Archived publication
This publication is no longer current or has been superseded.
2 Monitoring Methods for Particles in New Zealand
2.1 Total suspended particulate
Total suspended particulate (TSP) samplers have no size selective inlets and therefore include measurements of all airborne particles, which are typically in the range of 0-40 µm. The measurement methods are traditionally high-volume gravimetric sampling. Historically, samples of TSP have been collected in New Zealand for the purposes of analysing lead concentrations and have generally been collected as seven-day average concentrations.
Total suspended particulate include both fine and coarse mode particles. Particles greater and less than 2.5 microns in diameter are generally described as coarse and fine, respectively. Fine mode particles typically arise from combustion processes and atmospheric chemistry, whereas coarse mode particles tend to arise from mechanical processes such as grinding, sanding and abrasion. However, there is generally some overlap, with some of the finer mechanically derived particles existing in the fine fraction and conversely some of the larger combustion particles in the coarse fraction. Wind blown dusts, pollens and sea spray typically reside in the coarse fraction.
The relationship between TSP and PM10 concentrations will typically vary with location, season and meteorology. This is because of the differences in the processes giving rise to elevated concentrations of fine versus coarse mode particles. Consequently it does not follow that trends in TSP concentrations will reflect similar variations in PM10 concentrations.
2.2 Smoke monitoring
The British Standard definition for black smoke (BS 1747) is based on a non-gravimetric reflectance method in which air is sampled through a filter and the resulting blackening measured. A conversion from black smoke to mass equivalent was established in the United Kingdom in the 1960s, based on domestic coal smoke emissions, as that was the main source of emissions at that time.
It is likely, given the apparent absence of other conversion factors, that smoke monitoring carried out in New Zealand has been converted into mass concentration data based on the relationship derived in the UK for coal smoke. In most New Zealand locations this is likely to be inappropriate as coal burning is not likely to be a significant contributor to measured particle mass. Because the relationship between smoke blackness and mass will vary with different sources and compositions of particles, it is unlikely that the conversion derived for black smoke in the UK is suitable for New Zealand. Consequently historical smoke monitoring data for most New Zealand locations are unlikely to be indicative of actual PM10 concentrations.
2.3 Gravimetric sampling for PM10
Gravimetric sampling for TSP, PM10 or PM2.5 includes a number of methods that draw air across a filter for a specified period, typically 24-hours, and measure the mass collected by weighing the filter prior to and following the sampling period. The filter is preconditioned and reconditioned prior to weighing and reweighing.
These include both high-volume sampling methods that have flows of around 1133 litres per minute and low volume methods that have much lower flow rates. Filter sizes and materials also vary between methods. Samplers should be operated strictly in accordance with manufacturer's specifications as variations in filter media and flow rates can impact on collection efficiencies and the selection of appropriately sized particles.
The main advantages of the gravimetric methods are the relatively low sampler costs and the consistency with the USEPA reference method specifications. Disadvantages include no information on temporal distributions of data and high labour costs associated with changing, conditioning and weighing filters.
The USEPA reference method for PM10 described in US 40 CFR Part 50, Appendix J specifies gravimetric sampling. Alternative methods meeting equivalency specifications are also allowed for PM10 measurements. Gravimetric sampling for PM10 concentrations is one of the most common methods used for measuring PM10 concentrations in New Zealand.
2.4 Beta attenuation
The beta attenuation or beta gauge method operates by drawing air through a continuous glass or PTFE tape at a rate of 16.7 litres per minute. Beta particles are passed through the particles deposited on the tape and the attenuation of these particles is measured in a sensor located above the tape. The attenuation is converted to an estimate of mass based on the absorption coefficient.
Although the absorption coefficient will vary with different particle compositions, the instrument is typically calibrated using the mass absorption coefficient for quartz. In practice the mass absorption coefficient may vary by up to 20% (QUARG, 1996).
Advantages of the beta attenuation method include the provision of data at an hourly time resolution and the low labour and material costs associated with ongoing operation.
A number of different beta attenuation analysers have gained USEPA 'equivalency' status for measuring PM10 concentrations. Beta attenuation monitors are used in a number of ambient air quality monitoring programmes throughout New Zealand.
2.5 Tapered elemental oscillating microbalance (TEOM®)
The tapered elemental oscillating microbalance (TEOM) method provides a direct measure of the mass concentration of particles. The measurement is based on the frequency of mechanical oscillation of a tapered glass element. The element contains a filter upon which particles are deposited. Air is drawn through the analyser at a rate of 16.7 litres per minute to ensure an accurate cut point is achieved and 3 litres per minute of the air is passed across the filter.
The TEOM sensing system is heated to drive off water and to minimise the effects of thermal expansion and contraction of the mass sensor. This heating also results in loss of volatiles including nitrates and low molecular weight organic compounds. The amount of material volatilised will depend on the composition of the particulate aerosol and the temperature setting of the TEOM. In New Zealand, the standard temperature setting for this system is 40°C. The Ministry for the Environment recommends that where a TEOM is used to measure concentrations of PM10, another recommended method shall be co-located with the TEOM for a period of one year to determine an adjustment factor for comparing results to guideline value concentrations (MfE, 2000).
The TEOM is one of the more technologically advanced instrumentation systems in place in New Zealand. Its operation is computerised and can be interrogated remotely for information on status, filter loadings and general operation. The filter requires changing about once every 2-4 weeks depending on loading. Data are also available at a 10-minute time resolution allowing detailed comparison with variations in concentrations of other contaminants and meteorological conditions.
The TEOM method is used extensively throughout the United Kingdom, Canada and Australia and is the main monitoring method used by at least two regional councils within New Zealand.
2.6 Optical sampling
Optical samplers, which use principles of light scattering to estimate particle mass include models such as the GRIMM, Data Ram and GT 1020. As there is not a direct relationship between light scattering and mass, the method is not overly accurate. In addition, problems can arise as a result of light scattering by atmospheric moisture and particle coincidence (coincidence dead-time) in heavily polluted areas.
No samplers that estimate mass based on light scattering by particles have met the USEPA equivalency status for PM10 sampling. Optical samplers have not been used extensively throughout New Zealand.
