Page 3081 - Week 11 - Tuesday, 11 September 1990
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(2) On the days when calm conditions occur the dispersal of air pollutants is limited. These occur mainly in the autumn and winter months, April to September. These calm days often cause temperature inversions which trap cold air and pollutants close to the earth. The worst episodes of air pollution occur on these days.
The sources of airborne particulates in Canberra are broadly divided among pollens, soils, motor vehicles, domestic solid fuel heating and backyard burning. Controlled burning or beautifiers occasionally add to airborne particulates in Canberra.
Canberra is built in a system of valleys largely surrounded by hills. These provide a barrier to the movement of air in the region and contribute to the potential for poor air quality. Winds in the ACT vary with the seasons but tend to be predominantly from the north west. In autumn and winter, however, very still night air drains down the valleys from the south towards inner Canberra and out towards Belconnen and the Lower Molonglo Valley.
The basins such as Tuggeranong and Belconnen create their own cold air drainage which either works with the regional drainage or against it. This "cold air drainage" carries with it pollutants generated during these winter nights. When the drainage patterns are flowing in opposite directions pools of cool air containing pollutants are formed. These are obvious in the evening, overnight, and in the morning.
Nephelometer readings indicate the extent airborne particulate varies from hour to hour and from season to season in Canberra. The highest airborne particulates concentrations are normally found in autumn and winter at night, between 19.00-03.00 hours. This is due to a combination of calm weather conditions and a mixture of emissions from motor vehicles, domestic solid fuel heating and backyard burning. In autumn and winter the--7.atxxr two causes are dominant. A small peak occurs in true- early morning, between 07.00 and 10.00 hours. This is most probably due to vehicle emissions as people drive or are driven to work. These peaks are generally dispersed by midafternoon on most days of the year.
(3) Airborne particulate concentrations monitored in Canberra during 1989 are quite low when compared with the recognised health guidelines such as those prepared by the National Health and Medical Research Council (NHMRC) one year rolling average and the United States Environment Protection Agency (USER) 24 hour reading. These are given on Page 3.
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