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62 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.4.5 An Indoor Test Campaign of the Tomography Long Path Differential Optical Absorption Spectroscopy (DOAS) Technique Participating scientists Kai Uwe Mettendorf, Andreas Hartl and Irene Pundt Abstract For the validation of the LP-DOAS-tomography method and the test of the Multibeam DOAS instrument an indoor validation campaign was performed. Known concentration distributions were successfully measured by 39 intersecting light paths and reconstructed using the SIRT-method. Figure 2.29: Reconstruction of a concentration distribution by SIRT on a 12×12 pixel grid and with additional grid shifting. The upper panel shows the original concentration distribution, the lower left panel the result from modelled column densities and the lower right panel the result from measured column densities. Background Long path DOAS tomography is a novel application of the DOAS-method which was not been validated before. Funding DOAS Tomography (BMBF) Methods and results In this study the twodimensional long path DOAS tomography measurement technique was validated by an indoor experiment with well-known concentration distributions. The experiment was conducted over an area of 10m×15m using one and two cylindrical containers of diameter 2 m, respectively, filled with NO2. The setup was realized with three of the recently developed Multibeam instruments (Pundt and Mettendorf, 2005), which allow the simultaneous measurement along at least four light paths each. The configuration consisted of twelve simultaneous light beams, 39 horizontal light paths in total, and 16 different cylinder positions inside the field. It was found that for the discretization and inversion technique shown here reconstructions of the concentration distributions from experimental data agree well with simulated reconstructions. In order to draw conclusions for atmospheric applications, numerical studies including instrumental errors were carried out. It was concluded that with the presented measurement setup it should be possible to measure and reconstruct one or two NO2 plumes of 600 m diameter and average concentrations above 4.2 ppbv each, to a scale of 13.5 km 2 . Theoretical investigations show that it should be possible to localize and quantify 600 m diameter plumes of SO2 > 1.5 ppbv, H2CO > 6.3 ppbv, HONO > 3.2 ppbv, and O3 > 46.2 ppbv. Main publication Mettendorf [2005]
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]
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Institut für Umweltphysik und Arbe
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CONTENTS 7 3.2.1 Glaciological pilo
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