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Evaluation of Optical Instruments for Real-Time Continuous Monitoring of Smoke Particulates
The Andersen RTAA 800 aethalometer (figure 5) collects "elemental" or "black" carbon (BC) on a quartz fiber tape, measuring the optical absorption continuously while the spot of aerosol accumulates. The optical absorption estimations are converted to estimate the mass concentration of BC in µg/m³. A display on the front of the instrument shows the calculated BC concentration. A keypad allows the user to interact with the embedded computer. Data is automatically stored on an internal floppy disk. The instrument also has an RS-232 port and an analog output. The instrument can be configured with a particulate-size cutoff device for selective measurements. The instrument runs on 110 V ac.
Figure 5—The Andersen Instruments RTAA 800 aethalometer.
Gravimetric-based instruments were used in the evaluation to compare their results to those of the real-time samplers. Gravimetric- or filter-based instruments work by drawing air at a controlled rate through a filter that collects the fine particulate matter. The filter is carefully weighed at a special facility before and after sampling the air. This method provides very accurate results of the quantity of particulate that was collected during the test period. The particulate mass value is divided by the total volume of air drawn through the filter, yielding the average mass concentration for the test period, typically in µg/m³. The gravimetric instruments can be configured with impactor or cyclone size-selector devices to remove large particles. Three different gravimetric instruments were used in the evaluation; two Federal Reference Method PM2.5 air samplers and a PM2.5 sampler developed by the Rocky Mountain Research Station's Fire Sciences Laboratory (Fire Sciences Laboratory) in Missoula, MT.
Two different manufacturers of FRM PM2.5 samplers were used in the evaluation: the BGI, Inc., PQ200 (figure 6) and the Partisol FRM Model 2000 manufactured by Rupprecht and Patachnick (R&P). They both have similar design, performance characteristics, and operational requirements. Both are microprocessor-controlled, volumetric flow rate air-sampling instruments that obtain a valid PM2.5 air sample. The particulates are collected on 47-mm Teflon membrane at a volumetric sample rate of 16.67 L/min after being size discriminated through two inertial separators designed by the U.S. Environmental Protection Agency.
Figure 6—The BGI PQ200 Federal
Reference Method PM2.5 sampler.
Ambient temperature and barometric pressure measurements are made at actual sample conditions. A microprocessor and volumetric flow control system are integrated to maintain sampling parameters while sampling data are continuously logged into the processor memory. Memory stores 5-minute actual ambient temperature and pressure conditions along with volumetric sample flow rate, filter temperature, and pressure. The operator recovers measured values and flags indicating anomalies by downloading the summary to a laptop computer. The instruments may be powered by an internal battery, by external batteries, or by solar power.
Filters collected by the BGI PQ 200 were weighed at the Montana Department of Public Health and Human Services Environmental Laboratory in Helena, MT. Filters collected by the R&P Partisol Model 2000 sampler were weighed at IML Air Science in Sheridan, WY.
This gravimetric instrument was developed by the Fire Sciences Laboratory for conducting airborne smoke studies. The instrument uses a computer-controlled volumetric airflow controller to draw air over a 37-mm Teflon filter at a rate of 28.8 L/min. The air is first drawn through a size-selected cyclone device to remove particulate larger than 2.5 µm. The filters were weighed in a special environmentally controlled facility located at the Fire Sciences Laboratory.
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