Imaging measurements of volcanic BrO using Fabry-Pérot interferometer correlation spectroscopy

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.

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 (SO₂) imaging and laboratory measurements of formaldehyde (HCHO).

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^-3) from other effects within the complex environment of the volcanic plume. The camera prototype has a detection limit of 1x10¹⁴ BrO molec cm^-2 at a time resolution of 10 s and a spatial resolution of approximately 200 x 200 pixels. Using a second IFPICS instrument for SO₂ measurements, an estimate for the BrO to SO₂ ratio in the plume is given.