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Proceedings Paper

MAX-DOAS observations from ground, ship, and research aircraft: maximizing signal-to-noise to measure 'weak' absorbers
Author(s): Rainer Volkamer; Sean Coburn; Barbara Dix; Roman Sinreich
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Paper Abstract

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.

Paper Details

Date Published: 26 August 2009
PDF: 9 pages
Proc. SPIE 7462, Ultraviolet and Visible Ground- and Space-based Measurements, Trace Gases, Aerosols and Effects VI, 746203 (26 August 2009); doi: 10.1117/12.826792
Show Author Affiliations
Rainer Volkamer, Univ. of Colorado at Boulder (United States)
Sean Coburn, Univ. of Colorado at Boulder (United States)
Barbara Dix, Univ. of Colorado at Boulder (United States)
Roman Sinreich, Univ. of Colorado at Boulder (United States)

Published in SPIE Proceedings Vol. 7462:
Ultraviolet and Visible Ground- and Space-based Measurements, Trace Gases, Aerosols and Effects VI
Jay R. Herman; Wei Gao, Editor(s)

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