Airborne Multi Axis DOAS measurements (AMAXDOAS)
As part of the German contribution to the validation of the SCIAMACHY instrument on board of ENVISAT, a set of instruments was flown on the DLR FALCON in several campaigns in high, middle and low latitudes. The instruments involved are the ozone and aerosol LIDAR OLEX (operated by DLR Oberpfaffenhofen), the microwave radiometer ASUR (operated by the University of Bremen) and the multi axis DOAS instrument AMAXDOAS (operated by the Universities of Bremen and Heidelberg).
The AMAXDOAS instrument is similar to the DOAS instruments used on the
ground but observes the atmosphere simultaneously in different directions, both upwards and downwards. The rationale for using
different viewing directions is to derive information not only on the total column of absorbers, but also on their vertical
distribution. The most straightforward information comes from the difference between nadir and zenith viewing measurements that
enables tropospheric and stratospheric columns to be separated. These quantities are of particular interest as SCIAMACHY will
measure total columns and stratospheric profiles of O3, NO2, BrO, H2O and other trace species
quasi simultaneously. The difference between total column and integrated profile yields the tropospheric column. The AMAXDOAS
instrument provides direct validation of these partial columns. Additional information can be retrieved from the slant viewing
geometries, but this is a more experimental measuring mode that still has to be developed further.
The experimental set-up of the AMAXDOAS consist of two imaging grating
spectrometers, one for the UV and one for the visible wavelength region. Two domes are mounted on top and below the FALCON, each
housing several small telescopes pointing in the atmosphere at different angles. Quartz fibres conduct the light from the
telescopes to the spectrometers, where the spectra from the individual viewing directions are projected on different parts of the
CCD detectors. With this set-up, spectra from light with very different optical paths through the atmosphere can be measured
Apart from the possibility to separate tropospheric from stratospheric
columns, air borne measurements have other important advantages for the validation of SCIAMACHY measurements. In contrast to balloon borne
experiments, measurements from the FALCON can provide continuous time series along the satellite track. Also, the horizontal
inhomogeneity of SCIAMACHY ground pixels can be explored by selecting a flight track perpendicular to the satellite orbit. To
optimise the output of the experiments, FALCON measurements were closely co-ordinated in space and time with balloon launches during
the SCIAMACHY validation campaigns.