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Real-Time
Smoke
Particulate Sampling
Fire Storm 2000
Three categories of instruments are discussed in this report: optical real-time, continuous particulate monitors, mass quantifying real-time continuous particulate monitors, and gravimetric (filter-based) samplers.
Two types of optical real-time, continuous particulate monitors were used in this evaluation: predominantly light-scattering instruments and predominantly light-absorbing instruments. Light-scattering instruments, called nephelometers, measure the amount of light scattered over a known path length and use a mathematical relationship to estimate the aerosol mass concentration. The light-scattering coefficient (bsp) is determined by illuminating particles, individually or as a group, and measuring the scattered intensity at different orientations from an incident light source. The orientation of the light source to the particles and receiver will determine whether the instrument is primarily back scattering (sun is at the viewer’s back and light is scattered back to viewer), forward scattering (sun is at the viewer’s front where light is scattered to the viewer), or total integrating (considers both back and forward scattering).
Light-absorbing instruments, called aethalometers, quantify the light-absorbing aerosol (black carbon, for example) by depositing the aerosol on a quartz-fiber filter and measuring the light transmission or reflectivity. Aethalometers also compute measured light attenuation due to black carbon.
Met One GT-640
The Met One GT-640 particulate monitor (figure 9) is a complete ambient air sampler using a forward light-scattering detector and built-in data logger. A laser optical sensor detects and measures particulate concentrations up to 10,000 µg/m³. Built-in calibration functions are included. The unit has an internal relative humidity sensor that turns on an inlet heater at 55-percent relative humidity.
Figure 9—The Met One GT-640 particulate monitor
with meteorological instrumentation.
The monitor can be configured with either a PM10 (particulate matter finer than 10 µm) or PM2.5 (particulate matter finer than 2.5 µm) cutoff inlet. Total suspended particulate concentrations can be estimated by removing the cutoff device. Data are digitally recorded and stored with time and date information. Stored data are retrieved through an RS-232 port connected to a laptop computer or through an external modem.
The logger will record concentrations automatically, along with date and time, whenever power is applied. Optional connections on the bottom of the GT-640 allow various meteorological sensors to be attached. All internal components are housed in a weatherproof enclosure. The unit can be powered by an ac or dc power source.
The DataRam (figure 10) is a compact, self-contained instrument that internally estimates mass concentration from the measured scattering of light. The instrument can measure particulate concentrations from 0.1 to 400,000 µg/m³, according to the vendor. The instrument continuously displays the current and time-weighted average mass concentration while logging up to 10,000 data points. Data can be downloaded from the instrument through an RS-232 port. The DataRam can be configured with either a PM2.5 or PM10 impactor head to prevent particles larger than 2.5 or 10 µm from entering the optical chamber. For custom calibrations, or to analyze the chemical composition of particulates, the particulates can be collected on a 37-mm filter located in the instrument’s base. An inline heater may also be installed for monitoring in humid conditions (the manufacturer suggests using the heater when the relative humidity is higher than 70 percent). The instrument’s tubular heater is designed to heat the sampled air stream to evaporate liquid water from airborne particles or to eliminate fog droplets. The DataRam is powered by an internal rechargeable battery or by an external dc or ac power source. The DataRam has a built-in, internal calibration device.
Figure 10—The MIE DataRam.
The Optec NGN-3 PM2.5 size-cut nephelometer (figure 11) 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 has been used by the IMPROVE interagency 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 back scattering is converted to mass concentration using a region-specific, user-selected empirical conversion factor. The NGN-3 continuously outputs both back scattering and a fine-mass concentration estimate with a minimum integration time of 2 min. 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.
Figure 11—The Optec NGN-3 PM2.5 size-cut nephelometer.
The NGN-3 draws ambient air through a sample inlet line. A spiral inlet manufactured by SKC 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.
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