Abstract. This paper presents a procedure for performing and optimizing inversions for DOAS tomography and its application to measurement data. DOAS tomography is a new technique to determine 2-and 3-dimensional concentration fields of air pollutants or other trace gases by combining differential optical absorption spectroscopy (DOAS) with tomographic inversion techniques. Due to the limited amount of measured data, the resulting concentration fields are sensitive to the inversion process. Therefore detailed error estimations are needed to determine the quality of the reconstruction. In this paper we compare different row acting methods for the inversion, present a procedure for optimizing the parameters of the reconstruction process and propose a way to estimate the error-fields by numerical studies. The procedure was applied to data from the motorway emission campaign BAB II. Two dimensional NO 2 cross sections at right angles to the motorway could be reconstructed qualitatively well at different meteorological situations. Additionally we present error fields for the reconstructions which show the problems and skills of the used measurement setup. Numerical studies on an improved setup for future motorway campaigns show, that DOAS tomography is able to produce high quality concentration maps.
A novel long-path differential optical absorption spectroscopy (DOAS) apparatus for measuring tropospheric trace gases and the first results from its use are presented: We call it the multibeam instrument. It is the first active DOAS device that emits several light beams simultaneously through only one telescope and with only one lamp as a light source, allowing simultaneous measurement along multiple light paths. In contrast to conventional DOAS instruments, several small mirrors are positioned near the lamp, creating multiple virtual light sources that emit one light beam each in one specific direction. The possibility of error due to scattering between the light beams is negligible. The trace-gas detection limits of NO2, SO2, O3, and H2CO are similar to those of the traditional long-path DOAS instrument.
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