Air quality reports reveal some improvements – but more work needed

Thursday 2 May 2024

Several air quality reports were recently presented to Council, outlining the current State of the Environment when it comes to air quality.

The Annual Air Quality Report 2023 outlined monitoring efforts over the past year and compared data gathered against the current National Environmental Standards for Air Quality (NESAQ) limit.

“One of the biggest improvements is the reducing number of air quality exceedances in Arrowtown over the past six years,’’ says ORC Air Scientist, Sarah Harrison.

These have continued to decrease with just 10 exceedances last year, compared to almost 30 recorded in 2018— although there is still plenty of room for improvement,” she says. The NESAQ allows for only one exceedance per year per airshed.

The annual report also noted improvements being made to the State of the Environment monitoring network to include monitoring for PM2.5 particles which are smaller and therefore can be harmful to health.

Upon validating last year’s winter data, Otago recorded only 17 exceedances of the NESAQ for PM10 (particulate matter with a diameter of less than 10 micrometres) during the winter months in 2023.

Otago has several towns where air quality is considered degraded during winter, namely Alexandra, Arrowtown, Clyde, Cromwell, and Milton.

In Otago, the main source of PM is home heating emissions in winter. Long term exposure to PM10 and PM2.5 (particulate matter with a diameter of less than 2.5 micrometres) contribute to the risks of cardiovascular and respiratory conditions.

Furthermore, recent research provides evidence that air pollution is dangerous at lower concentrations than previously thought and supports the lowering of existing guidelines.

A report detailing air activities undertaken in 2023 was also shared and focused on two air studies in particular; ORC’s trial study of black carbon monitoring in Arrowtown between March and October last year, and a spatial study by NIWA using temporary sensors to understand more about particulate matter concentrations in Alexandra.

A third report showed findings from an air quality screening study at Port Otago in 2022 and 2023.

Annual Air Quality Report 2023

Continuous monitoring of PM10 was carried out in the Alexandra, Arrowtown, Central Dunedin and Mosgiel airsheds during 2023.

Significant reductions in emissions are required to meet the NESAQ for PM10 in many Otago towns.

All sites except for Central Dunedin recorded exceedances of the NESAQ during the winter months; the limit for 24-hour PM10 being 50 micrograms per cubic metre of air (µg/m3).

PM2.5 was monitored in Arrowtown, Central Dunedin, Clyde, Cromwell, Milton and Wānaka.

Arrowtown, followed by Milton, had the highest annual and winter average concentrations for PM2.5.

ORC’s State of the Environment monitoring network is currently being upgraded to include PM2.5 monitoring. The upgrade process includes a period of co-location of both old and new instruments for equivalence testing.


A visible inversion layer over Alexandra from outdoor burning (2023)


Further comparison data will still be required to correct for the new instruments and accurately report their data, which can then be compared to existing guidelines and the proposed NESAQ for PM2.5.

PM2.5 data will also be available to public later in 2024 via ORC’s environmental data portal and the LAWA (Land Air Water Aotearoa) website.

Black carbon monitoring in Arrowtown

Black carbon is an ultra-fine component of particulate matter that is both a climate change pollutant and a human health hazard.

It hasn’t been monitored in the past however ORC had the opportunity to conduct a trial study in Arrowtown and monitor black carbon from March to October 2023, using a Magee Aethalometer AE33 on loan from Benchmark Monitoring.

The AE33 is a continuous monitoring instrument which uses optical technology to calculate the concentration of black carbon as the particles accumulate on a filter tape.

Data collected showed a strong seasonal pattern, with higher concentrations during winter, similar to that of PM10 in Arrowtown, but on a much smaller scale.

Interestingly, the two time periods in the day where black carbon was highest was between 8am and 9:am and between 7pm and 9pm, which is similar to home heating patterns found in PM10 monitoring.

“These results are to be expected as they have the same source — home heating emissions,” says Ms Harrison.

Black carbon comes from incomplete combustion and measures just 10-30 nanometres. That’s around 1000 times smaller than PM10 particles. A nanometre is a billionth of a metre; PM10 and PM2.5 are closer to a millionth of a metre.

The size, shape and composition of black carbon particles can change as they age and react to other chemicals in the atmosphere.

Because it is black, black carbon absorbs light and heat, contributing to warming the atmosphere.

It is a short-lived pollutant, spending about one week in the atmosphere. It can also impact cloud formation and increases ice and snow melt when it settles on these surfaces.

New Zealand research shows that black carbon concentrations are likely to be highest in areas of high traffic and/or high use of solid fuel burning for home heating.

Spatial studies

NIWA conducted a spatial study in Alexandra using 42 temporary sensors to understand more about the variation in PM concentrations across the town.

These sensors measure particulate matter (PM2.5) from combustion-related sources, such as wood burners, industry, and traffic. The data gathered gives a better idea of where PM2.5 concentrations are highest and at what time of the day the peak concentrations occur.

NIWA found peak average concentrations to be in the northwest of the town. These concentrations are around double the concentrations in other parts of Alexandra and quite far away from ORC’s monitoring site. This is likely due to topography and wind patterns in Alexandra.

A possible new monitoring site and origins of the concentrations are being investigated as well as data from the current and previous site being analysed.

ORC also ran a number of spatial studies last year in Oamaru, South Dunedin, Hawea, Ranfurly, Luggate and Kingston, also using temporary sensors to understand more about the spatial variation of particulate matter.

The information from these studies can be used to help identify potential future locations for further monitoring.


Temporary air quality sensor being installed in South Dunedin


Port Otago air quality screening study

Monitoring for sulphur dioxide (SO2) was undertaken within and surrounding the Port Otago operational areas between 6 December 2022 and 3 August 2023.

The results indicated that SO2 was generally quite low in the Port Otago area, and no trends identified. Staff recommended no further monitoring unless conditions changed that might increase SO2.

Burn Dry, Breathe Easy firewood campaign

Air quality often degrades during winter due to increased domestic heating emissions, cold calm weather, and the occurrence of inversion layers.

Given the frequency of home heating emissions in our air quality data, ORC runs an annual Burn Dry, Breathe Easy campaign to highlight better ways to manage home heating emissions, including wood is dry before burning, and not burning rubbish in domestic appliances.

Woodsmoke pollution can be reduced when residents burn only dry wood and keep their fire hot and not smouldering. Further woodsmoke reductions could potentially be made through thermally efficient homes with good insulation, she says.

Further information

Everyone has responsibilities whether at home or as landowners, industry, or businesses to play their part to improving air quality.

The Council hopes this campaign will encourage residents to think about the health of their families and community and use wood burners responsibly or consider the installation of ultra-low emission burners or heat pumps.