Imaging Fabry-Perot interferometer correlation spectroscopy - improving the accuracy of SO2 flux measurements

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.

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 SO₂-Cameras, as they rely on band-pass filters with transmission spectra that are about 20 nm apart.

We will show results of a first study on the influence of cloudy backgrounds on measurement results by determining the amount of SO₂ caused by meteorological clouds in the field of view.

We also present measurements from July 2021 of SO₂ 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.