The Tomographic Differential Optical Absorption Spectroscopy (Tom-DOAS) technique and results from a first application are presented. This novel technique serves to measure 2-3-dimensional concentration fields of different trace gases (e.g. NO 2, SO 2, ozone) in the planetary boundary layer on a small scale (100-10 000 m) by the use of multiple light paths. Ways to use telescopes, lamps, retroreflector arrays and spectrometers for the simultaneous measurement of many light-paths are proposed. The results from a first Long-path Tom-DOAS experiment next to a German motorway, which was part of the BAB II campaign April/May 2001 [Edition of the Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe/Universität Karlsruhe, pp. 45, 2001), are presented. From the measurements along 16 different light paths vertical profiles on both sides of the motorway as well as a two-dimensional map of the NO 2 vehicle emission plume are derived. The concentration distributions agree qualitatively with model simulations of an earlier campaign. © 2004 Elsevier Ltd. All rights reserved.