7 Tier 2 Conservative Assessment Process
The tier 2 assessment is a relatively simple screening exercise to determine whether a proposal is likely to result in a significant air quality effect. This is determined by comparing the absolute level of change with assessment criteria, regardless of the existing air quality (consideration of the existing air quality is made in tier 3 assessments).
The aim of a screening assessment is to provide conservative estimates of air quality effects. These may not necessarily be accurate but can provide confidence that a project will not result in significant air quality effects. There are a number of screening models available internationally, but these have not been validated or used in New Zealand. The recommended screening assessment process is based on the methods most commonly used in New Zealand.
The tier 2 and tier 3 assessments use essentially the same tools and techniques. However, this section suggests conservative default values for traffic data, emission factors and dispersion modelling, so that a tier 2 assessment can be undertaken relatively quickly and easily. Each of these aspects is explained and discussed further in section 8 (tier 3 assessment). It is strongly recommended that users who are not familiar with the assessment process read this section in conjunction with section 8.
In general, the tier 2 assessment should be undertaken for any links and intersections identified in the tier 1 assessment. For projects that affect multiple links or intersections, the worst cases can be assessed to determine whether problems are likely at other locations. The worst case will be the location with the closest receptors and highest emissions.
For changes to existing roads or networks, the assessment must be undertaken with, and without, the proposal so that the impacts of the proposal can be clearly established.
7.1 National Environmental Standards (for Air Quality)
Figure 7-1 outlines the approach for a tier 2 assessment. Special care may be needed for any assessment in an NES airshed that breaches the ambient air quality standards. Consultation with the regional council is recommended prior to finalising any detailed tier 2 assessment, particularly in an NES airshed that already breaches the ambient air quality standards. [Breaches of the standard are required to be published by the council in local newspapers.]
7.2 Characterising the discharges to air
Characterising the discharges to air for a tier 2 assessment includes:
- estimating the effect of the proposal on traffic
- using emission factors to estimate the change in emissions as a result of the proposal.
The following sections provide guidance on estimating traffic effects and emissions for the purposes of a tier 2 air quality assessment.
7.2.1 Traffic data requirements
Traffic flow
The assessment ideally requires an estimate of 24-hour, eight-hour and one-hour traffic flows for a high-traffic day. Currently, annual average daily traffic (AADT) is often the only available traffic data. AADT is an estimate of the average traffic count over a year, and on any given day traffic may be considerably higher or lower than the AADT. Newer traffic models being employed are capable of giving more detailed and accurate information, down to an hourly or less resolution, for each link at various times of the day, week or year. Whenever possible, the best data should be used, preferably at least at hourly resolution.
In some cases, peak and inter-peak hourly data are available, but this is generally the peak and inter-peak hourly traffic for the annual average day. The aim of a screening assessment is to predict worst-case air quality. Therefore, the traffic analyst should provide an estimate of daily one-hour and eight-hour traffic flows on a high-traffic day based on local traffic count information, or on traffic count information from a similar location. Traffic count data are generally available from territorial local authorities (or Transit for state highways).
It is essential that the air quality assessor understands the basis, meaning and limitations of any traffic data.
Example
For an assessment where only daily traffic flow data are available, the one-hour peak and worst eight hours should be estimated from traffic count data.
Auckland traffic count data, which were compiled for the Auckland Regional Council emissions inventory, showed the highest proportion of vehicle kilometres travelled (VKT) over one hour was approximately 7-8 percent of the 24-hour traffic count, occurring from 5pm to 6pm. The highest proportion of VKT in eight hours was approximately 54 percent of the 24-hour traffic count, occurring between 11am and 7pm. Therefore, for the purposes of assessment, one-hour and eight-hour traffic could be estimated as 10 percent and 60 percent of the 24-hour traffic flow, respectively.
Average speed
Vehicle emission factors are sensitive to speed and level of service (congestion level). In most cases, a tier 2 screening assessment can be based on worst-case emission factors, so speed data are not required. Where speed data are readily available, they can be used to help select the most appropriate emission factors. However, the assessor should justify the emission factors are conservative. Selection of emission factors is discussed in section 7.2.2 below.
Vehicle fleet composition
Vehicle fleet composition is an important parameter in air quality assessments because of the relative health importance of different pollutants. The health effects of air pollution are dominated by particulates, and emissions of particulates from transport are dominated by heavy commercial vehicles (HCVs).
For a tier 2 screening assessment, vehicle fleet composition data are typically not required unless a high proportion of HCVs is expected compared to the default of 7.6 percent HCVs over 3.5 tonnes (ARC 2005a). This might occur, for example, on busways, port access roads and quarry access roads. Default emission factors are provided below in Table 7-1 for the national average fleet composition. For assessments where there is a higher proportion of HCVs (compared to the default), emission factors will need to be derived from NZTER, as described in section 8.1.3.
Table 7-1: Fleet weighted exhaust emissions factors for 2004
| Road Type | Congestion Level | Fleet-weighted Exhaust Emission Factors (g/km) | |||||
|---|---|---|---|---|---|---|---|
| CO | CO2 | NOx | SO2 | TSP | VOC | ||
|
Central urban |
Free |
7.80 |
326 |
2.12 |
0.097 |
0.158 |
1.16 |
|
Interrupted |
12.8 |
492 |
2.74 |
0.132 |
0.213 |
1.61 |
|
|
Congested |
24.3 |
684 |
3.32 |
0.176 |
0.314 |
4.03 |
|
|
Cold start |
43.7 |
693 |
2.83 |
0.169 |
0.332 |
4.96 |
|
|
Motorway |
Free |
3.82 |
261 |
2.54 |
0.080 |
0.182 |
0.53 |
|
Interrupted |
5.10 |
239 |
1.77 |
0.072 |
0.119 |
0.57 |
|
|
Congested |
7.90 |
287 |
1.82 |
0.083 |
0.148 |
0.99 |
|
|
Cold start |
22.8 |
330 |
1.85 |
0.091 |
0.173 |
3.92 |
|
|
Suburban |
Free |
6.60 |
310 |
2.03 |
0.087 |
0.138 |
1.06 |
|
Interrupted |
10.2 |
355 |
2.20 |
0.097 |
0.163 |
1.24 |
|
|
Congested |
14.8 |
428 |
2.32 |
0.119 |
0.230 |
1.95 |
|
|
Cold start |
33.3 |
498 |
2.47 |
0.124 |
0.261 |
5.40 |
|
|
Fleet Weighted Average |
11.8 |
344 |
2.17 |
0.095 |
0.171 |
1.64 |
|
7.2.2 Emission factors
The Ministry of Transport's Vehicle Fleet Model and associated New Zealand Traffic Emissions Rate database (NZTER) is currently the preferred source of emissions information for the New Zealand fleet. The limitations and applicability of NZTER are discussed in section 8.1.3. Updated emission factors are being developed, but until these become available NZTER is considered appropriate for tier 2 screening assessments.
For a tier 2 screening assessment, the most conservative approach for calculating one-hour emissions from a transport corridor would be to assume the worst-case level of service (LOS) for the type of road being assessed. LOS is a representation of the level of congestion on a road. The appropriate (highest) emission factors are for congested conditions, except for NOx and PM10 emission factors for motorways. Higher speed limits on motorways mean that NOx and PM10 emissions are higher under free-flow conditions.
A conservative prediction of eight-hour and 24-hour emissions in an urban area therefore assumes congested emission factors for peak hours (for example, four hours of the eight-hour and 24-hour averaging periods) and interrupted emission factors for the remainder of the eight- hour or 24-hour period. Ultimately, the assessor will need to use his or her judgement, and justify that the emission factors selected are appropriately conservative for a screening assessment, recognising the assessment should consider worst-case locations. So, for example, if the air quality near an intersection is being assessed, the emission factor should be selected for the expected LOS in that location, as opposed to the average speed or LOS for the link. In all cases, the assumptions used and their justification should be clearly reported.
Cold-start emission factors represent emissions from vehicles that have not warmed up. This effect is most significant in catalyst-equipped petrol vehicles. The emission factors are high because emissions control equipment does not function well until the vehicle is warm, and the combustion process is not as efficient as it can be. Cold-start emission factors should generally apply for the first three minutes of vehicle travel (or at least 2.5 km). For the purposes of a screening assessment it should be assumed that 20 percent of all vehicles are operating under cold start conditions in urban areas (Auckland Regional Council, 2005a). In practice, cold-start rates may be lower depending on the route and the feeder roads; for instance, the Auckland-wide average rate, from the regional traffic model, is 17 percent. In a study making a comparison between modelled emission rates and those measured using an on-road remote sensing device, better agreement was obtained when assuming cold-start rates of 10 percent or greater (Bluett and Fisher, 2005). In the absence of specific data on this factor from a reliable traffic model, the 20 percent figure is an appropriate conservative choice.
Use of speed-dependent emission factors
It is likely that NZTER emission factors will be updated or replaced with speed-dependent emission factors for the New Zealand fleet. For a screening assessment using speed-dependent emission factors, it will be necessary to select an appropriate speed. Vehicles generally emit lower amounts of pollutants when they are travelling at their optimal speed (30 to 70 km/hr for most vehicles). Emission rates tend to increase at higher and lower speeds.
Speed data may be available from the results of traffic modelling or monitoring. However, the assessor needs to demonstrate the average speed selected is conservative for the location and averaging period being considered. Where speed data are not available, a worst-case assumption can be made based on the typical speed ranges given in Table 7-2. When there is doubt about the likely speed, the lowest speed within the range should be used.
Table 7-2: Typical speed ranges for use with speed-dependent emission factors
| Traffic condition/location | Speed |
|---|---|
|
Around intersections |
10 km/hr |
|
Urban or suburban areas that experience congested conditions during rush hour |
10-25 km/hr for a one-hour average |
|
Urban or suburban roads with interrupted flow (some traffic lights, etc) |
25-45 km/hr |
|
Free-flowing urban or suburban roads |
45 km/hr |
|
Free-flowing motorway |
80 km/hr |
7.2.3 Traffic data and emission factors for future years
Predicting the likely future impacts of transport projects is difficult. Any prediction requires an estimate of likely future traffic, as well as the fleet composition and emission factors. It is expected that fleet-weighted emission factors will reduce over time as more modern, lower-emission vehicles enter the fleet and older vehicles retire. However, there is a great deal of uncertainty about the likely rate of reduction.
In general, it is typically assumed the rate of reduction in fleet-weighted emission factors will be higher than the overall rate of growth in traffic (except for CO2). A tier 2 assessment for future years should only be necessary if it is expected that the rate of traffic growth (after the assessment year) on the road or link under consideration will be significantly greater than expected overall traffic growth rates (say two to three percent per annum), or if it is expected that the LOS will significantly decrease, or the proportion of heavy-duty vehicles will significantly increase for some reason.
For future years, a tier 2 assessment using the latest 2004 emission factors will provide a conservative assessment of likely impacts. If this assessment indicates that significant impacts are likely, then emission factors from NZTER for the year being assessed could be adopted. However, this should be done with caution.
The analyst should document predicted traffic for future years (with and without the project, where appropriate) and justify whether to assess likely future impacts. The future assessment years may depend on the availability of traffic demand models, but should include a long-term forecast (at least 10 to 20 years).
Where future emission factors are adopted, the limitations of these need to be recognised and discussed. This also applies to any projects currently being considered which will not be constructed immediately.
7.3 Exposure estimates
Appendix 1 shows a screening method for estimating the traffic-derived, ground-level concentrations of carbon monoxide (CO), nitrogen dioxide (NO2) and fine particulate (PM10). This method uses simple techniques and relatively crude assumptions, with the aim of ensuring conservative estimates. Alternatively, a near-road dispersion model, such as CALINE4, may be used with worst-case meteorology to estimate the traffic-derived, ground-level concentrations of air pollutants. The use of CALINE4, as well as other corridor dispersion models, is discussed in the Good Practice Guide for Dispersion Modelling (Ministry for the Environment, 2004).
7.4 Assessing the effects
7.4.1 Tier 2 assessment of effects
To assess whether significant air quality impacts are likely, the estimated traffic-derived concentrations of CO, NO2 and PM10 are compared to assessment criteria. In general, the tier 2 assessment should be undertaken for all links and intersections identified in the tier 1 assessment. For projects that affect multiple links or intersections, the worst cases can be assessed to determine whether problems are likely at other locations. The worst case will be the location with the closest receptors and highest emissions.
Other pollutants do not need to be assessed for a tier 2 screening assessment. CO, NO2 and PM10 provide a good indicator of transport effects, and are the pollutants of most concern. If the assessment shows that levels of these contaminants are within acceptable criteria, then the analyst can be confident levels of other traffic-related pollutants will be acceptable.
The primary reference criteria are the ambient air quality standards provided by the Resource Management (National Environmental Standards Relating to Certain Air Pollutants, Dioxins and Other Toxics) Regulations 2004. However, for tier 2 assessments, compliance against the Standards is not made explicitly. This is because of the difficultly of assessing the existing (or background) concentrations. These are either (a) usually not known at the location of interest, (b) highly variable and may not have peaks at the same time as traffic-related peaks, or (c) may already include contributions from the transport project being assessed, resulting in double counting. A full assessment of air quality effects against the Standards, or other criteria, is left to tier 3 assessments.
The recommended approach for tier 2 is to determine whether the quantitatively derived, adverse effects are likely to be "significant". This is an absolute criterion that is not related to the existing air quality, nor to whether the area already has high concentrations.
7.4.2 Significance
The concept of significance allows for a level of increase in the air pollution due to a transport emission source, as long as the maximum quantum of that increase is below some specified significance criterion for each pollutant being assessed. (See also the discussion on "reasonableness" in section 4.6). Table 7-3 recommends criteria for determining whether effects are likely to be significant, based on such an incremental analysis, followed by a discussion on their derivation.
Table 7-3: Significance criteria for incremental analysis only
| Pollutant | Standard/guideline | Significance criterion | Time average |
|---|---|---|---|
|
Fine particles (PM10) |
50 µg/m3 |
1 µg/m3 |
24 hours |
|
20 µg/m3 |
0.2 µg/m3 |
Annual |
|
|
Nitrogen dioxide (NO2) |
200 µg/m3 |
20 µg/m3 |
1 hour |
|
100 µg/m3 |
5 µg/m3 |
24 hours |
|
|
Carbon monoxide (CO) |
10 mg/m3 |
1 mg/m3 |
8 hours |
Derivation of significance criteria
The selection of appropriate criteria for significance is based to some extent on the relative effects of the increase, and to some extent on the inherent variability in the measurements due to space and time variability in the concentration of the pollutant. The recommended criteria are tighter for those parameters having greater health effects.
PM10 24-hour: This is set at 1 µg/m3, or two percent of the standard. Day-to-day variations can be of the order of several tens of micrograms, as can place-to-place variations over just a few tens of metres (see eg, Fisher et al, 2005). The health effects of short-term elevated concentrations are not as severe as a longer-term increase (see PM10 annual below).
PM10 annual: This is set at 0.2 µg/m3, or one percent of the New Zealand ambient air quality guideline. This has a more serious health effect, and is consequently set at a relatively low percentage. Many councils do not report annual average PM10 concentrations to any better resolution.
NO2 one-hour: This is set at 20 µg/m3, or 10 percent of the New Zealand ambient air quality guideline. There is currently no annual standard for NO2. This is around the same order as the background value found in many houses (those with electric heating − it can be over 100 for those with gas, wood or coal heating), and is lower than that found inside most vehicles. The health effects of short-term increases of this order will be very small.
NO2 24-hour: This is set at 5 µg/m3, or five percent of the standard. It is set lower for the same reasons as PM10; ie, longer exposures can have a greater health effect.
CO 8-hour: This is set at 1 mg/m3, or 10 percent of the standard. Short-term exposures to CO tend to be reversible. Exposure to elevated values over periods longer than eight hours may have some long-term effects, as may exposure to very high levels (greater than 10 mg/m3). The general background through any large city tends be around 1-2 mg/m3.
Summary: These assessment criteria are somewhat subjective, but are based on practical considerations. The effects on increases of this order − either in health indicators, ecosystem effects or even in well-run monitoring records − would be difficult to identify.
Assessment methodology
The maximum ground-level concentration predicted by the screening methodology should be compared to these criteria. This comparison should be made for locations where people may be exposed for the relevant averaging period. A tier 3 assessment should be undertaken if the predicted concentration of PM10, NO2 or CO due to the proposal exceeds any of the assessment criteria. A tier 3 assessment should also be undertaken if the tier 2 assessment is inconclusive for any reason.
The tier 2 assessment is intended to provide a conservative estimate of the likely air quality impacts of a project. If these criteria are exceeded, this does not necessarily mean the air quality impacts will be significant. It simply means a more accurate assessment should be undertaken. Mitigation options, or alternative options that do not exceed the criteria, could be considered at this stage, but it should be noted that further assessment may show these are not required.
7.5 Reporting a tier 2 assessment
The results of a tier 2 assessment should be documented for inclusion into any AEE, and to provide the basis for a tier 3 assessment, where necessary. The report should summarise the findings of the tier 2 assessment, including the basis of the traffic information, air quality information, any assumptions and their justification. Recommended reporting requirements are given in section 4.5.
