More than twenty years have passed since the concept of combining Computed Tomography (CT) and Optical Remote Sensing (ORS) was first suggested to map air contaminants. However, there have been no commercial applications of CT-ORS due to a variety of reasons including hardware limitations and slow acceptance by the occupational and environmental scientific communities. A CT-ORS monitoring system provides the potential for near real-time mapping of multiple gases over large areas. Not just another nifty tool, this technology represents a major departure from conventional sampling methods and could allow us to understand chemical transport and exposure in ways, which are unavailable using conventional methods. Yet critical questions remain unanswered: which contaminant sources are most appropriate for this technology and how does this data apply to human exposure assessment? We discuss potential applications of CT-ORS, such a using open-path Fourier Transform Infrared spectroscopy for mapping leaks and evaluating worker exposures and quantifying emission flux from a process facility. Large scale (greater than 1 km) CT reconstructions could be obtained from a variety of ORS devices (Tunable Diode Laser, Differential Optical Absorption Spectroscopy, or Differential Absorption Lidar). Reconstructions could help locate industrial emissions and provide improved estimates of pollutant transport.
This paper presents a new approach to quantify emissions from fugitive gaseous air pollution sources. We combine Computed Tomography (CT) with Path-Integrated Optical Remote Sensing (PI-ORS) concentration data in a new field beam geometry. Path integrated concentrations are sampled in a vertical plane downwind from the source along several radial beam paths. An innovative CT technique, which applies the Smooth Basis Function Minimization (SBFM) method to the beam data in conjunction with measured wind data, is used to estimate the total flux from an area source. We conducted simulation study to evaluate the proposed methodology under two beam geometry and configurations. This approach was found to be robust for a wide range of fluctuating wind directions. In the very sparse beam geometry we examined (5 beam paths), successful emission rates were retrieved over a 70 degree range of wind directions.
We conducted a preliminary experiment in controlled ventilation chamber where a single source of Nitrous Oxide was released. A scanning Open Path Fourier Transform Infrared (OP- FTIR) system acquired Path Integrated Concentration (PIC) data of 19 beams scanned in a radial non-overlapping beam geometry. Prior to the experiment we conducted a calibration procedure by creating a homogeneous atmosphere inside the ventilation chamber. The Smooth Basis Function Minimization (SBFM) algorithm, which fits parametric distributions rather than fitting individual pixel concentrations, was used to reconstruct two-dimensional concentration maps from this beam geometry. The preliminary results show that good reconstructions are possible with this approach. Further, our calibration procedure could be suitable for any open path optical remote sensing instruments. In contrast to the complex beam geometries proposed in the past for CT, this radial scanning technique could be applied directly to air monitoring data from a variety of current optical sensing instruments. This development could vastly broaden the application of CT to obtain rapid reconstructions of ambient air pollution data.
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