An indoor test campaign of the tomography long path differential optical absorption spectroscopy technique

Abstract

In this study we validate the two-dimensional long path DOAS tomography measurement technique by means of an indoor experiment with well-known concentration distributions. The experiment was conducted over an area of 10 m × 15 m using one and two cylindrical polycarbonate containers of diameter 2 m, respectively, filled with NO₂. The setup was realized with three of the multibeam instruments recently developed by Pundt and Mettendorf (Appl. Opt., 2005, in press), 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 18 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 found that with the presented measurement setup it is possible to measure and reconstruct one or two NO₂ plumes of 600 m diameter and average concentrations above 4.2 ppbv each, on a scale of 13.5 km². 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 ozone > 46.2 ppbv. Larger plumes can be measured with higher precision.