Motor vehicles contribute the majority (80%) of the NOx emissions in Auckland, with industry producing 12% and other mobile sources, domestic fuel combustion and other domestic sources contributing the remainder (Figure 4.1). Like CO, however, the actual contribution of different sources at any location will vary, depending on factors such as proximity to roadways. Monitoring data from some of the roadside sites such as Queen Street or Khyber Pass Road will primarily represent motor vehicle emissions.
The main source of NOx emissions in the Wellington region is motor vehicles (68%), with other mobile sources also a significant contributor at 28%. This latter source includes aviation and commercial shipping. Domestic and commercial combustion and industry are only minor sources of NOx in the region each contributing 2% of total NOx emissions. These results are estimates of the relative contribution of each source averaged across the whole of the Wellington region, including both urban and rural areas.
Motor vehicles are responsible for the majority of the NOx emissions in Hamilton, Tokoroa and Taupo (Figures 4.3 and 4.4). In Hamilton, industry contributes less than 1% of the NOx emissions. The industrial contribution is greater in Tokoroa and Taupo at 9% of the NOx emissions. The domestic heating contribution is minor in all areas at around 6–7%.
4.4 Taranaki, Northland, Gisborne and Bay of Plenty
Unlike most areas, the main source of NOx emissions in Taranaki is industry, which contributes around 72% of the NOx emissions (Figure 4.5). The NOx emissions in Taranaki occur primarily within the remaining pastoral land and New Plymouth Bell Block urban areas. Similarly, in Northland industry contributes 62% of the NOx emissions, with motor vehicles responsible for 37% (Figure 4.6). In Gisborne, burn-offs are the main source of NOx emissions, with transport contributing 32%. Motor vehicles are the main source of NOx emissions in the Bay of Plenty contributing around 87% of the NOx emissions (Figure 4.7).
The relative contributions to NOx emissions in Taranaki, Northland, Gisborne and Bay of Plenty are based on annual averages, rather than winter specific data. The main source contribution likely to change during the winter months is domestic home heating, although this is not typically a major source of NOx emissions in New Zealand.
Motor vehicles are the dominant source of NOx emissions in Christchurch, Timaru, Rangiora, Ashburton and Kaiapoi (Figures 4.8 and 4.9). In Waimate, domestic fires and motor vehicles each contribute around 40% of the NOx emissions with industry responsible for the remainder.
The main source of NOx in Dunedin and the urban areas of Otago are motor vehicles contributing around 60% of emissions (Figure 4.10). In many of the smaller urban areas, the contribution from motor vehicles is less, with industry and domestic heating showing some dominance (Figures 4.11–4.13). For example, in Arrowtown, Clyde and Mosgiel, motor vehicles contribute less than 30% of the NOx emissions. Mosgiel shows the greatest industrial impact with around half of the NOx emissions from this source.
4.7 Nelson and Richmond
Motor vehicles are the main source of NOx emissions in both Nelson and Richmond, contributing about two-thirds of the emissions. Industry contributes 20–25%, with domestic heating and outdoor burning comprising the remainder (Figure 4.14).
4.8 Trends in NOx emissions in New Zealand
Emission inventory studies show the main source of NOx emissions in most urban areas of New Zealand is motor vehicles, although industry is dominant in a number of locations. Changes in emissions from motor vehicles and industry will therefore be key drivers in trends in NO2 concentrations within New Zealand.
An estimate of the impact of improved vehicle engine technology on NOx emissions in New Zealand has been provided by the Ministry of Transport. This suggests a significant decrease in NOx emissions in New Zealand over the next 20 years. Figure 4.15, which is based on the Ministry of Transport’s New Zealand Transport Emission Rate model (NZTER), shows the estimated improvement in motor vehicle NOx emissions with time. The three levels of service (LOS) categories represent emission rates for different levels of congestion.
Future trends in NOx emissions from industry are difficult to assess. In some areas, these are likely to be dominated by one or two major sources, for example gas fired power generation. Growth in industry is likely to be area specific and may depend on the extent of existing regulation control measures.
There is very limited trend information from emission inventory studies as Timaru and Christchurch are the only areas to have published a second inventory assessment. The Timaru inventory shows no changes in total NOx emissions from 1996 to 2000, as a slight decrease in transport NOx emissions was offset by an increase in the industrial component. A trend is also apparent in a comparison between the 1996 and 1999 Christchurch emission inventory. It is possible, however, that the increase in the industrial component reflects methodological differences in the assessment of emissions from this source between the two inventories.
4.9 Summary of sources of NOx in New Zealand
The results of the emission inventory investigations into sources of NOx in New Zealand are shown in Table 4.1. With the exception of Bay of Plenty, Taranaki and Gisborne, these data represent average wintertime emission sources. The dominant source of NOx in most urban areas is motor vehicle emissions.
In Auckland, around 100 tonnes of NOx is estimated compared to around 16 tonnes for Christchurch and 35 tonnes for the Wellington region. In most of the smaller urban areas NOx emissions of less than one tonne per day are estimated. While motor vehicles are still the dominant contributor in some of these smaller towns, the industrial contribution is sometimes greater.
The relative contributions to NOx emissions shown in Table 4.1 are based on assumptions relating to emission rates and fuel use and contain some degree of uncertainty. There is some variation from area to area in the approach taken and the subsequent confidence in results. Further information on the limitations associated with different inventories is provided in Table 2.1.
|Domestic heating |
|Other mobile |
|Domestic/ commercial heating |