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Air Good Practice Guide for Assessing Discharges to Air from Industry Online version
7 Tier 2 Screening Assessment
The Tier 2 assessment is a relatively simple screening exercise to determine whether a proposal is likely to result in exceedances of ambient air quality criteria, in particular the national ambient air quality standards.
The aim of a screening assessment is to provide conservative estimates of air quality impacts, which may not be completely accurate but can provide confidence that a project will not result in significant air quality impacts. 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, but a Tier 2 assessment uses conservative assumptions so it can be undertaken relatively quickly and easily. Users who are not familiar with the assessment process should read this section in conjunction with section 8 (Tier 3 assessments).
A Tier 2 assessment (see Figure 7.1) would be suitable for a small-scale activity, which may be located in an area where there are sensitive receptors nearby or close to a more sensitively zoned area. Discharges to air may have the potential to give rise to off-site effects if poorly managed or during process upsets. Monitoring of emissions or processes may be required for existing facilities to ensure that offsite human health, environmental or amenity effects are avoided. Air pollution control equipment may need to be installed, along with the implementation of best practice operating procedures.
7.1 National environmental standards for air quality
Special care is needed for any assessment in an airshed that breaches the national ambient air quality standards. In particular, within an airshed that breaches the PM10 standard, it is likely that any proposal resulting in a significant increase in PM10 emissions will require mitigation or not be allowed. Any significant increase in emissions at a location that already breaches a national ambient air quality standard would generally require a detailed Tier 3 assessment. Consultation with the regional council is recommended before undertaking any detailed Tier 3 assessment, however, particularly in an airshed that breaches the Standards.
7.2 Characterising the discharges to air
Characterising the discharges to air for a Tier 2 assessment is essentially the same as for a Tier 3 assessment (see section 8.1), with a level of detail appropriate to the nature and scale of the proposal. However, to ensure conservatism, a Tier 2 assessment should be based on maximum proposed emission rates, and the assessment should include enough detail to demonstrate that the emission rates assumed are conservative. This may involve:
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developing an emissions inventory for the site, and noting the nature and composition of the air discharges, including (where available) an estimation of the quantities of pollutants emitted - some emission testing may be required to assist in compiling this inventory
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assessing the engineering and operational controls in place to control discharges to air - this may require some compliance testing for existing plant to show that the equipment is operating satisfactorily
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reviewing the performance standards for proposed new control equipment included as part of the development.
See figure at its full size (including text description)7.3 Exposure estimates
7.3.1 Estimating ground-level concentration of air pollutants
Atmospheric dispersion modelling is used to estimate the maximum ground-level concentration of pollutants. The Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a) includes detailed guidance on all aspects of dispersion modelling.
A Tier 2 screening assessment will generally be based on simple techniques and relatively crude assumptions, with the aim of ensuring conservative estimates. It is recommended that a Tier 2 assessment should:
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use a steady-state Gaussian plume dispersion model, such as Ausplume
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use worst-case meteorology – either a screening meteorological data set or measured worst-case conditions
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be based on maximum normal emission rates.
Note that the use of screening meteorological data has limitations that need to be understood. They will generally only give an indication of the theoretical worst case one-hour concentration. They cannot give (a) concentrations at other averaging times, (b) any idea of the frequency of occurrence, (c) anything but a very crude idea of the location of the peak values, (d) possible concentrations in extreme geographical or meteorological situations. It is always preferable to use a local, validated, meteorological data set, and these are available for many regions, particularly the major cities.
7.3.2 Background concentrations
Although it is important to assess the concentration of air pollutants as a result of the proposal, the RMA requires an assessment of the overall end result – the cumulative effect. This means that the modelled concentrations must be added to background concentrations discharged by other sources.
The Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a) includes guidance for accounting for background concentrations (in section 6.4 of that guide). For a Tier 2 assessment it is generally appropriate to make worst-case assumptions about background air quality. On this basis, a recommended approach for a Tier 2 assessment is outlined below. This approach is very conservative, and a more detailed assessment may be required if this results in a prediction of unacceptable air quality effects. This is discussed further for the Tier 3 assessment procedure.
See figure at its full size (including text description). Local monitoring data may be available from the local regional council. If such data are not available, the most straightforward options are as follows.
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1. Compare the location with somewhere similar
If the area does not have significant large sources, and does not have any complex geographical or meteorological features, then it can be assumed that the air quality will be similar to another area of similar population density, emission sources and meteorology. This method requires that such an area can be identified, and that monitoring data are available. - 2. Make a worst-case assumption
In the absence of air quality data from local or similar areas, it might be necessary to simply guess the existing air quality. The safest guess is to assume a concentration at the upper end of what might be feasible. Some examples are provided in Table 7.1. It is important to ensure that the 'existing air quality without project' concentration selected is appropriate and relevant to the location of the proposed project. For example, to avoid 'double counting' it is important to select existing air quality data from an area that is not influenced by the discharge under consideration, if the assessment includes dispersion modelling to predict the impact of any existing and proposed additional discharges.
Table 7.1: Examples of existing NO2, PM10 and CO concentration 'without project'
Area where estimate of background air quality is required |
Pollutant |
Value to assume |
Justification for worst-case assumption, based on review of data to 2004 (extracted from various council monitoring reports and web site data in mid-2005) |
|---|---|---|---|
An urban area with a significant wood- or coal- burning problem (eg, a gazetted airshed) |
NO2 |
150 (µg/m3) |
10-year average of maxima, Packe Street, Christchurch = 124. |
PM10 |
100 (µg/m3) |
Christchurch, Nelson, Timaru, Masterton, Mosgiel, Arrowtown, Richmond and Kaiapoi have all recorded peaks of over 100 (the highest is 252 in Christchurch in 2002). |
|
CO |
8 (mg/m3) |
The highest values recorded in Christchurch have been slightly above 8. |
|
Area with poor dispersion (eg, urban canyon) within 5 m of a busy intersection or congested area (with over 10,000 vehicles per day and/or wood or coal burning) |
NO2 |
340 (µg/m3) |
4-year average of maxima Khyber Pass = 343. Khyber Pass is a peak traffic monitoring data for NO2 (traffic approx 30,000 vehicles/day, air quality monitoring < 5 m from roadside) |
PM10 |
80 (µg/m3) |
Even smaller centres that have poor dispersion can record high values (Reefton 55, Nelson 165, Wainuiomata 57, Upper Hutt 60). |
|
CO |
10 (mg/m3) |
The highest values recorded in Auckland have been slightly above 10. |
|
Area within 20 m of vehicle routes of over 10,000 per day, or within 100 m of a motorway |
NO2 1 hr |
140 (µg/m3) |
10-year average of maxima, Auckland Penrose = 139. |
PM10 |
70 (µg/m3) |
There are not many sites in this category with monitoring results, but Auckland’s Khyber Pass has recorded 81, almost certainly largely due to traffic. |
|
CO |
5 (mg/m3) |
4-year average of maxima, Peachgrove Road, Hamilton = 4.75. |
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Urban area that doesn’t have significant wood-burning problem and no vehicle routes of over 10,000 vehicles per day within 20 m, or motorways within 100 m |
NO2 |
50 (µg/m3) |
Hastings, less than 1 year of data, maximum = 36. |
PM10 |
40 (µg/m3) |
Residential neighbourhood monitoring sites in Hawke’s Bay and Bay of Plenty have recorded occasional exceedances of the PM10 standard, although averages of maxima taken over several years tend to be lower than 40. |
|
CO |
2 (mg/m3) |
Maximum concentrations measured at neighbourhood sites in Upper Hutt, Lower Hutt and Masterton are typically 2 or less. |
|
Rural area, or urban area that is very open with low population density |
NO2 |
15 (µg/m3) |
Masterton 2-year average of maxima = 13.5. There are no results available from rural monitoring sites. Masterton is the lowest result for a 'residential neighbourhood' site, so this is a worst-case assumption for a rural area. |
PM10 |
15 (µg/m3) |
This is a typical maximum concentration when no obvious sources occur upwind. |
|
CO |
0 (mg/m3) |
With no local sources, CO concentrations are generally very low, and can be taken as effectively zero. |
[Some units in the web version of this table were corrected from milligrammes to microgrammes on 3 November 2008 to match the print and PDF published versions.]
7.4 Assessing the effects
To assess whether significant air quality impacts are likely, the estimated concentrations of pollutants are compared to assessment criteria (discussed in section 5). For the most common pollutants, the primary criteria are the national environmental standards for air quality and the New Zealand Ambient Air Quality Guidelines. As noted in section 5, air quality objectives in regional plans should also be considered as these take precedence if they are more stringent than the national ambient air quality standards or guidelines.
The maximum ground-level concentration (including background) predicted by the screening methodology should be compared to the relevant assessment 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:
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the predicted concentration (including background) of nitrogen dioxide or carbon monoxide exceeds the assessment criteria, and the industry-derived concentration is a 'principal source' of the exceedance (the meaning of principal source is discussed in section 5), or
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the proposal is within an airshed that does not already breach the PM10 standard, and the predicted concentration (including background) exceeds the assessment criteria for PM10, or
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the proposal is within an airshed that already breaches the PM10 standard, and the proposal is predicted to significantly increase the concentration of PM10 in the airshed; and/or the predicted concentration, including background, exceeds the straight-line path (or curved-line path) (factors to consider when determining significance are discussed in section 5), or
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the predicted concentration (including background) of any other contaminant exceeds the relevant assessment criteria.
The requirements of the Standards are summarised in previous sections. If there is doubt (eg, about whether the predicted increase in PM10 is significant), a Tier 3 assessment should be undertaken. A Tier 3 assessment should also be undertaken if the Tier 2 assessment is inconclusive for any reason.
The aim of a Tier 2 assessment is to provide a conservative estimate of the likely air quality impacts of a proposal. If these criteria are exceeded, this does not necessarily mean the air quality impacts will be unacceptable. 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, although 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 assessment of environmental effects, 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 for the emission information, air quality information, any assumptions, and their justification. Recommended reporting requirements are given in section 4.4.
Tier 2 screening assessment − recommendations
Tier 2 provides for the next level of assessment. It is distinguished from Tier 1 in that most Tier 1 processes will be permitted activities or have trivial discharges. Tier 2 processes, on the other hand, are likely to be discretionary activities and need a more quantified level of assessment. Some guidance is given on the likely features of a Tier 2 assessment, although different regions may have different specific requirements. If there is any doubt, discuss with the council consenting officer.
