with the minima of the absorption in the second setting. Since the difference in wavelength between these two settings is only about 1nm, it is theorised that measurements with cloudy backgrounds become possible as their scattering properties aren't expected to differ much between the measurement settings and thus allow for cancellation. This is not the case for a conventional SO<sub>2</sub>-Cameras, as they rely on band-pass filters with transmission spectra that are about 20 nm apart.</p><p>We will show results of a first study on the influence of cloudy backgrounds on measurement results by determining the amount of SO<sub>2</sub> caused by meteorological clouds in the field of view.</p><p>We also present measurements from July 2021 of SO<sub>2</sub> fluxes at Mt. Etna with an IFPICS instrument with a detection limit of ≈ 5e17 molec/cm² at 4 Megapixel spatial resolution and 1 s temporal resolution and discuss uncertainties and challenges of the technique. </p>

<p>Imaging of atmospheric trace gases gives insights into physical and chemical processes in the atmosphere on the scale of seconds and metres. This is of particular importance when observing point sources with highly variable emission, like smoke stacks or volcanoes, and the chemical processes therein. In particular for volcanic plume measurements, instruments are required that not only combine a high spatio-temporal resolution with a high trace gas selectivity, but that are also sufficiently robust and compact to be used under field conditions and in remote locations.</p><p>Imaging Fabry-Perot interferometer (FPI) correlation spectroscopy (IFPICS) is a novel imaging technique for atmospheric trace gases. Atmospheric trace gas column densities are quantified with a high spatial and temporal resolution by matching the periodic spectral transmission of a FPI to the close to periodic vibronic absorption features of the target trace gas in the ultraviolet or visible wavelength range. So far, IFPICS has been applied to volcanic sulphur dioxide (SO<sub>2</sub>) imaging and laboratory measurements of formaldehyde (HCHO).</p><p>In this study, we present measurements of volcanic bromine monoxide (BrO) from a field campaign at Mt. Etna in July 2021. BrO is a very reactive species and thus only present in low amounts (some tens of ppt) in volcanic emission plumes, however, it is important as (1) indicator for degassing processes and (2) agent in plume chemistry. We discuss the challenges associated with separating the weak absorption signal of BrO (typical optical density around 10<sup>-3</sup>) from other effects within the complex environment of the volcanic plume. The camera prototype has a detection limit of 1x10<sup>14 </sup>BrO molec cm<sup>-2</sup> at a time resolution of 10 s and a spatial resolution of approximately 200 x 200 pixels. Using a second IFPICS instrument for SO<sub>2</sub> measurements, an estimate for the BrO to SO<sub>2</sub> ratio in the plume is given.</p>