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26 August 2009 MAX-DOAS observations from ground, ship, and research aircraft: maximizing signal-to-noise to measure 'weak' absorbers
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Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments, as solar straylight satellites, require an accurate characterization and elimination of Fraunhofer lines from solar straylight spectra to measure the atmospheric column abundance of reactive gases that destroy toxic and heat trapping ozone and form climate cooling aerosols, like glyoxal (CHOCHO), iodine oxide (IO), or bromine oxide (BrO). The currently achievable noise levels with state-of-the-art DOAS instruments are limited to δ'DL ≈ 10-4 (noise equivalent differential optical density, δ'); further noise reductions are typically not straightforward, and the reason for this barrier is not well understood. Here we demonstrate that the nonlinearity of state-of-the-art CCD detectors poses a limitation to accurately characterize Fraunhofer lines; the incomplete elimination of Fraunhofer lines is found to cause residual structures of δ' ≈ 10-4, and only partially accounted by fitting of an "offset" spectrum. We have developed a novel software tool, the CU Data Acquisition Code that overcomes this barrier by actively controlling the CCD saturation level, and demonstrates that δ'DL on the order of 10-5 are possible without apparent limitations from the presence of Fraunhofer lines. The software also implements active control of the elevation angle (angle with respect to the horizon) by means of a Motion Compensation System for use with mobile MAX-DOAS deployments from ships and aircraft. Finally, a novel approach to convert slant column densities into line-of-sight averaged concentrations is discussed.
© (2009) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Rainer Volkamer, Sean Coburn, Barbara Dix, and Roman Sinreich "MAX-DOAS observations from ground, ship, and research aircraft: maximizing signal-to-noise to measure 'weak' absorbers", Proc. SPIE 7462, Ultraviolet and Visible Ground- and Space-based Measurements, Trace Gases, Aerosols and Effects VI, 746203 (26 August 2009);

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