Abstract
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.
Highlights
Within the last decades, progress in optical remote sensing of atmospheric trace gases has led to a better understanding of many important processes including air pollution, ozone, and halogen chemistry and the evolution of volcanic plumes
The Fabry–Pérot interferometers (FPIs) correlation spectroscopy technique allows for numerous different realisations regarding the used spectral window and FPI instrument parameters that can be chosen according to, for example, measurement conditions or availability of optical components (FPI, band pass filters (BPFs))
Many locally variable atmospheric processes are difficult to quantify with state-of-the-art UV and visible spectral range (UV–Vis) remote-sensing methods (e.g. Differential optical absorption spectroscopy (DOAS)) due to the limited spatio-temporal resolution
Summary
Progress in optical remote sensing of atmospheric trace gases has led to a better understanding of many important processes including air pollution, ozone, and halogen chemistry and the evolution of volcanic plumes. Determination of two-dimensional atmospheric trace gas distributions with high time resolution at timescales of the order of seconds, i.e. fast imaging, of atmospheric trace gases is possible with techniques recording all spatial pixels of an image at once for a low number of spectral channels Mori and Burton, 2006; Bluth et al, 2007; Kern et al, 2010; Platt et al, 2018) or a tuneable BPF as a wavelength selective element (e.g. NO2 camera, ∼ 3 min per image for stack emissions of power plants; Dekemper et al, 2016) These techniques either involve intricate optical set-ups with low light throughput or yield a rather coarse spectral resolution, which might result in strong cross interferences (see e.g. Lübcke et al, 2013; Kuhn et al, 2014). We show that SO2 CDs can be accurately retrieved from the recorded data without calibration (Sect. 4)
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