TotalBrO: First results of a small solar occultation instrument for the stratosphere

Spectroscopic remote sensing in solar occultation geometry offers an important tool for determining atmospheric trace gas concentrations in the middle atmosphere. Monitoring ozone-depleting substances such as halogen oxides is essential to watch the ozone layer throughout a changing climate. The new TotalBrO instrument consists of an active solar tracker (LxWxH ~ 0.40 x 0.40 x 0.50 m, weight ~ 12 kg) and a spectrometer unit (LxWxH ~ 0.45 x 0.40 x 0.40 m, weight ~ 25 kg) with two temperature-stabilized grating spectrometers for the UV/visible spectral range. The instrument is compact and designed to measure bromine and iodine monoxide  (BrO and IO) in addition to other gases such as ozone (O₃) and nitrogen dioxide (NO₂) by means of Differential Optical Absorption Spectroscopy (DOAS). Sets of spectra collected during balloon ascent, sunset and sunrise allow for inferring vertical profiles of the gases.

Here, we report on the first deployment of TotalBrO on a stratospheric balloon launched from Kiruna, Sweden, during the Klimat campaign in August 2021. The solar tracker was able to track the sun once the balloon gondola was azimuthally stabilized above altitudes of about 25 km. TotalBrO collected UV/visible absorption spectra throughout solar occultation during sunset and sunrise on August 21/22, 2021. For the solar occultation periods, the tracking deviation with respect to the center of the solar disk was in the targeted regime of < 0.05°, and the solar tracker was able to catch the sun down to solar zenith angles (SZA) of around 95°, corresponding to tangent heights of about 10 km. We show preliminary results for profile retrievals of O₃ and NO₂ and for DOAS analyses of BrO and IO. The latter currently suffer from an unexplained oscillatory spectral pattern, for which we report on extensive sensitivity studies.