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Background

Implications

Regional aspects

Assessing and measuring the current situation

Management responses

Discussion

Future directions

Recommendations

Related issues

Background

There are many meteorological conditions that can exacerbate air quality problems. The most common are temperature inversions trapping pollution near the ground.

Temperature inversions are particularly evident during still winter conditions. The lack of cloud at night, which is common when a high-pressure system passes over the State, results in a net loss of radiation (heat) from the ground, causing the ground to cool dramatically. The cold surface cools the air above it, so the coldest temperatures are closest to the ground.

Cold air is denser than warm air, so just like water, it flows downhill (i.e. cold air drainage) and pools in topographic low spots (frost hollows).

Cold air drainage in a valley

Below the sudden change in temperature (the inversion) the air is very stable, with the difference in air densities above and below the inversion reducing mixing in the air column. Hence the inversion acts as a lid, keeping any pollutants released into the lower layer close to the surface. The pollution problem can be increased when inversions occur in valleys, as contaminants are fully contained by the valley sides and the inversion above. Under such conditions, contaminants such as those from wood heaters, can become quite concentrated. It is therefore particularly important to operate wood heaters efficiently to minimise particulate (smoke) pollution.

Launceston and the northern and western suburbs of Hobart often experience inversions during the winter months, but they regularly occur throughout Tasmania. The amount of fog typically indicates the severity of the inversion.

Implications

Certain meteorological conditions do not automatically lead to poor air quality. It generally just increases the chances of poor quality, or increases the severity of existing conditions. It is important to know what factors increase the potential for poor air quality so that modifications to behaviour/management can be made.

There are number of human health effects linked to poor air quality. Increases in respiratory problems such as asthma and in diseases from pollution containing lead, can be caused or exacerbated by air pollution.

The small population and small number of large industries in Tasmania means that large-scale air pollution is unlikely in the State. It is only in urban areas and near some large industrial sites that air pollution is regularly a problem. By international comparisons the existing air quality is good, but Tasmania prides itself on its 'pristine' environment, and markets its produce and sells itself as a tourist destination with this 'clean, green' image. For these reasons, the State must maintain very high standards of air quality, above those that would be considered acceptable elsewhere.

Regional aspects

The meteorological conditions that exacerbate poor air quality occur regularly throughout the State, however they are generally a greater problem in the colder months. It is only in urban areas and near some large industrial sites that air pollution is regularly a problem.

Assessing and measuring the current situation

In Tasmania there are no directly relevant indicators for this issue that can be supported at this stage. The following is a brief discussion of the conditions in Launceston and Hobart that exacerbate poor air quality condition from a report by Carnovale (2001). There are also other parts of the State that would also experience similar conditions.

Launceston topography and meteorology

Launceston is located on the upper reaches of the Tamar River, in a well-defined valley, approximately 50 kms from the ocean. The valley axis is mostly aligned in a north-west to south-east orientation and is flanked by hills which reach heights of up to 400 m.

Northerly winds prevail throughout the year in Launceston. Calm conditions occur about 34% of the time in winter, 20% in autumn and 12% in spring. In addition, minimum daily temperatures are below 5°C on approximately 120 days/yr.

Launceston (Tea Tree Bend) wind roses

Calm atmospheric conditions, coupled with low overnight temperatures in Launceston are also associated with thermal inversions. Thermal inversions act as barriers to the atmospheric mixing of pollutants, resulting in pollutants being trapped close to the ground.

At night and in the early hours of the morning, there is a downward flow of air from the upper sections of the valley towards the Tamar River and down the valley axis to the north-west of Launceston. Such drainage flows (katabatic) sweep pollutants, emitted in the more densely populated areas to the east, south and west of the Launceston CBD, in the direction of DPIWE's monitoring station at Ti Tree Bend (see image below).

Cold air drainage flows in Launceston

The seasonal variation in air pollution potential (APP) has been modelled for the Tamar Valley (see maps below). APP describes the atmosphere's ability to disperse any hypothetical contaminants that are released into it at a particular location or time. It is not a measure of actual air pollution concentrations. The model takes into account the spatial and seasonal variability of strongly stable and strongly unstable conditions; stagnation and ventilation events lasting 24 hours or more; strong winds (wind speed >= 8 ms-1); and atmospheric recirculation events (Power 2001).

Air pollution potential (APP) in summer, Tamar Valley

Air pollution potential (APP) in autumn, Tamar Valley

Air pollution potential (APP) in winter, Tamar Valley

Air pollution potential (APP) in spring, Tamar Valley

Hobart topography and meteorology

Hobart is located in a well-defined valley with the Derwent Estuary running through its axis. The valley axis is mostly aligned in a north-west to south-east orientation with the dominant topographical feature being Mt. Wellington, approximately seven kilometres to the south-west of the Hobart CBD.

Hobart is documented to have two dominant mesoscale windflows, namely a sea breeze and katabatic drainage flows.

The dominant daytime wind regime during winter is a drainage flow down the valley axis referred to as the "mountain wind". This wind increases in strength and frequency with distance down the valley. The mountain wind is fed by down-slope drainage winds (katabatics) flowing off the valley walls on to the Estuary. Light winds are generally associated with either the mountain wind or katabatics.

Hobart (Ellerslie Road) wind roses

High concentrations of particulate pollution in Hobart are frequently associated with the occurrence of highly stable atmospheric conditions and light winds that are unable to disperse pollutants. These conditions are linked to the passing of an anticyclone. Clear skies during calm nights result in the cooling of air in the upper slopes of the Derwent Valley. The air slowly drains down the Valley (katabatic winds) entraining pollutants within them. As a result, relatively high pollutant concentrations are likely to be found in topographic hollows and basins, and on low-lying land often located near the coast.

Cold air drainage flows in Hobart

Management responses

  • DPIWE has established air pollution monitoring sites in Launceston, and more recently in Hobart. In addition to air quality parameters, some limited meteorological parameters are measured at each site. Where necessary, Bureau of Meteorology data are used to aid in data interpretation. The Bureau of Meteorology's Launceston site is adjacent to DPIWE's air monitoring site.
     

  • A good knowledge of the meteorological conditions is vital for understanding and predicting occurrences of potential poor air quality. Launceston in particular has been the subject of significant studies, and the Bureau of Meteorology now provides forecasts of poor air quality during winter. This is based on an analysis of past air quality results and the available suite of meteorological parameters that are forecast daily. Forecasts are produced at 3:00pm and cover the next day.
     

Discussion

Poor air quality is not only generated by a source of pollutants, but also by the meteorological conditions under which those pollutants are released. Calm, cold atmospheric conditions during the cooler months are particularly relevant for Tasmania. In these situations, cold air inversions develop, trapping any pollutants released close to the ground. The prevalence of wood heaters in Tasmania is a major source of particulate pollution into this layer.

While there is little that can be done about the meteorological conditions that can lead to poor air quality incidences, there is a significant amount that can be done about the release of pollutants during such times. Under such circumstances, it is particularly important to operate wood heaters efficiently to minimise particulate (smoke) pollution.

Future directions

  • The Bureau of Meteorology (BoM) has produced an atmospheric dispersion model based on local emission sources, forecast weather parameters and topography. This model is currently being tested in Sydney and Melbourne. It is hoped that the Bureau's model will be applied to the whole of Tasmania, allowing daily winter time PM10 forecasts to be made.
     

  • The installation of meteorological towers at DPIWE's air pollution sites would aid in data interpretation, and would provide useful information for atmospheric dispersion modelling.
     

  • There are limited meteorological datasets available in Tasmania suitable for use in atmospheric dispersion models. These models are used widely elsewhere to assess pollution dispersion from proposed or existing sources. A short-term consultancy could produce appropriate datasets for a large number of areas within Tasmania, using existing BoM data and a meteorological model. DPIWE could then distribute this information throughout the atmospheric dispersion modelling community.
     

Tasmania Together and the RMPS

Relevant Tasmania Together goals and standards for 'Atmosphere' are listed in the linked file. The Tasmania Together Progress Board reported on progress toward targets for benchmarks set (Tasmania Together Progress Board 2003). Indicators, targets and baseline data are available in the latest Progress Report June 2003. Further information, including progress report updates, is available from Tasmania Together.

Involvement of the community, and the fair and orderly use of resources are also fundamental principles of the RMPS. The RMPS objectives have been developed to advance the principles of sustainable development.

Recommendations

2003

Chapter Title

Recommendation Title

Atmosphere

Total Exposure to Air Pollution

Ambient Air Quality Monitoring

Related issues

Atmosphere

Particulate Pollution

Climate Variability

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