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Differential optical absorption spectroscopy instrument for stratospheric balloonborne trace-gas studies.

A newly developed UV-visible instrument for differential optical absorption spectroscopic measurements of atmospheric trace gases from balloon platforms is described. Direct solar light at daytime in the near-ultraviolet (320.6-422.6-nm) and the visible (417.6-670.7-nm) spectral ranges can be simultaneously analyzed for the atmospheric column abundances or profiles of O(3), NO(2), NO(3), BrO, OClO, O(4), H(2)O, and possibly other species (HNO(2), IO, CH(2)O). Compared with previously used balloonborne UV-visible spectrometers, the instrument has the superior properties of low mass (42 kg), low power consumption (30 W), decreased spectral drift that is caused by temperature and pressure changes, low detector dark current, and low spectrometer stray light. The three last-named characteristics are achieved by enclosure of the entire spectrometer in a pressurized and thermostated container and by inclusion of separately thermostated photodiode array detectors. The optical setup is simplified to reduce its weight. The spectral stray light is reduced by suppression of the higher-order and zero-order grating reflections by use of light traps and in the UV by addition of a dispersive prism preanalyzer. The major instrumental design characteristics and the instrumental performance as tested in the laboratory and during several stratospheric balloon flights are reported.