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Mosgiel air pollution exceeded the national limit four times last month, according to information form the Otago Regional Council.
The council has been monitoring PM10 (particulate matter in the air smaller than 10 micrometers in diameter) levels continuously in Mosgiel since 2006. National Environmental Standards set the air quality threshold at a concentration of 505 micrograms per cubic metre.
The PM10 level in Mosgiel exceeded this threshold four times last month. One day a year over this threshold is permitted.
ORC engineering, hazards and science director Gavin Palmer said Mosgiel usually exceeded the standard between four and nine times per year. The goal was to reduce this to once a year by 2020.
The high PM10 levels came from emissions from domestic solid-fuel burners, industry, and occasionally outdoor burn-offs, he said.
''On June 24 [last Tuesday week], Mosgiel recorded 1085mcg/cu m, a very high result for the day. Our monitors do take readings every hour and on that day, PM10 was high (over 505mcg/cu m) every hour from 6am through the end of the day.
This would seem to indicate that there may have been a burn-off occurring. However, no complaints were received about the event,'' he said.
Typically, winter had the highest PM10 levels due to increased emissions from domestic burners.
PM10 concentrations were also highly dependent on the weather and colder, calmer weather often created conditions that limited dispersion of smoke.
The council monitored PM10 in nine towns around Otago and Mosgiel had neither the best nor the worst air quality. Alexandra, Milton, and Cromwell all had worse air quality in winter. Mosgiel skies often appeared hazy but this was not always due to smog.
Occasionally, true ''fogs'' (consisting of harmless water droplets) covered Mosgiel and much of the Taieri Plain, Mr Palmer said.
Under the Otago regional plan, all newly installed domestic solid fuel burners within the Mosgiel air zone boundary had meet emission standardsThe council had been operating the Clean Heat/Clean Air scheme since 2007, which provided subsidies for people to upgrade burners and insulate.
- Jonathan Chilton-Towle