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Evaluation of Optical Instruments for Real-Time Continuous Monitoring of Smoke Particulates
We surveyed commercial off-the-shelf instruments that would meet the Forest Service's needs for real-time continuous smoke sampling at a reasonable cost in remote locations. Our survey originally included gravimetric and optical devices. We narrowed our evaluation to optical devices because they can provide smoke or air quality managers with continuous, real-time estimates of particle concentrations.
Optical instruments use light absorption, forward-scattering, or backscattering to infer the presence of particles. Based on the size, shape, and refractive index of the particles, a series of equations relates the amount of light scattered or absorbed to the particulate concentration. Aerosols having a consistent size and shape distribution can be estimated fairly accurately with these equations. However, smoke particles have a wide variety of sizes and shapes and their physical characteristics are dependent on the fuel characteristics and on whether the particles were formed during flaming or smoldering combustion. Once in the atmosphere, the particles continue to change with temperature, moisture content of the air, sunlight exposure, and other factors.
The greatest amount of change occurs immediately after combustion, but the process continues for days at a reduced rate. Because of this variability in the size and shape of the smoke particles, it is difficult using current optical techniques to define a highly accurate equation for determining particulate matter concentrations. Another difficulty is that aerosols have different light-scattering and absorption responses, depending on the orientation of the light receiver to the particles and to the light source. The response from a forward-scattering device may differ from that of a backscattering device. We did not evaluate whether the algorithms selected by the instrument vendors were appropriate for a particular biomass smoke event or not.
Our standard measure during most of the field tests and during the 2000 test was provided by two Federal Reference Method monitors, the Partisol Model 2000 and the BGI PQ200. The FRM monitor is gravimetric and has been selected by the U.S. Environmental Protection Agency as the approved method for measuring fine particles (2.5 µm and smaller). For the purpose of this study, the two monitors compared well against each other, showing less than 3-percent variability. A gravimetric device built and used by Forest Service research for aerial particle sampling was used as the standard for the 1998 high concentration evaluation. On average, it reported concentrations about 10 percent lower than the FRM.
The five brands of optical instruments we evaluated were similar in many ways. All of them were more or less portable for one person and rugged enough to withstand frequent relocation in the field. All but the aethalometer were battery powered. Each instrument was reasonably easy to operate and acceptably reliable. The brands for which we had two identical instruments showed good measurement consistency between instruments. In other words, there were few fundamental discriminating characteristics other than accuracy that showed one or more instruments to be clearly superior to the others. Our summary of some instrument features (table 2) and our subjective rating of characteristics other than accuracy may help users choose one instrument over another for a particular application (table 3).
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