From zenith to the horizon
For decades passive differential optical absorption spectroscopy (DOAS) was essentially based on the observation of light scattered in the zenith (elevation angle = 90°). These zenith scattered sunlight measurements in the visible/near ultraviolet spectral range were used to study several stratospheric species such as, e.g., O3, NO2, OClO, and BrO. To gain sensitivity for tropospheric species (e.g., NO2, HCHO, glyoxal, and SO2), a new type of DOAS instruments was developed allowing for off-axis (elevation angle = 0-90° for a fixed azimuth angle) observations in addition to the zenith geometry. These multi-axis DOAS (MAXDOAS) measurements were employed to retrieve stratospheric and tropospheric trace gas total columns and profiles. Recently we developed a "new generation" of our MAXDOAS instruments. New features were implemented, among them a capability for sun tracking. The new instruments do not only allow for full flexibility in the choise of azimuth (0-360°) and elevation angles (0-90°) but can also perform direct sun measurements. First tests indicate that with these new features we could retrieve improved information on tropospheric aerosol (extinction profile, phase function) allowing us to improve the tropospheric trace gas profile retrievals. In the following section we will give a brief description of the instrument.
The BIRA-IASB MAXDOAS instrument
The newly developed MAXDOAS instrument, schematically represented in the preceding figure, consists of three main parts: a thermo-regulated box (A) containing two spectrometers (B and C) located inside, the optical head (D) mounted on a suntracker (E) located outside connected to the spectrometers via optical fibers, and the controlling and acquisition unit ( F and G).
The BIRA-IASB MAXDOAS instrument is a dual-channel system. A two-way splitter fiber optic bundle with rectangular terminations links the output of the optical head with the two spectrometers. The optical head design is such that the telescope (D.1) can be moved in a wide range of elevation (0-90°) as well as azimuth directions (0-360°). In addition the optical head is mounted on a commercial sun tracker from the BRUSAG company (INTRA). This set-up enables not only scattered light, but also direct-sun measurements. The sky radiances are collected by an off-axis parabolic mirror (D.2) within a 1° field of view. The optical head also includes a 6-position filter wheel (D.3) equipped with transmission diffuser plates, (eventually) linear polarizers, and band pass filters.
The optical head collects the direct-sun and scattered light at various elevation and azimuth angles. The light is guided to the two spectrometers through a two branches fiber optic assembly. The first spectrometer (B) covers the UV region from 300 to 390 nm, while the second spectrometer (C) extends from 400 nm to 720 nm. The output of each spectrometer is connected to a low-noise thermo-electrically cooled CCD detector (I and H). The whole system is mounted inside a container (A) thermally regulated by a Pelletier cooling system (J) to minimize thermal stress on mechanical and optical parts.