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2.4. DOAS TOMOGRAPHY GROUP 63
2.4.6 2D and 3D tomographic LP-DOAS measurements of trace gas distributions
in Heidelberg over an area of 4 ∗ 4 km 2
Participating scientists Denis Poehler, Alexander Stelzer, Irene Pundt
Abstract A configuration of three Multibeam LP-DOAS Instruments and 20 different light paths
to retro reflector arrays was established for long term 2D and 3D measurements of NO2, SO2, O3,
HCHO, and HONO over an area of a 4 ∗ 4 km 2 above the city centre of Heidelberg.
Figure 2.30: 3 Dimensional view on all light paths of the measurement over an area of 4 ∗ 4 km 2 with
three telescope locations: IUP, SAS and HD-Druck.
Background LP-DOAS (Long Path Differential
Absorption Spectroscopy) is a well known
technique for measuring average concentrations of
tropospheric trace gases along a light path extending
between a telescope and a retro reflector. But
single path measurements are often not sufficient
(as well as single point measurements), if smallscale
variations exist or transport is important.
To obtain informations of the dispersion, it is necessary
to measure along a couple of light paths.
The development of the ”Multibeam LP-DOAS
telescope” (Pundt and Mettendorf, 2005), allows
the measurement along up to 6 light paths simultaneously.
With the suitable combination of
two or more Multibeam LP-DOAS telescopes and
the use of tomographic reconstruction techniques
good quality 2D and 3D reconstructions of the
measured trace gases can be achieved.
Funding German Ministry of Research and Education
, AFO 2000-C, project 07 ATC-03
Methods and results The experimental setup
consists of 3 Multibeam telescopes located on top
of high buildings in the city (IUP, HD-Druck and
SAS) and 20 retro reflector arrays. This work focuses
on the one hand on the telescope location
HD-Druck, where a Multibeam Instrument was
installed and modified, to allow automatic measurements
from this place.
Each of the three telescopes uses a Xe-Arc lamp as
light source, which creates up to 6 parallel light
beams using different mirrors. Each light beam
can be redirected to different retro reflector arrays
which reflect parts of the light back into the
telescope. The received light is measured with a
Czerny Turner Spectrograph and contains information’s
of the trace gas absorptions. With the
measurement between 285 nm and 365 nm and
the use of the DOAS technique reconstruction of
NO2, SO2, O3, HCHO, and HONO are made
along each light path.
During the first measurements it was mostly impossible
to run all three telescopes simultaneously,
due to many breakdowns of the sensitive experimental
setup. But since simultaneous measurements
of all light paths are necessary for tomographic
inversions, the hardware was modified and
new controller software was coded to improve the
stability. First simultaneous measurements were
performed in September 2005, but are not completely
analysed yet.
Outlook/Future work The system will be run
continuously over a time scale of several months.
For the periods of simultaneous measurement
data, 2 dimensional trace gas distributions will
be reconstructed.
Main publication Poehler et al. [2005]