Tomographic Reconstruction of 2-D Atmospheric Trace Gas Distributions from Active DOAS Measurements

Applying the tomographic principle to active DOAS remote sensing leads to a novel technique for the measurement of atmospheric trace gas distributions. Standard analytical methods for the reconstruction of a scalar field from its line integrals cannot be used due to low numbers of light paths (10-50) and their irregular arrangement, so that the concentration field is expanded into a limited number of local (piecewise constant or linear) basis functions instead. The resulting discrete linear inverse problem is solved by a least squares-minimum norm principle. For sharp 2-D concentration peaks it is shown systematically with respect to their extension how the optimal choice of parametrisation (in terms of number and kind of basis functions) and a priori can tremendously improve the reconstruction. Regularisation plays a minor role. Proposals for retrieving peak distributions by combining different parametrisations are again examined systematically showing that their usefulness heavily depends on the features one is most interested in. Comparison of different 2-D light path geometries reveals that linear independency within the associated systems is pivotal. A detailed analysis of the reconstruction error points out special issues of tomography with only few integration paths and argues that a complete error estimation is not possible without a priori assumptions. Based on this discussion a numerical scheme for calculating the reconstruction error is suggested. The findings are applied to an indoor experiment simulating narrow emission puffs and 2-D model distributions above a street canyon, respectively. For the latter case it is demonstrated how model evaluation can be possible even with a relatively small number of light paths. Contrary to the reconstruction of peak distributions regularisation becomes crucial.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Heidelberg

Year

2007

Author dc:creator

• Hartl, Andreas

Contributors dc:contributor

• Platt, Ulrich

Repository record source_url

http://www.ub.uni-heidelberg.de/archiv/7173

OAI identifier oai:identifier

oai:archiv.ub.uni-heidelberg.de:7173