Imaging of volcanic trace gases by Fabry-Perot interferometer correlation spectroscopy
Imaging of atmospheric trace gas distributions by optical remote sensing allows for a direct assessment of the dynamics of both physical and chemical processes. In particular, the fast evolution of trace gases emitted by point sources (e.g., volcanic plumes) can be studied comprehensively by imaging approaches. Presently applied imaging techniques still either lack in spatio-temporal resolution like, e.g., Imaging DOAS or in selectivity and sensitivity like sulphur dioxide (SO2) cameras. Throughout this thesis, a novel imaging approach based on Fabry-Perot interferometer correlation spectroscopy (FPI CS) is presented. The technique exploits the periodic transmission proﬁle of a Fabry-Perot interferometer (FPI) and its correlation to the (approximately periodically varying) target trace gas spectral absorbance. The feasibility of the novel technique for SO2, bromine monoxide (BrO), and formaldehyde (HCHO) is examined in a model study. A prototype of a one-pixel FPI instrument for HCHO is characterised and tested in the laboratory yielding a good agreement between the modelled and measured sensitivity for HCHO. The sensitivity (weighted mean trace gas absorption cross section) of the one-pixel HCHO instrument is 2.28 × 10−20 cm2 molec−1. From the HCHO sensitivity, a BrO sensitivity of 6.21 × 10−18 cm2 molec−1 can be inferred due to the spectral similarity of the BrO and HCHO absorption cross section. Finally, an imaging FPI CS prototype is designed, built, and tested in ﬁeld measurements. The SO2 detection limit of 3.8 × 1017 moleccm−2 s−1/2 is comparable to present SO2 cameras, however, the selectivity is drastically increased. FPI CS therefore shows a promising potential to allow for fast imaging measurements of most of the trace gases that can be measured by DOAS.
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