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3 Other methods for assessing sources of particles

An alternative method for assessing sources of PM10 that is increasing in popularity in New Zealand is the use of receptor modelling. This source apportionment method involves the analysis of the composition of the particles collected on a filter and the subsequent statistical assessment of sources based on the relative clustering of elements on the filters.

Typical filter measurements include elements of molecular weight greater than sodium, elemental and in some cases organic carbon, and inorganic ions such as nitrates and sulphates. The elemental analysis in New Zealand is typically done using proton induced x-ray emission (PIXE), the elemental carbon using light absorption methods and ion chromatography is used to measure inorganic ions.

One of the advantages of receptor modelling methods is that they allow for the assessment of natural sources such as sea spray, which may contribute to measured particle concentrations, but is unlikely to be adequately estimated using emission inventory methods. It provides estimates of daily variations in sources contributing to particle concentrations, for example with wind direction or other daily variations in source emissions or meteorology.

Limitations of receptor modelling methods are that a large numbers of filter samples are required and it is not always possible to distinguish between sources. In particular, combustion processes emit similar ratios of elements making it difficult to assess the motor vehicle contribution relative to the domestic fire contribution. It is probable that existing source information for New Zealand will improve as more studies are carried out and that more advanced techniques will assist in improving the outputs from this source apportionment method.