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Executive summary

Air quality monitoring throughout New Zealand indicates that concentrations of suspended particles (PM10) in the air exceed the ambient air quality guideline values in many urban areas during the winter months.

Assessments to determine sources of PM10 emissions using the emission inventory methodology have been carried out since around 1995. At least one inventory has been carried out for the main cities as well as a number for smaller towns. All inventories have included an assessment of sources of particles with PM10 being the most common size fraction reported. Other common contaminants included are carbon monoxide (CO), nitrogen oxides (NOx), sulphur oxides (SOx), volatile organic compounds (VOC) and carbon dioxide (CO2).

Differences in methodologies between inventories can make comparisons between inventories for some regions difficult. Common variations include source categorisations, the presentation of data and the quality of data collection. In some areas, data are presented as annual average contributions only, whereas in other regions data are presented as daily (typically wintertime) or daily and annual estimates. Notwithstanding these differences, a general comparison of contributions in different areas can be made.

Results indicate that the main source of PM10 emissions in most areas during the winter months is solid fuel burning for domestic heating, although industrial contributions may also be significant in a number of locations. Domestic home heating is also responsible for the majority of the PM2.5 emissions in most locations as most of the PM10 emissions from this source are the smaller PM2.5 size fraction. Motor vehicle emissions may also be a major source of PM10 and PM2.5 in Auckland, although further work is being carried out to assess this.

Limited information is available on trends in sources of PM10 emissions in New Zealand owing to the relatively recent nature of the use of emission inventories. Some data for Christchurch and Timaru suggest little changes in emissions between the years 1996 and 2000. Estimated future trends in PM10 emissions are likely to be dominated by variations in home heating methods and growth in household numbers in most locations. Home heating methods are likely to be influenced by cost and availability of alternative sources such as electricity and gas as well as any air quality management measures. Future trends in industrial PM10 emissions are likely to be location dependent whereas a nationwide decrease in PM10 emissions from motor vehicles is predicted from 2001 to 2021 (MOT, 1998).

Another technique, which is used for assessing sources of PM10 in New Zealand, is source apportionment using receptor modelling. At this stage, the method has mostly been used in the Wellington region, although preliminary studies have been carried out in Auckland and Christchurch. The advantage of receptor modelling for determining sources of particles is the inclusion of natural sources such as sea spray and wind blown dusts, which are difficult to quantify using the emission inventory methodology.