Airborne imaging DOAS measurements (iDOAS)
A new imaging DOAS (iDOAS) instrument has been developed at IUP Bremen in 2011. The instrument records scattered and reflected solar radiation in the visible wavelength region. From the measurements, molecular absorption structures are identified and analysed by applying the DOAS technique. The iDOAS instrument is capable of measuring amounts and spatial distributions of tropospheric trace gases, e.g., nitrogen dioxide, NO2. This is useful for pollution mapping, identification of source regions and source strengths, emission fluxes as well as satellite data validation.
Imaging DOAS observations from an airborne platform have many advantages. Several viewing directions are observed at the same time, i.e., a broad stripe below the aircraft across track is imaged onto the instrument detector. iDOAS provides good spatial resolution on the order of 100 m, at useful spatial coverage. In general, aircraft observations form a link between ground-based observations and satellite measurements.
There are two major specialities about the iDOAS instrument. A frame transfer CCD detector is used so that subsequent measurements are recorded without temporal gaps in between. In addition, an optical light guide with sorted fibres is used which allows flexible integration of the instrument in the aircraft.
First test flights have been conducted during a flight campaign in June 2011 onboard the AWI Bremerhaven aircraft Polar-5. The new iDOAS instrument performed well and operated reliably. It proves to be a suitable device for the observation of pollution sources and similar features. Further flight campaigns are foreseen for the future.
Figures: (Left) Polar-5 aircraft at the hangar in Bremerhaven. (Right) iDOAS installed inside the Polar-5 aircraft in Jun 2011.
The special setup allows gap-free measurements (due to frame transfer CCD) and flexible positioning in aircraft (due to sorted fibre bundle).
Sketch showing the influence of the aircraft angles (pitch, roll and yaw) on the ground geolocation
Influence of aircraft angles on geolocation:
The imaging quality of the iDOAS instrument is very good and allows recording of small scale sources. In the figure above, the left figure shows the CCD intensity image (in absolute Counts). Spatial information is distributed along the vertical axis, spectral information along the horizontal axis. The stripes are generated by the individual fibres. The right image shows the photograph of the nadir picture camera above a brightly reflecting motorway in between darker fields to the sides. The blue box marks the field of view corresponding to the instant when the CCD image on the left was recorded. The high radiation intensity on the CCD image is nicely visible in one of the fibres only.
During a flight on June 4th in 2011, a power plant exhaust plume has been observed emerging from a black coal power plant. The recorded spectra have been analysed for the absorption signal of NO2. The settings for the DOAS retrieval are listed below. The enhanced NO2 amounts above the power plant are clearly visible in the NO2 map. Large variations of the NO2 amounts across and along track are observed
Figure: Preliminary result of NO2 slant columns above a black coal power plant. The exhaust plume is clearly visible with enhanced NO2 amounts downwind of the power plant stack. From the slant columns, the vertical columns and respective volume mixing ratios are computed by taking into account the radiative transfer conditions as well as an NO2 altitude profile.