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3 Assessing Discharges to Air: General Considerations

This section includes background information on assessing the air quality effects of transport projects, including:

  • contaminants of concern (including indicator contaminants)

  • factors influencing vehicle emissions

  • information sources in local government

  • consultation.

3.1 Contaminants of concern

The energy to propel motor vehicles comes from burning fuel in an engine. Discharges to air from vehicles arise from by-products of the combustion process (emitted via the exhaust system), the evaporation of fuel itself, and particulate matter from brakes and tyre wear, among others. There are a large number of contaminants, many of which have some effect on health, the ecosystem or the environment. It is beyond the scope of this document to address all of these, but a more detailed summary is given in Appendix 3.

Most of the effects of transport air pollution emissions can be assessed by examining a few of the key indicator pollutants. These indicator pollutants and the background to their selection are discussed below.

3.1.1 Health effects of motor vehicle pollution

Emissions from motor vehicles known to cause adverse health effects are the gases carbon monoxide, nitrogen oxides, volatile organic compounds and sulphur dioxide, as well as solid particulate matter. Other gases (such as ozone) and secondary particulate (sulphates and nitrates) can form in the atmosphere from reactions involving some of these primary emissions.

There is extensive information available about the health effects of these pollutants. A brief summary is provided in Appendix 3, but further information is available in the Ambient Air Quality Guidelines (Ministry for the Environment, 2002), technical reports (Ministry for the Environment 2003a, 2003b), and New Zealand research from the Health and Air Pollution in New Zealand programme (

3.1.2 Ecosystem effects

Air pollution can also have effects on the environment and ecosystem. These are usually less of a concern than effects on public health, but in some circumstances they may warrant attention. A detailed discussion of ecosystem effects is not repeated here, but can be found in another technical publication The Effects of Air Pollution on New Zealand Ecosystems (Ministry of Transport, 1998b).

Some effects include damage to plants and animals due to both direct emissions to air and secondary effects through deposition and run-off. However, relatively high levels of air pollution are usually required to cause effects of concern, and these rarely occur in New Zealand at such levels. In most cases, public health effects dominate the assessment criteria.

3.1.3 Indicator contaminants

Such a large range of contaminants with synergistic effects can usefully be reduced to a series of indicator contaminants. These are the contaminants with the highest potential to cause adverse effects, and as such provide a general indication of emissions from vehicles. The Ministry of Transport has identified five indicators of emissions from vehicles. They are:1

  • carbon monoxide (CO)

  • oxides of nitrogen (NOx, including NO2)

  • photochemical oxidants, including ozone (O3)

  • particulate matter (PM10 and PM2.5)

  • volatile organic compounds (VOCs), including benzene and 1,3 butadiene.

These indicators are described briefly below.

Carbon monoxide (CO)

Vehicle traffic is the single largest source of CO in most urban areas. CO disperses rapidly from the discharge source, with the highest potential exposure levels being immediately adjacent to roads. CO concentrations measured near New Zealand roads have frequently approached, or exceeded, guideline levels, and so CO is a key indicator for assessing local effects.

Oxides of nitrogen (NOx)

As with CO, vehicle traffic is the largest source of oxides of nitrogen in urban areas. Most is emitted as nitric oxide (NO) but is subsequently oxidised in air to nitrogen dioxide (NO2). Emission rate characteristics are different to those for CO, and local dispersion of NO2 is often slower due to oxidation of NO to NO2. Concentrations of NO2 measured near busy roads in Auckland have shown levels approaching, and exceeding, guidelines.

Ozone (O3)

Ozone (O3) and other secondary pollutants are primarily associated with regional effects because the rate of formation is such that they result from long-range dispersion. Their formation also depends on the mix of chemicals within the urban airshed. An assessment of O3 will generally only be necessary for major projects where an assessment of regional effects is required. Most assessments will provide adequate coverage of all potential effects if based on CO, particulate matter and NOx. However, the selection of the contaminants of concern is dependent on the scale and size of the development and the sensitivity of the receiving environment. The assessor needs to justify the selection of contaminants used in the assessment of environmental effects and provide information to support the decision.

Particulate matter (PM)

Sources of particulate matter (PM10 and PM2.5) include exhaust emissions, re-suspension of road surface dust, tyre wear, and brake and road surface wear. In considering the combustion emissions of PM, essentially all PM is less than 2.5 µm (micrometres).

Diesel exhausts contain much higher particulate concentrations than petrol exhausts, and the contribution of transport to urban discharges of particulate matter (PM10 and PM2.5) may be growing with increasing numbers of heavy diesel vehicles. Diesel particulate is especially concerning because it has been identified as a potential carcinogen, although at present all particulate matter is assessed in the same way regardless of its source. In many parts of New Zealand existing concentrations of particulate are high due to discharges principally from domestic sources. This means it may sometimes be difficult to identify the particulate contribution from a road as distinct from other sources. Nevertheless, particulate matter is a crucial contaminant for assessing the effects of transport sources, particularly if the fleet in question contains a large proportion of diesel vehicles.

Volatile organic compounds (VOCs)

Significant sources of VOCs include home heating and industry, as well as motor vehicles. VOCs are difficult to assess because there is very limited information on emissions and existing background levels. Also, VOCs comprise a large number of compounds and it is difficult and expensive to measure them all. For most assessments it is valid to assume that CO, PM10 and NOx are good indicators of the likely effects.

In some instances individual VOCs can have specific effects that may need to be assessed. One example is benzene, which is a significant component of petrol and diesel. Long-term exposure to elevated benzene concentrations has a carcinogenic effect. Another VOC that is becoming a concern is 1,3 butadiene, also a carcinogen. Motor vehicles are the main sources of benzene and 1,3 butadiene in urban areas, and there are ambient air quality guidelines for both of these compounds. An assessment of benzene and 1,3 butadiene should be considered for major projects or in special circumstances; for example, if monitoring has shown high existing concentrations.


As detailed in Appendix 3, vehicles and other transport-related sources can emit a wide range of air pollutants. In some very special cases these may need to be explicitly assessed, but such instances would be extremely rare. In general, if the above indicators pass any assessment criteria, then so will all the other types of emissions.

3.1.4 Greenhouse gases

The widest interpretation of the term 'air pollution' would include greenhouse gases such as carbon dioxide (CO2). Greenhouse gas emissions are not a formal part of any consenting process for road development, but they are of increasing concern to the community and government. These gases are subject to national mitigation strategies, and (increasingly) to local government initiatives. However, these emissions from transport are not included in this Good Practice Guide. The focus here is on local effects on public health and the environment associated with air pollutants that are known to have some toxic effect. Greenhouses gases do not have direct toxic effects, are not covered by current standards or guidelines, and are currently outside the scope of the policies and regulations being considered here.

3.2 Factors influencing vehicle emissions

Factors influencing vehicle emissions are complex and are only discussed briefly below. Detailed information is available from technical reports associated with the Vehicle Fleet Emissions Control Strategy (Ministry of Transport, 1998d). More recently, the results of on-road remote sensing of emissions from more than 40,000 vehicles in the Auckland region has shown the real world effects of some of these factors (Auckland Regional Council, 2003). A more comprehensive review of the influences on vehicle emissions in New Zealand is included in the Pilot Project Report for Petrol Vehicles (Ministry of Transport, 2006b) and the Pilot Project Report for Diesel Vehicles (Ministry of Transport, 2006a).

3.2.1 Vehicle fleet composition

Vehicle fleet composition has a significant influence on emissions, because different types and sizes of vehicles emit very different amounts of pollution. The proportion of diesel heavy commercial vehicles is particularly important because they are the most significant source of PM10. Assessments and modelling often assume an average fleet profile, but local variations can be substantial. Vehicle fleet composition is discussed further in the Tier 2 and Tier 3 assessment procedures of this guide (sections 7 and 8).

3.2.2 Vehicle technology and fuel standards

Most of the New Zealand vehicle fleet comes from countries that have well-defined emissions standards for vehicles, including Japan, the USA, Australia and Europe. These emissions standards reflect the environmental goals of the source countries and are achieved through various technologies that affect either engine performance or after-combustion treatment. In New Zealand, the Ministry of Transport recently introduced the Vehicle Exhaust Emissions Rule 2003. From the time the rule takes effect for the various vehicle types, vehicles entering the fleet will be required to be built to an international emissions standard (including Australian, Japanese, US [federal] and the European Union [EU/UN-ECE] jurisdiction standards). The emissions standards in different countries vary, and so there is no one particular standard that is applicable to New Zealand, although the largest fraction of New Zealand vehicles originate from Japan. Further information on vehicle emissions standards and their applicability in New Zealand is available from the Ministry of Transport website (

To illustrate the point, some details of the European emissions standards are shown below, for the simple reason that they are well defined and easily obtainable. Figure 3.1 shows the influence of improving vehicle emissions standards. Emissions are shown as a function of speed for Euro 1 to Euro 4 vehicles; Euro 1 was the European emissions standard for vehicles manufactured between 1992 and 1995, and Euro 4 is the emissions standard from 2005.

The way the fleet changes with time has significant effects, especially when considering options more than a few years into the future. It is expected that fleet-weighted emission factors will reduce over time as more modern, lower-emissions vehicles enter the fleet and older vehicles retire.

Fuel standards are also improving over time, and fuel improvements can lead to some direct emissions reductions. More importantly, they are a prerequisite for modern low-emissions vehicles. However, the overall rate of reduction in emissions from vehicles is uncertain because of uncertainties in the likely future fleet composition, vehicle maintenance requirements, etc.

Figure 3.1: Emissions as a function of speed for several European standards

3.2.3 Vehicle maintenance

Similarly, the state of tuning of vehicles determines their emissions. At any given time many vehicles will be out of tune to various degrees. Again, an average profile is used, but this can be different locally and can change substantially over time. Common overseas experience with vehicle emissions testing shows that in many cases the total air pollution emissions on a road can be dominated by a small percentage of vehicles - the 'gross emitters'. In New Zealand, on-road remote sensing has found that the worst 10% of vehicles may be responsible for over 50% of emissions (Auckland Regional Council, 2003). This factor may be one cause of the high variability in monitoring results, even when total vehicle counts are consistent. One poorly tuned, large diesel truck or bus can emit as much particulate air pollution as 100 well-tuned, private diesel cars.

3.2.4 Cold start

A cold engine is inefficient. This means that before the engine warms up, vehicles emit significantly higher amounts of CO and hydrocarbons and have higher fuel consumption than when they achieve their normal operating temperature. This effect is much more significant in catalyst-equipped vehicles. Catalysts do not begin to work until their temperature reaches a 'light off' value of around 300°C. This delay varies for different vehicles, but is generally within the first three minutes of the trip. This factor needs to be accounted for, and can be significant - many surveys show that a high proportion of trips in New Zealand are less than five kilometres. The cold-start performance differs between petrol and diesel vehicles and can also be influenced by the vehicle’s state of tune, the level of service on the road (congestion) and driver behaviour.

3.2.5 Speed and level of service

Vehicles generally emit lower amounts of pollutants when they are travelling steadily at their optimal design speed, which is around 30 to 70 km/hour for most vehicles. Emission rates tend to increase at higher and lower average speeds.

Emission rates under stop-start driving conditions (often associated with congested traffic conditions) are much higher than those when vehicles are driven more smoothly. This means that in stop-start traffic, emissions of some pollutants (eg, CO and VOCs) can be substantially higher than in free-flow conditions. For example, studies have shown hydrocarbon emissions from a car travelling at a steady speed to be only half of those measured at the same average speed but with the car driven in a more typical way − over drive cycles containing accelerations, decelerations and periods of idling (National Roads Authority and DEFRA, 1992/2003).

Emission factors provide an estimate of emissions for a typical drive cycle. The effect of speed and level of service (LOS, or extent of stop-start) is implicitly considered in most emission factors (including the New Zealand Transport Emissions Rate database - NZTER) because the variability of speed during a trip is closely related to the average speed. Slow-speed journeys in towns involve frequent speed changes in response to the traffic conditions, while higher-speed trips are normally driven more smoothly (National Roads Authority and DEFRA, 1992/2003). Nevertheless, for two trips at the same average speed (or for the same LOS / road combination in the NZTER), emissions can vary substantially depending on speed variability.

This effect is not usually significant over the averaging times being considered. For an air quality assessment, average emissions over an hour or longer are needed, so an emission factor for an average drive cycle is generally appropriate.

Graphs illustrating typical variations in emission rates as a function of average speed are presented in Figure 3.1 for catalyst-equipped vehicles. Figure 3.1 illustrates the relationship between average vehicle speed and emissions for four key pollutants. The performance of vehicles complying with other standards (eg, Japanese and US) will not be identical, but they do show similar patterns.

3.2.6 Road design

Road design can significantly affect emissions from vehicles. For example, emissions are significantly higher on steep gradients and sharp bends, where braking and acceleration are required. Another significant influence on air pollution levels is the location of the road. Roads located in an area with reduced dispersion will result in higher levels of air pollution. This could include valleys or street canyons (where high buildings reduce dispersion).

The influence of average vehicle speed / level of service on emissions and reduced dispersion characteristics on pollution levels can be assessed. However, more detailed assessments of the impacts of road design are unusual. This is discussed further in section 8.1.3 (micro-simulation of emissions).

3.3 Information sources in local government

Air discharge assessments can be complex, and are typically very site- or case-specific. It is recommended the relevant council with responsibility for air quality management in the region be consulted before undertaking any significant air quality investigation. The regional authority can provide advice on issues such as relevant policies or rules, existing air quality, and local air quality issues and concerns.

Where a notice of requirement or resource consent is required, establishing what information is available can assist the process greatly, particularly if this is established early in the process. Councils with air quality responsibility will be interested in the proposals and may make submissions or even be formally involved in the process. In particular, councils will have:

  • information on current air quality in the area

  • knowledge of development plans and potential reverse sensitivity issues

  • experience of community concerns

  • information on the locations of potentially sensitive receptors

  • experience of what is required in air quality assessments.

For projects that may have a number of options, such as alternative routes, the council will be interested in providing early input to these from the viewpoint of air quality issues (as well as other environmental issues within their jurisdiction).

3.4 Consultation

It is important to consider air quality as part of any consultation process for a land transport or land-use project that will have air quality impacts. Detailed guidance on consultation and affected persons is provided on the Ministry for the Environment website.2

Assessing discharges - recommendations

To ensure all relevant information is considered, the relevant council with responsibility for air quality management in the region should be consulted before undertaking any significant air quality investigation.

Air quality should be considered as part of any consultation process for a land transport or land-use project that will have air quality impacts.

1 Vehicle Fleet Emissions Control Strategy for Local Air Quality Management: Final Report (Ministry of Transport, 1998d) and supporting technical publications.