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Evaluation of Optical Instruments for Real-Time Continuous Monitoring of Smoke Particulates
The Radiance Research Portable Nephelometer, Model M903 (figure 3) is a lightweight, low-power instrument designed for portable operation as well as general environmental monitoring. The M903 measures and displays Bscat (backscattering). It does not display a computed estimate of mass concentration as do the other nephelometers in the evaluation. Mass concentrations can be estimated from the Bscat readings. The instrument has a particulate measurement range of approximately 1 to 1,000 µg/m³ when mass concentration is estimated from Bscat. The instrument has an internal data logger that will store scattering coefficient averages and the operating parameters that are used to estimate the Bscat. The stored data can be retrieved using a personal computer through an RS-232 port. Different averaging times and log intervals may be set. The instrument can store approximately 2 weeks of 5-minute averages. A source of a gas, such as Freon, is required for calibration. The unit can be powered by an ac or dc power source.
Figure 3—The Radiance Research nephelometer,
Model M903, with an attached inlet heater.
The Optec NGN-3 PM2.5 size-cut nephelometer (figure 4) is a self-contained instrument developed to estimate PM2.5 aerosol scattering and mass concentrations. The NGN-3 is based on the Optec NGN-2 ambient nephelometer that is used by the IMPROVE program for visibility studies and in other applications. The NGN-3 integrates the optical design of the NGN-2 ambient nephelometer with both a PM2.5 size-cut separator and an inline sample heater to measure the dry scattering fraction of extinction by fine-mass aerosols. Once measured, the Bscat is converted to mass concentration using a region-specific, user-selected empirical conversion factor. The NGN-3 continuously outputs both Bscat and a fine-mass concentration estimate with a minimum integration time of 2 minutes. The NGN-3 has no internal data storage capabilities, outputting data in serial or analog form. Portable remote monitoring data loggers such as the Campbell Scientific CR-23X or a laptop computer are needed for data storage.
The NGN-3 draws ambient air through a sample inlet line. An SKC spiral inlet mounted at the sample air inlet removes coarse particulates from the sample stream. The sample air is heated (to lower the relative humidity) as it enters the nephelometer. The temperature of the heated air is output continuously. To ensure monitoring accuracy, the instrument performs automatic zero calibrations at user-defined intervals. Manual zero and span calibration checks may be performed at any time. An external span gas such as Freon is required for calibration. The NGN-3 can be powered by either an ac or dc power source.
Figure 4—The Optec NGN-3 PM2.5
size-cut nephelometer.
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