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Towards imaging of atmospheric trace gases using Fabry–Pérot interferometer correlation spectroscopy in the UV and visible spectral range

Abstract. Many processes in the lower atmosphere including transport,
turbulent mixing and chemical conversions happen on timescales of the order
of seconds (e.g. at point sources). Remote sensing of atmospheric trace gases
in the UV and visible spectral range (UV–Vis) commonly uses dispersive
spectroscopy (e.g. differential optical absorption spectroscopy, DOAS). The
recorded spectra allow for the direct identification, separation and
quantification of narrow-band absorption of trace gases. However, these
techniques are typically limited to a single viewing direction and limited by
the light throughput of the spectrometer set-up. While two-dimensional imaging
is possible by spatial scanning, the temporal resolution remains poor (often
several minutes per image). Therefore, processes on timescales of seconds
cannot be directly resolved by state-of-the-art dispersive methods. We investigate the application of Fabry–Pérot interferometers (FPIs) for
the optical remote sensing of atmospheric trace gases in the UV–Vis spectral range. By
choosing a FPI transmission spectrum, which is optimised to correlate with
narrow-band (ideally periodic) absorption structures of the target trace gas,
column densities of the trace gas can be determined with a sensitivity and
selectivity comparable to dispersive spectroscopy, using only a small number
of spectral channels (FPI tuning settings). Different from dispersive optical
elements, the FPI can be implemented in full-frame imaging set-ups (cameras),
which can reach high spatio-temporal resolution. In principle, FPI
correlation spectroscopy can be applied for any trace gas with distinct
absorption structures in the UV–Vis range. We present calculations for the application of FPI correlation spectroscopy
to SO2, BrO and NO2 for exemplary measurement
scenarios. In addition to high sensitivity and selectivity we find that the spatio
temporal resolution of FPI correlation spectroscopy can be more than 2
orders of magnitude higher than state-of-the-art DOAS measurements. As proof
of concept we built a 1-pixel prototype implementing the technique for
SO2 in the UV. Good agreement with our calculations and conventional
measurement techniques is demonstrated and no cross sensitivities to other
trace gases are observed.
