Ground level ozone is a secondary pollutant, which is formed when the primary pollutants nitrogen oxides (NOx) and volatile organic compounds (VOC) combine in the presence of sunlight (MfE, 1994). The main VOC of interest are those with eight or less carbon atoms per molecule as these are the most reactive in the atmosphere. The exception is methane, which has one carbon atom and is relatively stable. Because sources of nitrogen oxide are considered in Section four, the focus of this section is on emissions of VOC.
Not all inventories have included estimates of VOC. Of those that have, not all specify that emission estimates have been for non-methane VOC. However, it has been assumed that unless specified, estimates of VOC in the inventories refer to non-methane VOC.
Figures 6.1 to 6.7 show the relative contribution of different sources to VOC emissions in different areas of New Zealand. In most areas, domestic home heating and motor vehicles are estimated to be the main contributors to VOC emissions. In Gisborne, natural emissions are the dominant source of VOC emissions. However, in most other areas natural source emissions were not included in the emission inventory assessment. Consequently, this source may also be a significant contributor in other areas. An estimate of natural VOC emissions for New Zealand has been made (NIWA, 1996). These results cannot easily be extended to the existing inventories because of differences in the spatial resolutions.
In Taupo, Christchurch and Kaiapoi, industry is a significant contributor to VOC emissions. Table 6.1 shows summary data for VOC emissions for different areas of New Zealand.
Figure 6.2: Sources of VOC in Northland (non-specific year) and Gisborne (1996 inventory)
Figure 6.3: Sources of VOC in Taupo and Tokoroa (2001 inventory)
Figure 6.4: Sources of VOC in Hamilton (2001 inventory) and Christchurch
|Domestic heating |
|Motor vehicles |
|Other domestic |
|Other mobile |
Domestic/ commercial heating