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Appendix D: Particulate Monitoring Instruments Commonly Used in New Zealand

 

Instrument Sampling Features
Beta attenuation monitor (BAM) Sampler fitted with PM10 or PM2.5 selective inlet. Some instruments fitted with dichotomous sampler to allow simultaneous monitoring of PM10 and PM2.5. Air is passed through the paper tape for a fixed time (a few minutes to 24 hours). Absorbance of ß–radiation by particles on paper tape.Instrument calibrated to give direct reading of the particle mass concentration in μg/m3.

Automated method. Continuous monitor suitable for unattended operation. Accuracy of measurements depends on geometry of the measuring head, strength of the beta source, and uniformity of thickness of the filter tape.
Tapered element oscillating microbalance (TEOM) Ambient air (heated to 40–50°C to prevent condensation) is drawn onto a filter through a standard PM10 sampling head. The filter is attached to the top of a vibrating, tapered glass tube. If required, the PM10 sampling head can be replaced with either a PM2.5 or PM1 head. Particles collected on the surface of the filter reduce the frequency of oscillation. Instrumental measurements of frequency are converted to mass. May be supplemented with a filter cartridge collection unit if subsequent chemical analysis required.

The measurements are made sequentially, giving continuous data. Measurements are usually made over a sampling average period of 15 minutes. Some semi-volatile particulates could be lost at the operating temperature. Relatively expensive. Main applications in well-funded and long-term air quality monitoring networks.
Partisol Sequential air sampler that could monitor different particle size fractions. A built-in pump draws ambient air through the sample inlet. The air passes through the filter for a specified period. Filters are automatically changed and stored. Gravimetric determination: the mass of particles collected is determined by the difference between the weight of the filter before and after exposure.

Filter cassettes can hold up to 16 filters. Continuous and unattended monitoring can be done for a period of up to two weeks. Has an RS232 interface for data retrieval and remote operation.
Hi-vol Hi-vol sampler fitted with size-selective inlet. Multi-stage cascade impactor inlets can also be used to determine the full particle-size distribution. Gravimetric determination: the mass of particles collected is determined by the difference between the weight of the filter before and after exposure. Membrane or glass fibre filters can be used depending on whether gravimetry alone or further analysis is required.

Continuous monitoring is possible but compliance monitoring requirements (changing of filters at midnight) may present operational difficulties. Some samplers can be connected to a datalogger and programmed to start/stop under specific wind directions and/or speeds. Many can be configured for either PM2.5 or PM10 fractions by changing the inlet head.
Light scattering method: optical particle counters Technique requires an appropriate flow rate to avoid erroneously low results. Most instruments operate at about 1 L/min. Examples are Casella APM950 Ambient Particulate Monitor, SKC Ltd Haz-Dust II and Grimm GmbH Series 1.200 Ambient Particle Size Dust Monitor. Light striking a particle within the measurement cell is scattered. The photomultiplier tube output is proportional to the size of the particle; also affected by shape, colour and the refractive index of the particle. Results obtained relate only to equivalent polystyrene spheres (used for calibration) giving the same magnitude of light pulses and should be referred to as equivalent optical particle sizes rather than true particle sizes. Some operate using infrared light or laser light.