The cooler climate and lower population of Tasmania means that the cities are virtually free of the petrochemical smog experienced in the mainland cities and gaseous pollutants like ozone, sulphur dioxide and oxides of nitrogen are close to background levels.
Instead the major source of air pollution in Tasmania’s cities is fine airborne particles (particularly winter wood smoke from domestic wood heaters).
The concentration of airborne particles is measured using a High Volume Air Sampler (HVAS), a Tapered Element Oscillating Microbalance (TEOM) or an optical particle counter. Observed concentrations of PM10 particles (particles less than 10 micrometres [µm] in diameter) in the ambient air in Tasmania vary from less than 5 µg/m3 on a fine summer day, to over 100 µg/m3 on a smoky winter’s night.
High Volume Air Sampler
A High Volume Air Sampler (HVAS) is essentially a large vacuum cleaner fitted with a special air intake to remove the coarse particles and a filter to trap the respirable particles. A typical sampler draws 70 cubic metres of air per hour through the filter for 24 hours (from midnight to midnight).
Although the HVAS is the primary reference method for measuring PM10 particle air pollution, it has a number of limitations. Firstly, the HVAS can only provide daily average values for the particle concentration as the sampler must be run for 24 hours to accumulate sufficient mass on the filter to be weighed with a reasonable degree of accuracy.
Secondly, the mass of material collected on the filter is typically less than 1% of the mass of the filter, and each filter must be aged and weighed under carefully controlled climatic conditions, leading to delays of several days (or weeks) before the data becomes available. These instruments have been phased out during 2005, in favour of more modern Low Volume Sequential Samplers.
Tapered Element Oscillating Microbalance
The Tapered Element Oscillating Microbalance (TEOM) is a low volume air sampler with its filter mounted on the end of a tapered quartz tube, which acts like a tuning fork whose resonant frequency is very sensitive to small changes in the mass of the filter. By making the quartz tube the frequency control element in an electronic oscillator, it is possible to continuously monitor the filter mass via the oscillation frequency.
A TEOM permits the measurements of airborne particles time scales as short as ten minutes, providing information about the variation of particle concentrations during the day and their correlation with potential sources.
To maintain the frequency stability of the instrument and prevent condensation of moisture on the filter, the TEOM element must be operated at a constant temperature above the highest ambient air temperature (in Australia, this has been set at 50°C).
Over several hours, this can lead to a significant loss of volatile components in the particles as they “boil off” from the heated filter. This can result in an underestimation of particle concentrations under some conditions, especially when monitoring winter wood smoke, which contains a high proportion of fine droplets of condensed tars and other unburned fuel vapours.
For this reason it is important that the TEOM is associated with a co-located high volume sampler to provide the reference data necessary to calculate the appropriate site-specific TEOM adjustment factor to account for the loss of volatile material. The magnitude of the adjustment varies with the nature of the particle matter and ambient weather conditions.
Low Volume Sequential Sampler
The inclusion of PM2.5 Standards and Goals into the National Environment Protection (Air Quality) Measure in May 2003 mandated the adoption of a different approach to monitoring these tiny particles.
High Volume Air Samplers are not suitable for measuring PM2.5 so Australia has adopted the method approved by the US EPA, using much smaller filters and very much lower airflows. The principle is the same except that the vacuum cleaner is much smaller and more modern.
Air is drawn through a pre-weighed filter at a known rate, which collects any particles on the surface. The filter is weighed again to determine the mass of particles collected, and by dividing the mass by the total volume of air, we can calculate the concentration of the particles.
Only very small masses of particles are collected on the smaller filters, so a special, very sensitive balance is used, that can weigh things as small as 0.1 micrograms. That is one ten-millionth of a gram, a very tiny amount. To do this, the balance and the filters are kept in a special room that has an air conditioner that controls both the temperature and the amount of moisture in the air.
In Tasmania, we have decided to measure both PM2.5 and PM10 particles in this way. So we have installed some of the new samplers with air intakes that allow only PM2.5 particles to pass through and be captured on the filter. We have also replaced our old PM10 high volume air samplers with the new samplers. These new samplers have air intakes that allow all PM10 particles to pass through and be captured on the filter.
A "Dust Monitor" 180 designed by Grimm was installed at the George Town Monitoring Station (GAMS). The instrument was decommissioned in 2011. The Grimm monitor measures the concentration of particles in the air, namely PM10, PM2.5 and PM1. The instrument is a non-gravimetric, low-volume sampler, which uses a light scattering technique to measure the number of particles in the air.
The air drawn into the monitor is directed past a laser beam. As the particles in the air stream pass through the beam they reflect some of the light. This reflected light is then reflected by a mirror to a receiver.
The amount of light energy reflected to the mirror has an indicative relationship to the number of particles and their size in the air. The mass of particles in the surrounding ambient air can then be estimated from the intensity of the received signal.
It should be noted however, given the fact that the Grimm data provides only estimates of particle concentrations, it can not be used to determine compliance with the air quality criteria.