Currently, no standard protocol exists for making nonpoint source measurements of air emission flux. An accurate and cost-effective method is needed to quantify nonpoint air emissions from Department of Defense (DoD) sources. Current estimation techniques for nonpoint sources based on emission factors are imprecise and typically overestimate emissions. Multiple point measurements are locally accurate but may not be representative of the entire plume, and it is costly to obtain the quantities required. The objective of this project was to demonstrate standardized nonpoint source measurement methods based on path-integrated optical remote sensing (PI-ORS) and computed tomography developed by ARCADIS.
PI-ORS (e.g., open-path Fourier-transform infrared [OP-FTIR] spectroscopy) uses multiple beam paths and optimizing algorithms to give a time-averaged, mass-equivalent concentration field across a plume of contaminant, from which the emission rate can be determined without dispersion modeling. Reflectors are deployed in a radial asymmetric pattern that includes the emission area source, from which the approximate boundaries of the plume's origin can be determined. When the plume is located, the array of reflectors is redeployed in a vertical plane immediately downwind and centered across the plume's origin. The PI-ORS system scans from reflector to reflector in a constant pattern, separately accumulating values for each reflector to generate a long-term average in each spatial element. The novelty is in placing a two-dimensional array of reflectors so the absorption information can be directly translated, by a tomographic algorithm, into time-averaged area concentrations without using dispersion model estimates. Source strength is effectively the product of the sum of the area concentration elements multiplied by the average wind speed during the determinations. This technology is also referred to as Radial Plume Mapping (RPM).
This project demonstrated that the RPM method is capable of measuring nonpoint sources to within 10% of the known mass of a release during stable atmospheric conditions. During unstable atmospheric conditions, measured values are generally within 20% of the true values. The horizontal RPM method also located hot-spots to within a distance that is less than 10% of the diagonal of the area being scanned. The RPM method was used successfully to quantify the emission flux from a variety of sources on DoD installations, including landfills, wastewater treatment plants, gas stations, and industrial maintenance complexes. To facilitate regulatory approval, an approved Environmental Protection Agency (EPA) protocol and method (OTM 10) were developed in collaboration with the EPA Emission Measurement Center.
Advantages of the PI-ORS/tomography technology include accuracy, low cost, and the ability to continuously track the flux in real-time. Real-time tracking of a nonpoint flux allows accurate measurement of intermittent or variable sources. The alternatives, using an array of Summa canisters and reverse dispersion models, both require intensive and expensive post analyses. PI-ORS/tomography avoids inaccuracies introduced by assumptions required for the dispersion modeling. PI-ORS also allows samples to be taken at intervals of seconds rather than hours, providing information on temporal variations of emissions that is not available with a canister method. (Project Completed - 2008)