Rapid measurements and mapping of tracer gas concentrations in a large indoor space

Rapid mapping of gas concentrations in air benefits studies of atmospheric phenomena ranging from pollutant dispersion to surface layer meteorology. H ere we demonstrate a technique that combines multiple-open-path tunable-dio de-laser spectroscopy and computed tomography to map tracer gas concentrati ons with approximately 0.5 m spatial and 7 s temporal resolution. Releasing CH4 as a tracer gas in a large (7 m x 9 m x 11 m high) ventilated chamber, we measured path-integrated CH4 concentrations over a planar array of 28 " long" (2-10m) optical paths, recording a complete sequence of measurements every 7 a during the course of hour-long experiments. Maps of CH, concentra tion were reconstructed from the long path data using a computed tomography algorithm that employed simulated annealing to search for a best fit solut ion. The reconstructed maps were compared with simultaneous measurements fr om 25 "short" (0.5m) optical paths located in the same measurement plane. O n average, the reconstructed maps capture similar to 74% of the variance in the short path measurements. The accuracy of the reconstructed maps is lim ited, in large part, by the number of optical paths and the time required f or the measurement. Straightforward enhancements to the instrumentation wil l allow rapid mapping of three-dimensional gas concentrations in indoor and outdoor air, with sub-second temporal resolution. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.