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.
The combination of Cavity Enhanced Absorption Spectroscopy (CEAS) with broad-band light sources (e.g. Light-
Emitting Diodes, LEDs) lends itself to the application of cavity enhanced DOAS (CE-DOAS) to perform sensitive and
selective point measurements of multiple trace gases with a single instrument. In contrast to other broad-band CEAS
techniques, CE-DOAS relies only on the measurement of relative intensity changes, i.e., does not require knowledge of
the light intensity in the absence of trace gases and aerosols (I0). We have built a prototype LED-CE-DOAS instrument
in the blue spectral range (420-490nm) to measure nitrogen dioxide (NO2), glyoxal (CHOCHO), iodine monoxide (IO),
water (H2O) and oxygen dimers (O4). Aerosol extinction is retrieved at two wavelengths by means of observing water
and O4 and measuring pressure, temperature and relative humidity independently. The instrument components are
presented, and the approach to measure aerosol extinction is demonstrated by means of a set of experiments where
laboratory generated monodisperse aerosols are added to the cavity. The aerosol extinction cross section agrees well with
Mie calculations, demonstrating that our setup enables measurements of the above gases in open cavity mode.
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