Towards imaging of atmospheric trace gases using Fabry Perot
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 time scales 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 setup. While two dimensional imaging is possible by spatial scanning, the temporal resolution remains poor (often several minutes per image). Therefore, processes on time scales of seconds cannot be directly resolved by state of the art dispersive methods. We investigate the application of Fabry-Perot Interferometers (FPIs) for the optical remote sensing of atmospheric trace gases in the UV/Vis. 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 setups (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. We present calculations for the application of FPI Correlation Spectroscopy to SO2, BrO and NO2 for exemplary measurement scenarios. Besides high sensitivity and selectivity we find that the spatio temporal resolution of FPI Correlation Spectroscopy can be more than two orders of magnitude higher than state of the art DOAS measurements. As proof of concept we built a one-pixel prototype implementing the technique for SO2 in the UV. Good agreement with our calculations and conventional measurement techniques are demonstrated and no cross sensitivities to other trace gases are observed.