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 x 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.

2-D reconstruction of atmospheric concentration peaks from horizontal long path DOAS tomographic measurements: parametrisation and geometry within a discrete approach

“…Conventional LP-DOAS instruments emit one light beam only and are not very easily pointed at different targets. Here we refer to an indoor experiment (Mettendorf et al, 2006) designed to evaluate the properties of a novel instrument which emits up to six beams at once . Three telescopes emitting four beams were used to cover a horizontal area of 15×10 m 2 regularly by three 90 • -beam fans sitting in the corners of the area with 39 light paths in total (3×12 light paths plus one additional per telescope for instrumental reasons that are irrelevant for atmospheric measurements.…”