4 The Assessment Process
Transport or land-use projects have the potential to affect air quality if there are changes in traffic flows, speed, congestion or traffic composition. If these changes are significant, an air quality assessment may be required. This section provides guidance on assessing the air quality effects at a level of detail appropriate to the likely effects of the project.
Note projects involving maintenance works and/or minor safety upgrades are not likely to have significant air quality impacts and would not require assessment.
4.1 Key steps of the assessment process
An assessment of discharges to air from transport will typically involve four steps:
- characterise the receiving environment
- estimate emissions and future trends
- estimate concentrations
- assess the air quality effects.
Each of these steps should be undertaken to a level of detail that is appropriate to the nature and scale of the proposal.
Characterise the receiving environment
The potential effects of the proposal on air quality must be considered in light of existing air quality, the sensitivity of the receiving environment, and the local topography and meteorology.
Estimate emissions
Traffic estimates can be derived in two ways: from a traffic model, or from vehicle counts, which produce vehicle flows, traffic composition and driving conditions. The traffic data are combined with emission factors (from an emissions model or database), and emissions are calculated accordingly.
Estimate concentrations
A common method for determining environmental concentrations, and subsequent exposure, is with an atmospheric dispersion model. This calculates ambient air concentrations or deposition rates of the contaminants of concern, and sometimes incorporates an atmospheric chemistry model to account for the production of secondary pollutants. Transport projects requiring dispersion modelling would, in most cases, require the use of dispersion models that are designed specifically for transport emissions assessments.
Assess the air quality impacts
Transport projects can cause a range of environmental effects, but as we have seen, human health effects usually take the highest priority. These can be assessed by comparing the predictions of environmental exposure with established assessment criteria, such as ambient air quality standards or guidelines. In some cases, it may be necessary to undertake a detailed health risk assessment study, where predictions of human exposure and contaminant dose (to a part of the body) are compared with dose-response data to determine the increased probability of adverse health effects.
4.2 The level of assessment required
Section 88 of the RMA requires an Assessment of Environmental Effects (AEE) to be provided"in such detail as corresponds with the scale and significance of the effects that the activity may have on the environment".
This section provides guidance on the level of assessment that is appropriate for transport proposals. Figure 4.1 illustrates the overall three-tier assessment process described in this guideline. The three-tier approach is intended to ensure that the level of assessment undertaken reflects the likely level of effects from a proposal.
The three tiers of assessment are:
- tier 1 screening assessment of basic traffic and land-use information to determine whether changes in air discharges are likely to have a significant effect (for significant projects the tier 1 assessment includes a preliminary qualitative assessment to identify key air quality issues early in the process)
- tier 2 conservative assessment using more detailed traffic information and straightforward dispersion modelling techniques to determine the level of significance of air discharges more quantitatively
- tier 3 detailed (less conservative) assessment, which may involve increased complexity in modelling and requirements for site-specific data.
Tier 1 and tier 2 assessment procedures apply only to the assessment of emissions from motor vehicles. Assessment of other land transport projects (eg, railways and tunnels) requires specialist input. The focus of the tier 3 assessment is also emissions from motor vehicles, but the same general principals are applicable to any transport project or traffic-generating development.
Sections 6, 7 and 8 look at the tier 1, 2 and 3 assessment processes (respectively) in detail, but it is worthwhile here giving an outline of these levels of assessment.
4.2.1 Tier 1
Any transport or land-use project has the potential to affect air quality and exposure risk if there are changes in traffic flows, speed, vehicle fleet composition, congestion, or the location of traffic relative to receptors. The first step in undertaking an assessment of discharges to air from transport is to determine whether these changes are likely to have significant effects. The tier 1 assessment procedure provides quantitative criteria to determine whether a project requires further assessment. These criteria are intended as a guide only. The extent of the air quality assessment is a matter for judgement. For significant projects, the tier 1 assessment includes a preliminary qualitative assessment to identify key air quality issues early in the process. The tier 1 assessment procedure is outlined in section 6.
4.2.2 Tier 2
The tier 2 assessment procedure provides conservative estimates of likely air quality impacts. Tier 2 is a relatively quick and easy quantitative assessment. This means the assessment can provide confidence that a project will not result in significant air quality impacts, despite the relative uncertainty of the predictions. If the results of the tier 2 assessment are inconclusive, or are found to be close to, or above, thresholds, then a more detailed (tier 3) study is required.
A tier 2 assessment should be undertaken for any project (or parts of a project) that is identified in the tier 1 assessment to determine whether significant adverse effects on air quality are likely. For major projects it may be immediately obvious that a tier 3 assessment will be required. However, tier 2 may still be useful for doing a quantitative ranking of options and for identifying parts of the project that require detailed assessment. The tier 2 assessment procedure is outlined in section 7.
4.2.3 Tier 3
A more detailed tier 3 assessment is required if the screening assessment indicates potential, significant adverse effects, or if the project is sufficiently complex that a screening assessment is not appropriate. As stated earlier, for some projects the need for tier 3 assessment may be obvious during the preliminary assessment.
It is recommended that the relevant council with responsibility for air quality management be consulted prior to undertaking a tier 3 assessment.
A detailed assessment may require site-specific emission calculations, including ambient air monitoring data, wherever this is available (to confirm background concentrations), and atmospheric dispersion modelling. In many cases, the tools or models are the same as those used for a tier 2 screening assessment but use more detailed input data. For example, an atmospheric dispersion model may accommodate real-time meteorological data, or at least a representative meteorological frequency distribution, rather than simple worst-case assessments.
Detailed assessments may also be required to handle special localised features, which may affect the dispersion of contaminants that are not adequately examined by the screening models. This could include areas of complex terrain, severe street canyons, complex intersections and discharges from road or rail-tunnel vents.
A tier 3 assessment should be undertaken to a level of detail sufficient to determine the air quality effects in a robust and defensible manner. In practice, it is likely that the methods described in the tier 2 and tier 3 procedures would be combined for many assessments. For example, a scarcity of information on existing air quality may result in the need for air quality monitoring, whereas the dispersion modelling technique may be very straightforward due to simple topography.
In this respect, there is no guidance on the scope or size of the assessment in relation to the level of assessment required. In some situations, a fairly comprehensive exercise may be needed -such as for a new large road, near sensitive receptors, in a poor quality airshed. However, there may also be circumstances where a tier 3 assessment may be very straightforward and not involve much more work than a screening tier 2 assessment - such as for a modification on an existing road, in an area without sensitive receptors and in an airshed that does not experience problems.
The tier 3 assessment procedure is outlined in section 8.
4.3 Factors affecting the level of assessment
The factors affecting the level of assessment required include the:
- scale of the development
- extent of any increase in emissions
- scale of effect
- airshed designation under the National Environmental Standards for Air Quality (the Standards)
- existing air quality
- physical geography of the receiving environment
- land use of the receiving environment
- any legislative or consent requirements.
The tier 1 preliminary assessment process includes a qualitative assessment of these factors. In some cases, it will be obvious from the preliminary assessment and discussions with the consenting authority that a tier 3 assessment will be required. However, a tier 2 screening assessment may still be undertaken to identify the pollutants or sources of most concern before undertaking a tier 3 assessment.
4.3.1 Legislative and policy requirements
Legislative and policy requirements are outlined in section 2. It is important to consider any requirements of relevant legislation or policies in determining the appropriate level of assessment.
4.4 Scale of effect
An assessment should consider three potential levels of effect:
- corridor effects
- community effects
- airshed effects.
Corridor effects are effects in the immediate vicinity of the proposed road, or roads, affected by the proposal. Emissions from most transport sources disperse quickly with distance from the source, and so the contribution to ambient concentrations or deposition is generally minor beyond approximately 200 m from the source. At a local level, therefore, the assessment should focus on determining the environmental effect (usually concentration of contaminants) in the immediate vicinity of the proposed transport development.
Community effects are the effects on a wider geographical area, usually as a result of a large development producing a significant net change in emissions. For example, a new overpass might decrease emissions from its immediate surrounds but increase emissions in another (suddenly congested) area elsewhere. An assessment of community effects would determine whether the project has an overall air quality benefit when all of the corridor effects are taken into consideration. Assessment of community impacts is discussed in section 9.
An assessment of airshed air quality impacts would usually only be required for very large projects. For example, a new tunnel or motorway might require modifying the emissions inventory and undertaking urban airshed modelling to assess likely impacts on ozone and secondary particulates. Assessment of airshed air quality impacts is discussed further in section 8 (Tier 3 Detailed Assessment Process).
4.5 Reporting
There is considerable merit in developing a standard reporting method and format. Although specific projects will have specific requirements, any report should contain the following features.
- Executive summary: a one-page statement of the key features and results. This may be the only part of the report that some users read, so it should be succinct and clear.
- Scope: who has commissioned the project, and why, including the intended outcomes.
- Introduction: the background to the issues and relevance of any previous work.
- Description of proposal: a description of the project and any changes to existing activities. This section should include enough information to characterise the discharges and the receiving environment, as detailed in later sections of this guidance.
- Site description: the area being assessed, including maps with all relevant features (and photos if available). Show any facilities where there are likely to be sensitive members of the public (eg, young, old, sick), such as hospitals and schools, in relation to the proposal. Consider likely future development of the surrounding area, taking into account the relevant district plan provisions.
- Data used: the sources and validity of all input data, including traffic flow data, fleet profiles, emission factors, meteorology, existing concentrations and all assumptions made.
- Methodology: a description of the process used, any models employed, and any statistics or analysis used.
- Assessment of effects: the outcomes of the study, and all options assessed, as much as possible in summary tabular and graphic form. The emphasis should be on key results that can inform decision-making. Where possible, errors should be estimated and stated. Detailed results should be given in an appendix.
- Discussion: any implications, uncertainties and reliance on assumptions. Include discussion of possible mitigation options and associated emissions reduction.
- Conclusion: a summary of the scope, method, result and implications.
- References: all material used should be referenced explicitly, and should include web-based links where appropriate.
- Appendices: any detailed calculations or results that are used. This should include model control files.
The size and nature of each of these sections will depend on the project, but it is anticipated that for any tier 3-type assessments each section will be included and the report will run to 30 to 60 pages. Specific requirements for tier 1, 2 and 3 assessments are discussed in sections 6, 7 and 8 of this guidance.
Any assessment of effects for a resource consent application should address all matters outlined in the Fourth Schedule of the RMA, as well as any requirements of the consent authority.
4.6 Reasonableness
Assessing air quality effects is a complex task. Although a number of quantitative tools are applied, many of these have uncertainties that at times can be difficult to quantify. Many steps in the process require assumptions to be made, and this is where the concept of "reasonableness" can be applied.
For instance, although in theory it would be possible to describe and model all possible options (all types of traffic, all types of drivers, all weather conditions, etc) this would result in hundreds or even thousands of sets of results for an unreasonable cost. It is also remarkably difficult to gain accurate information on future emissions characteristics, particularly with evolving engine technologies and the sometimes unrealistic performance of transport models out beyond a few years.
Even if very accurate emissions and dispersion modelling for the project under consideration are possible, the situation is further compounded by the fact that the state of knowledge about other sources and their effects is imperfect, particularly in areas that are geographically complex and have many other sources and no ambient monitoring (ie, many New Zealand cities).
Calls are often made − particularly by the various engineering professions responsible for the design, implementation and operation of the transport networks − to provide quantified uncertainty estimates. However, the current state of knowledge about emissions and dispersion is such that this is simply impossible to do with the same rigour as most engineering calculations. Simple uncertainty estimates in each stage can compound to form enormous uncertainties in the final results. This is not so much looseness in the computations as a feature of not having developed the right statistical methods (for instance, many processes are far from being normally distributed and/or are often not independent, so common statistical measures of uncertainty are inappropriate).
This lack of rigorously quantified uncertainly does not detract from the value of the methodology used. Indeed, it is precisely why so much emphasis is placed on monitoring, validations and experience from similar projects.
As a result, any assessment of the air quality effects of transport needs to apply a factor of reasonableness. This is difficult to define, but should include consideration of the following factors.
- Fleet composition - Is it about right? Does it represent the conditions of interest?
- Transport corridor being assessed - How far does it go? Are the right number of feeder routes included?
- Project options - Are there a manageable number (not just one or two, but also not 50 or 60)?
- Time horizon - How far into the future is reasonable? One year is not enough, but 30 is probably too much.
- Contaminants - Are the right ones on the list? Are there not too many, but not too few?
- Existing air quality and effects - Are the time and spatial variability captured?
- Cumulative effects - Is the procedure for examining the additive effects of the project emissions to existing effects robust?
- Receiving environment - How far to go: right on the road, or 10 m, or 200 m?
- Reporting - Just enough to tell the story? Or should there be hundreds of pages of background and justification?
- Mitigation options to assess - Just what might be sensible and practical versus unrealistic or cost prohibitive?
The rest of this good practice guide should help with deciding these questions, but the answers to them will never be absolute.
