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4 Nitrogen oxide (NOx) emissions

4.1 Auckland

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.

Figure 4.1: Sources of NOx in Auckland (1993 inventory)

4.2 Wellington

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.

Figure 4.2: Sources of NOx in Wellington (1998 inventory)

 

 

4.3 Waikato Urban 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%.

Figure 4.3: Sources of NOx in Hamilton (2001 inventory)

 

 

Figure 4.4: Sources of NOx in Taupo and Tokoroa (2001 inventory)

 

 

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.

Figure 4.5: Sources of NOx in Taranaki (left) and the proportion of NOx from different areas within Taranaki (right)

 

 

Figure 4.6: Sources of NOx in Northland (non-specific year) and Gisborne (1996 inventory)

 

 

Figure 4.7: Sources of NOx in Bay of Plenty (left) and the proportion of NOx from different areas within the Bay of Plenty region (right)

 

 

4.5 Canterbury

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.

Figure 4.8: Sources of NOx in Christchurch (1999 inventory) and Timaru (2001 inventory)

 

Figure 4.9: Sources of NOx emissions in Rangiora, Kaiapoi, Ashburton and Waimate

 

4.6 Otago

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.

Figure 4.10: Sources of NOx in Dunedin and for urban areas of Otago (1999 inventory)

 

 

Figure 4.11: Sources of NOx in Alexandra, Arrowtown, Balclutha and Clyde (1999 inventory)

 

 

Figure 4.12: Sources of NOx in Cromwell, Milton, Mosgiel and Oamaru (1999 inventory)

 

 

Figure 4.13: Sources of NOx in Queenstown and Wanaka (1999 inventory)

 

 

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).

Figure 4.14: Sources of NOx emissions in Nelson (2001 inventory) and Richmond (2000 inventory)

 

 

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.

Figure 4.15: Predicted trends in NOx emissions from motor vehicles

 

 

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.

Table 4.1: Comparison of emission estimates of NOx for different regions of New Zealand

 

Domestic 
kg/day
Mobile sources
kg/day
Industry
kg/day
    Total 
kg/day

Alexandra

46

87

50

   

183

Arrowtown

18

10

9

   

37

Balclutha

18

87

10

   

115

Clyde

6

5

6

   

17

Cromwell

16

21

19

   

56

Milton

21

35

26

   

82

Mosgiel

54

77

135

   

266

Oamaru

111

394

182

   

687

Queenstown

101

327

78

   

506

Wanaka

19

34

30

   

83

Dunedin

462

2,225

945

   

3,632

  Domestic heating 
kg/day
Motor vehicles
kg/day
Industry
kg/day
Other domestic
kg/day
Other mobile 
kg/day
Total 
kg/day

Christchurch

988

13,180

1,795

   

15,963

Rangiora

56

247

20

   

323

Kaiapoi

34

486

11

   

531

Ashburton

82

517

147

   

746

Waimate

28

28

10

   

66

Timaru

91

968

46

   

1,105

Nelson

107

1,045

288

20

 

1,460

Northland

275

6,980

11,791

27

 

19,073

Auckland

2,000

82,000

12,000

2,140

4,500

102,640

Hamilton

397

5,019

9

   

5,425

Taupo

68

920

95

   

1,083

Tokaroa

109

1,323

148

   

1,580

  Domestic/ commercial heating 
kg/day
Motor vehicles
kg/day
Industry
kg/day
Other domestic
kg/day
Other mobile
kg/day
Total 
kg/day

Wellington

850

23,900

600

60

10,000

35,410

 

Domestic 
t/ year

Mobile sources
t/ year

Industry
t/ year

Burn-offs 
t/ year

 

Total 
t/ year

Taranaki

72

4,436

11,549

   

16,057

BOP

244

10,193

1,210

76

 

11,723

Gisborne

72

1,246

333

1,675

 

3,326