Introduction

The need to study photochemical and dynamical processes influencing the stratospheric ozone layer has been recognised for many years.

In order to study these questions in more detail, a new UV/vis DOAS (Differential Optical Absorption Spectroscopy) spectrometer has been built for atmospheric trace gas studies from balloon platforms. Direct solar spectra of the near/UV (321  - 422 nm) and visible spectral range (417 - 669 nm) are analyzed to measure the atmospheric abundances of O, NO, NO, HNO, BrO, OClO, CHO, O and HO along the instrumental line of sight. The instrumental design was optimized for low weight, low power consumption and constant pressure within the optical setup. The spectrometer forms an instrument package together with the LPMA gondola as described by C. Camy-Peyret [1995] (see also Poster 112).

Description of the instrument

The optical components of the novel DOAS-spectrograph consist of two transfer optics with quartz diffusers, filters and orifices, two quartz fibre bundles also forming the entrance slits, two holographic gratings and two windowless photodiode array detectors stabilized at -10.0C with on chip peltier elements. The input beam is stabilized by a sun-tracker installed on the gondola as described by Hawat et al., [1995].

Figure 1

The complete optics is included in an evacuated stainless steel housing thus minimizing shifts by changing ambient pressure (P=1 Bar). Thermal stabilisation of the instruments is provided by embedding the housing into a water-ice reservoir. The cooling of the warm side of the Peltier elements (about 4 W each) is provided by cycling a refrigerant.

Figure 2

The linearity of the detector is checked by the shape of Fraunhofer lines recorded at different saturation levels. Figure 2 displays the ratio of selected channels over detector saturation. Within the saturation levels during flights (up to 80%) the linearity is better than 10.

Figure 3

The spectral shifts relative to a Fraunhofer spectrum taken at Kiruna during the flights performed at Leon, Kiruna and Gap are shown in Figure 3. The instrument withstood the impacts without degradation and no readjustments between the flights were undertaken.

Figure 4

The effect of spectral shifts and possibly misaligned spectra are estimated in Figure 4. NO interference is low due to low changes in NO slant column relative to Fraunhofer reference.

Figure 5

Figure 5a) shows a fit of a Kiruna ascend spectrum (12:30:36 UT, 82.8 SZA, 6.9 km) to a Fraunhofer reference spectrum (28.8 km, 88.8 solar zenith angle (SZA)). Included in the fit are two O laboratory spectra recorded at -80 and -20C shown added in b), c) a NO laboratory spectrum recorded at -70C corrected for HONO, d) an O spectrum of Greenblatt et al. [1990] convoluted to instrument resolution, e) a 228K BrO spectrum of Wahner et al. [1990 priv. comm] convoluted to instrument resolution, f) the residuum and an artificial spectrum to account for spectrograph straylight. At present we estimate the retrieved BrO column densities to be accurate to about 22% (5% due to temperature dependency of BrO cross section, 10% due to fitted range, 12% due to shifts of reference spectra, 15% due topossible interference with other absorbers).

Figure 6

Figure 6 shows the BrO slant column densities during the flight from Kiruna with SZA 83 to 94 (Errorbars denote fiterrors). The accompanying Poster 75 continues with evaluation of BrO slant column densities. Poster 117 focusses on O and NO retrieval and Poster 217 on O evaluation.

Instrument condition (f.e. temperatures of photo diode array, peltier currents) is recorded together with the spectra and can online be monitored on ground. The total mass of the instrument is 42 kg, and its electrical power consumption is about 25 W.

Acknowledgements

The project is funded by BMBF (01LO9316/5) and the EU (contract No. ENV4-CT-95-0178). We are grateful to C. Camy-Peyret/LPMA, Paris, and his colleagues as well to D. Huguenin, Observatoire de Genève, and his team for their great support integrating the DOAS-instrument on the LPMA gondola and for their assistance in the balloon launches already conducted.

References

Camy-Peyret, C., P. Jesek, T. Hawat, G. Durry, S. Payan, G. Berube, L. Rochette, and D. Huguenin, The LPMA balloon-borne FTIR spectrometer for remote sensing of atmospheric constituents, Proc. 12th ESA Symp. European rocket and balloon programmes and related research, Lillehammer, 29 May - 1 June 1995, ESA SP-370, September 1995.

Greenblatt G. D., Orlando J. J., Burkholder J. B. and Ravishankara A. R., Absorption measurements of oxygen between 330 and 1140 nm, J. Geophys. Res. 95, 18577-18582, 1990

Hawat, T., C. Camy-Peyret, P. Jeseck, and R. Torguet, Description and performances of a balloon-borne heliostat for solar absorption measurements, Proc. 12th ESA Symp. European rocket and balloon programmes and related research, Lillehammer, 29 May - 1 June 1995, ESA SP-370, September 1995.

 
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