We present a new computed tomography method, the low third derivative (LTD) method, that is particularly suited for reconstructing the spatial distribution of gas concentrations from path-integral data for a small number of optical paths. The method finds a spatial distribution of gas concentrations that (1) has path integrals that agree with measured path integrals, and (2) has a low third spatial derivative in each direction, at every point. The trade-off between (1) and (2) is controlled by an adjustable parameter, which can be set based on analysis of the path-integral data. The method produces a set of linear equations, which can be solved with a single matrix multiplication if the constraint that all concentrations must be positive is ignored; the method is therefore extremely rapid. Analysis of experimental data from thousands of concentration distributions shows that the method works nearly as well as smooth basis function minimization (the best method previously available), yet is about 100 times faster.

10aAir Flow10acomputed tomography10aConcentration mapping10aOptical remote sensing10apollutant dispersion1 aPrice, Phillip, N.1 aFischer, Marc, L.1 aGadgil, Ashok, J.1 aSextro, Richard, G. uhttps://energyanalysis.lbl.gov/publications/algorithm-real-time-tomography-gas02318nas a2200253 4500008004100000245008700041210006900128300001400197490000700211520148700218653001301705653002401718653002701742653002701769100002201796700002301818700002401841700002501865700002301890700002001913700002401933700002201957856008501979 2001 eng d00aRapid Measurement and Mapping of Tracer Gas Concentrations in a Large Indoor Space0 aRapid Measurement and Mapping of Tracer Gas Concentrations in a a2837-28440 v353 aRapid mapping of gas concentrations in air benefits studies of atmospheric phenomena ranging from pollutant dispersion to surface layer meteorology. Here we demonstrate a technique that combines multiple-open-path tunable-diode-laser spectroscopy and computed tomography to map tracer gas concentrations with approximately 0.5 m spatial and 7 s temporal resolution. Releasing CH_{4} as a tracer gas in a large (7 m×9 m×11 m high) ventilated chamber, we measured path-integrated CH_{4} concentrations over a planar array of 28 “long” (2–10 m) optical paths, recording a complete sequence of measurements every 7 s during the course of hour-long experiments. Maps of CH_{4} concentration were reconstructed from the long path data using a computed tomography algorithm that employed simulated annealing to search for a best fit solution. The reconstructed maps were compared with simultaneous measurements from 28 “short” (0.5 m) optical paths located in the same measurement plane. On average, the reconstructed maps capture ∼74% of the variance in the short path measurements. The accuracy of the reconstructed maps is limited, in large part, by the number of optical paths and the time required for the measurement. Straightforward enhancements to the instrumentation will allow rapid mapping of three-dimensional gas concentrations in indoor and outdoor air, with sub-second temporal resolution.

We investigate the possibility of performing tomographic pollutant mapping using path-integral data from non-intersecting optical paths, and conclude that such a geometry does allow reconstruction of the pollutant distribution with the smooth basis function minimization method. The simulated optical data are derived from actual pollutant concentration distributions determined from previous experiments.

10aOptical remote sensing10aSmooth basis function minimization10aTomography1 aPrice, Phillip, N. uhttps://energyanalysis.lbl.gov/publications/pollutant-tomography-using-integrated