Monitoring of particulate matter in Balclutha over the past two years has revealed variations in air pollution levels in the town.

For each of the past two winters, monitoring has reported two days when average daily PM10 (small particles found in smoke) was over the current National Environmental Standard limit of 50 micrograms per cubic metre of air.

These and other results have emerged in an Otago Regional Council (ORC) investigation of PM10 patterns across Balclutha, carried out last winter to determine whether the continuous monitor's results are representative of the entire town.

ORC director of environmental information and science John Threlfall said the ORC Air Plan goal level of 35 micrograms per cubic metre of air was exceeded on a total of 15 days during the monitoring period.

The study results indicate low-to-moderate air pollution in that flat downtown area of Balclutha. However, on some nights these levels could be two to three times higher in the north-east and south-west hill suburbs.

Dr Threlfall said these two areas are mostly residential and have higher solid-fuel use densities than the downtown area, which has mixed land use. Variations in PM10 patterns across Balclutha also occur because of the specific wind and weather conditions prevalent at night.

Balclutha relies heavily on wood and coal as solid fuels for home heating. Similar to other towns in Otago, emissions from these fuels make up the vast majority (over 90%) of PM10 in the town.

Balclutha's location on a broad plain along a wide river allows good smoke dispersion, particularly across the flat middle section of the town adjacent to the Clutha River/Mata-Au.

"This probably helps clear some of the emissions and prevent higher levels from accumulating in central Balclutha," Dr Threlfall said.

"High emissions do not necessarily equate to high concentrations of PM10 because meteorology and topography play a major role in how emissions are transported in the atmosphere," he said.

"The study has provided some useful context for the current monitoring site in relation to the overall air quality in Balclutha. It has also improved our understanding of winter-time air quality problems in towns around Otago," Dr Threlfall said.

"While the current monitoring site effectively represents the downtown area of Balclutha, it may not accurately reflect pollution levels in the hill suburbs."

Dr Threlfall said it was unclear whether the higher hill suburb emissions stagnated or dispersed through the night. To answer this question, the monitor would be relocated to an area of higher emissions for this winter.

The new location will also satisfy the requirements of the NES that monitoring takes place in an area with the worst air quality.