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Good Practice Guide for Assessing Discharges to Air from Industry

Acknowledgements

Foreword by the Ministry

1 Introduction

2 Legislative Context

3 Assessing Discharges to Air: General Considerations

4 The Assessment Process

5 Air Quality Criteria

6 Tier 1 Preliminary Assessment

7 Tier 2 Screening Assessment

8 Tier 3 Full Assessment

Appendix 1: Assessment Checklist

Appendix 2: NO to NO₂ Conversion

References

8 Tier 3 Full Assessment

The aim of a Tier 3 assessment is to provide reasonably accurate estimates and a detailed assessment of the likely air quality impacts associated with a proposal. This is usually done through the use of detailed emission information, a topographical profile, dispersion modelling and background air quality and meteorological data. For any aspect of the assessment where detailed information is not available, or is not required, it is appropriate to adopt the conservative assumptions discussed for Tier 2 assessments.

An overview of the Tier 3 assessment is shown in Figure 8.1.

8.1 Characterising the discharges to air

Characterising the discharges to air includes:

• identifying all air discharges associated with the development (they may be point sources such as stacks and vents, area sources such as ponds or disturbed soils, or volume sources such as fugitive emissions from buildings)

• describing the sources of the discharge (including all process factors influencing the quantity and nature of the discharges)

• detailing the nature of the emissions (pollutant types, concentrations and mass emissions, emission temperatures, velocities, particle sizes, etc)

• detailing the measures employed to prevent and minimise the discharges.

The following discussion offers guidance on the type of information that should be provided to describe the proposed development and the potential sources of air emissions.

8.1.1 Description of the proposal

The description of the proposal should be sufficient to enable a full understanding of the application from an air discharge consent viewpoint, and should also provide sufficient information to ascertain whether any other consents are required.

The depth of information required will vary depending on the type of activity, although for industrial activities the following should generally be provided:

• a detailed process flow diagram or, if more appropriate, a process and instrumentation (P&D) diagram showing all existing and/or proposed plant and equipment included in the proposal, including emissions control equipment and emission points

• a written summary of the process flow from raw materials to final product, focusing on aspects that give rise to emissions to air

• details of alternatives considered and reasons for their rejection

• details of any relevant historical information, including past changes to the activity

• details of any proposed changes to an existing activity

• any relevant timeframes or constraints for undertaking the activity

• details of all emissions control equipment, including design-criteria calculations and design drawings

• details of potential upset/emergency conditions, including their influence on emissions

• any mitigation and/or preventive measures undertaken on site for both ordinary and accidental emissions to air, including management techniques (eg, management plans), alarms, interlocks, monitors and control equipment

• maximum and normal processing capacities

• maximum and normal ratings, capacities and throughput of all major plant equipment, including boilers, air discharge control equipment, driers, mixing tanks and crushing plant

• details of any materials-handling procedures and mitigation measures in place for raw, intermediate, by-product and finished materials.

Because the application is defined by the process description, consent can only be granted for what is applied for. Therefore, all details possible relating to current and proposed operations should be included to enable all matters to be properly considered. Any proposed changes to an existing process should be highlighted.

Consent applicants should recognise that data provided in an assessment will often form the basis for setting the consent conditions. For example, fuels in a combustion process may be limited to those identified in the application, or dispersion-modelled emission rates may be adopted as stack emission limits. It is therefore important that any application reasonably reflects all anticipated operational scenarios for a development in order to provide for the desired flexibility.

8.1.2 Defining the discharge for dispersion modelling

Section 4.1 of the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a) provides a description of the information required to define the variety of discharge points for dispersion modelling.

An assessment of emissions from industry to air should also address abnormal or uncommon emission scenarios, including start-up, shut-down, upset conditions and emergency release. These scenarios often result in elevated emissions, or emissions of chemical intermediates that would not normally be released. It is often appropriate to assess these using a risk-based (probabilistic) approach; that is, considering not only the consequences of the release but also the likelihood of it occurring. Consideration of these occurrences is often tailored to a specific situation; for example, considering only short-term average assessment criteria commensurate with the duration of exposure.

Where a project goes through different stages of development, such as in a mine or an industrial process with anticipated production growth, these project-life operating and emission changes should also be addressed in the assessment.

Compliance Monitoring and Emissions Testing of Discharges to Air (Ministry for the Environment, 1998) provides guidance for obtaining pollutant emission rate and concentration data by measuring an existing source. If the assessment will be based on previous monitoring results, the data collection method should be audited against the methods specified in the guide.

It is important to note that although measured emissions data are more representative than estimates from process data (eg, emissions factors or engineering calculations), they too are typically only accurate to within ±15%. Variations in process can further add another ±10%, so that what is measured and modelled as 1.0 g/s may in fact be as high as 1.3 g/s or as low as 0.8 g/s. This may have a significant impact on downwind concentration estimates. The Good Practice Guide for Atmospheric Dispersion Modelling addresses this by recommending the use of a maximum emission rate to cover the worst-case discharge of concern.

Proprietary process simulation software can also provide useful emissions data (eg, GT PRO for gas turbine emissions). A number of air quality professionals also make use of combustion calculation software.⁴

As noted above, the Ministry for the Environment provides guidance on both characterising release points for the purpose of dispersion modelling and on emissions monitoring (Ministry for the Environment, 2004a and 1998, respectively).

8.2 Characterising the receiving environment

8.2.1 Existing air quality

Information on existing air quality is essential for assessing the effects of new industrial developments. It is not just the increase in air quality attributed to the industry, but rather the cumulative impact of any increase to existing pollution levels and how that compares with the appropriate air quality criteria that requires assessment.

When existing air quality data are required

Existing air quality should be considered in all assessments of discharges to air. The level of detail and accuracy required is influenced by the:

• nature of the discharge – large discharges or discharges of pollutants of high toxicity, which may have the potential to adversely impact the environment, would be expected to require a thorough assessment of the existing air quality

• anticipated air quality – areas that are anticipated to have poor air quality due to a combination of existing emission sources and/or adverse terrain or meteorology would be expected to require a more robust definition of the existing air quality

• sensitivity of the receiving environment – where discharges have the potential to affect highly sensitive receiving environments (see Table 6.1), existing air quality would be expected to be well-defined.

It is the combination of these considerations that determines the extent to which existing air quality should be addressed. A small emission of a low-toxicity pollutant within a commercial/ light industrial area, for example, might only require a qualitative statement on existing air quality identifying the reasons existing air quality is anticipated to be good. Conversely, a large-scale industrial source with the potential to have an impact on residential suburbs might be expected to provide good-quality, representative and quantitative air quality data.

As well as the generic considerations identified above, in situations where a proposal might result in increased PM₁₀ emissions within a gazetted airshed (as defined under the Standards), existing air quality is likely to be given greater attention.

Identifying existing data

The range of options for generating air quality monitoring data, ranked in order by the increasing effort required to obtain the data, are:

• use surrogate data from locations with air quality characteristics representative of the area of interest

• use existing monitoring data sets for the area of interest

• use atmospheric dispersion modelling to predict air pollutant concentrations due to existing sources (as well as the concentrations due to the proposed development)

• commission a monitoring programme specifically for the purposes of the consent.

Pre-existing air quality data can be obtained from a range of sources. The consenting authority will usually have the best knowledge of the full range of data available within its region, and will also be able to provide an opinion as to whether the pre-existing data are sufficient for the assessment proposed.

The range of air quality data sources is identified in section 6.4.1 of the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a). Atmospheric dispersion modelling may be the preferred approach for estimating existing air quality where:

• there are a small number of existing emission sources in the area for which reliable emission data are available

• any contribution to ambient levels from other hard-to-characterise sources (such as vehicle emissions, domestic fires or dust from wind erosion) is negligible.

This situation would be unlikely to occur in urban areas of New Zealand. Again, the Good Practice Guide for Atmospheric Dispersion Modelling provides advice on the appropriate application of dispersion models.

Overall, the pollutants released from the proposal and the assessment criteria that are available for those pollutants determine the type of existing air quality data required for any assessment.

Reviewing existing data

Guidance on assessing the quality of currently available ambient air quality data as well as quality assurance and control procedures for the collection of new air quality data is provided in the Ministry for the Environment’s (2000) Guide to Air Quality Monitoring and Data Management. The assessment criteria often include requirements for the air quality monitoring technique to be used. For example, the Standards contain specific requirements for the monitoring of pollutants within gazetted airsheds. The Ministry for the Environment’s Ambient Air Quality Guidelines also contain recommended monitoring methods.

The use of such methods reduces uncertainty and minimises inaccuracy. Before using existing air quality data in an assessment, it is important that the monitoring technique and protocols be audited against the requirements of the Standards and the Guide to Air Quality Monitoring and Data Management to demonstrate that the existing air quality data are of appropriate quality.

The location of a monitoring site and the time of monitoring also affect how representative existing air quality data might be. The site should be representative in terms of location (ideally, within the affected airshed), but also representative in terms of land use and physical setting. The specific location of the monitoring site (eg, its proximity to major sources such as roads and other industry) will also be important.

The time of the monitoring is also relevant, in that data collected at the site in previous years may not be representative if the character of the area has changed markedly since monitoring was last undertaken. For example, historical data from an area that has experienced significant population growth and commercial expansion (and hence increased vehicle and potentially industrial emissions) may no longer be representative of current levels.

Trends in air quality should be considered, and it is preferable for several years of data to be analysed so that any improvement or deterioration of the air quality of an area can be ascertained. As a minimum, one year of data could be used if there are other longer-term monitoring sites in similar locations which can be used to provide an indication of long-term trends. Ideally, 10 years of data should be used to determine trends.

As noted above, monitoring data should be reviewed with reference to the monitoring method identified in Schedule 2 of the Standards and other Ministry for the Environment guidance (Ministry for the Environment, 2000a, 2002 and 2004a).

8.2.2 The built environment

Sensitivity to air quality will vary with land-use type. For example, residential land use (including schools) will typically have greater sensitivity than an industrial setting.

The land use surrounding a proposed development should be reviewed and described in any assessment of air quality impacts. The review will provide both an indication to any developer of the likely acceptability of, or objection to, a proposal and a guide to the depth of consultation required. The sensitivity will also be one factor that influences the level of assessment of environmental effects required for a proposed development.

Land-use zoning must also be reviewed in the relevant district plan to ascertain whether the proposal (in its proposed location) is permitted. District plans provide maps that generally zone the district by land-use types. District plans also provide rules that identify the limitations on land use, either within those zones or in the wider district.

Authors of district plans and developers should also be aware of the issue of 'reverse sensitivity'. Reverse sensitivity relates to sensitive land uses encroaching on, for example, industrial facilities. Allowing such encroachment is seen as having potentially adverse effects on the health, safety or amenity values of people, as well as potentially adversely affecting the economic and safe operations of industries.

Case precedents in the Environment Court (see ARC vs ACC, RMA10/97) mean that new designations of sensitive land uses within the vicinity of industry may be turned down on the basis of reverse sensitivity. The use of buffers minimises the effects of reverse sensitivity, and it is recommended that buffers be owned by the industry creating the discharge.

8.3 Exposure estimates

8.3.1 Dispersion modelling

Atmospheric dispersion models are used to estimate contaminant concentrations downwind of a discharge source. The prediction of pollutant concentrations using atmospheric dispersion models has been covered in detail in the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a). This contains guidance on the application of models for a range of emission types, and meteorological and terrain scenarios. Of particular note is Recommendation 59, which deals with the issue of model accuracy and how it should be dealt with in a regulatory context.

8.4 Assessing the effects

8.4.1 Incorporating background concentrations

Background air quality data and predicted pollutant concentrations must be considered together against the selected assessment criteria. Adding the background data and predicted results to provide an estimate of the cumulative impact for comparison with the selected assessment criteria is reasonable for annual average concentrations. For short-term concentrations, this simplistic approach is appropriate where the criteria are not breached, although it is a very conservative approach and a more accurate assessment may be necessary where compliance is an issue.

The above approach can lead to an overly conservative assessment due to issues relating to the spatial and temporal coincidence of background and predicted concentrations, as follows.

• Spatial co-incidence problems - it is often difficult to know whether the background data are representative of the point at which the modelled peak occurs. In general, they will not be located in the same place, so adding the two will overestimate actual future concentrations.

• Time co-incidence problems - both modelled and background concentrations vary with the time of day due to factors such as meteorological patterns, operational variations and changes in background emission sources (eg, winter emissions from home heating and/or peak traffic emissions). In most cases, the peak caused by a point-source emission does not occur at the same time as the background peak, so adding the two together may again overestimate the future concentration.

For the highest percentiles (ie, concentration values close to the peak short-term concentration of a year’s worth of such concentration predictions), simple addition can overestimate the source contribution, and in general the overestimate is more severe for the higher percentiles.

The best predictive assessment technique is to use hourly, sequential ambient air quality monitoring data that are recorded in the airshed of interest, and then add the hour-by-hour predicted concentrations. These predicted concentrations should be made using meteorological data recorded at the same time as the recorded air quality data. Where data are available, such an approach is recommended.

It is rare for all this data to be available, however, and the UK Environment Agency study has investigated some alternative approaches. A simpler approach, which gave better accuracy than some and equal accuracy to the best alternative statistical approach, was to add the predicted short-term average concentration to twice the annual average background concentration.

This approach will not generally be applicable for pollutants such as PM₁₀, for which more locally specific techniques could be used. For example, if the PM₁₀ levels are known through monitoring in some nearby location that has obvious geographical and emissions similarities, then this can be used as a proxy for the background concentration.

8.4.2 Accumulation

Many of the effects assessments discussed so far are based on various standards, guidelines and criteria relating to acceptable concentrations in the air. However, some discharges have the potential to deposit on the surface and accumulate over the entire lifetime of the discharge. These are often in the form of particulates, but they can be gaseous.

A common example is mercury, which is a component in coal and is discharged in both particulate and gaseous (vapour) phases. Although the current ambient concentration guideline may be met, under some circumstances − such as a very long lifetime for the discharge (20 years or more), in particularly sensitive areas (eg, near a market garden), or in particularly difficult terrain (eg, near a hill) − there may be accumulated effects due to deposition that allow mercury to build up in the soil to unacceptable levels.

Mercury is one example, but the situation applies to all other heavy metals, many toxic organics that do not degrade rapidly, and other contaminants that might have effects as well as being non-degradable. These factors should be evaluated, and modelling can be used to assess potential accumulation rates. Assessing the actual effects can be more difficult, since for some compounds the long-term effects are not well known. A more detailed account can be found in the Ministry for the Environment’s guide on ecosystem effects (Ministry for the Environment, 2000b).

8.4.3 Atmospheric chemistry

The chemical transformation of emissions during transport in the atmosphere can be another important consideration. For example, the perception of odour can change between the source and the receiving environment due to chemical transformation, although there is no practical way to assess this effect.

Perhaps the most commonly encountered issues with regard to atmospheric chemistry are:

• the oxidation of NO to NO₂ when NOx (oxides of nitrogen) are released from industrial emissions

• the formation of the secondary pollutant ozone (O₃) following release of NOx and volatile organic compounds (VOCs) from industrial and other anthropogenic sources

• the formation of secondary particulates from sulphur and nitrogen discharges.

Methods for assessing these issues were discussed and identified in section 4.3.6 and Appendix C of the Good Practice Guide for Atmospheric Dispersion Modelling (Ministry for the Environment, 2004a).

To estimate NO₂ concentrations from modelled NOx concentrations, the methodology proposed in Appendix C of that guide is recommended. This is a simply applied, conservative approach based on the US EPA O₃-limiting method, together with knowledge of the available background O₃ concentrations within air masses moving off the oceans and across New Zealand. Since the publication of the Good Practice Guide for Atmospheric Dispersion Modelling, further analysis has been conducted and a refined methodology developed. This is detailed in Appendix 2, with examples.

The formation of O₃ following the release of NOx and VOCs from anthropogenic sources is a large-scale regional effect, affecting rural areas surrounding major urban centres (where the precursor chemicals NOx and VOCs are released). The estimation of O₃ creation is technically complex due to the wide range of chemical reactions involved. Individual industrial emissions would not normally be expected to have a significant impact on O₃ creation in isolation from other urban sources, and only large industrial emissions sources of NOx and/or VOCs might be expected to have a discernible additional effect. Typically, only major industrial emission sources in, or near, large urban areas might be expected to assess such effects.

Complex models are available to assess the photochemical production of O₃, and consenting authorities might find such tools useful for airshed management of O₃. These tools are also available for developers for assessing potential effects of large-scale industrial emission sources of NOx and VOCs in major urban areas.

Section 4.3.6 of the Good Practice Guide for Atmospheric Dispersion Modelling recommends using models such as CALGRID and UAM-V. These are identified as having sufficiently complex chemistry schemes to enable examination of small changes in urban emissions generally associated with an individual industrial source.

A further model to consider is the (Australian) Commonwealth Scientific and Industrial Research Organisation (CSIRO) IER-Reactive Plume Model. The model is identified in the NSW Department of Environment and Conservation’s Approved Methods for the Modelling and Assessment of Air Pollutants in New South Wales (NSW DEC, 2001 and 2005). The model is identified as being suitable for predicting the effects of industrial source emission on ground-level O₃ concentrations.

8.4.4 Composition of minor constituents in New Zealand air

Some computer dispersion models require a knowledge of the baseline concentrations of constituents in the air. The standard global values can be found in many textbooks, but some of these are out of date and others are slightly different for New Zealand conditions. Table 8.1 gives some recent values (2003), many of which have been measured very accurately.

These values are based on measurements by NIWA at the Baring Head Clean Air Station, near Wellington, and are representative of values for uncontaminated air that has not passed over New Zealand (ie, only measurements made in clean southerly winds are included) (available from www.niwa.co.nz).

Table 8.1: Clean air background concentrations of some atmospheric gases in New Zealand

View clean air background concentrations of some atmospheric gases in New Zealand (large table).

8.4.5 Non-human health considerations

Air pollution effects on ecosystems

Section 4 of the Ambient Air Quality Guidelines (Ministry for the Environment, 2002) provides critical levels for protecting ecosystems from sulphur dioxide, sulphate particulate, nitrogen dioxide, ammonia, ozone and fluoride. The Effects of Air Contaminants on Ecosystems and Recommended Critical Levels and Critical Loads (Ministry for the Environment, 2000b) provides some guidance on methods for calculating pollutant deposition rates from predicted or ambient monitoring results, and guidance for assessing whether a discharge is likely to cause adverse effects on ecosystems.

In Europe and North America the effects on sensitive ecosystems of acid deposition and elevated pollutant concentrations from industrial and other anthropogenic sources have been subject to legislative controls for some decades. Similar effects in New Zealand are not so evident, although they have been reviewed in a number of Ministry for the Environment technical reports (eg, Ministry for the Environment, 1999c and 2000b).

Much of the work has drawn on knowledge of the effects on non-New Zealand (North American and northern European) plant species. The recommended critical levels and loads and the provisional guidance on assessing deposition and its effects given in The Effects of Air Contaminants on Ecosystems is based on this knowledge. There is scant information on either the effects of air pollutants on native New Zealand species or the current level of pollutant deposition or concentration in New Zealand’s natural environments. The robustness of any assessment of air pollution effects on ecosystems in New Zealand is therefore very vulnerable to these knowledge gaps. Despite these limitations, it is good practice to assess potential effects on ecosystems for any significant source that may have an impact on sensitive ecosystems.

Global climate change

Sections 104E and 104F of the RMA place climate change outside the remit of consenting authorities in their consideration of discharge consents. The assessment of effects of greenhouse gas emissions from industry on global climate change is therefore outside the scope of this document. Policy measures to control the emission of greenhouse gases are developed and led by the Ministry for the Environment.

Ozone depletion

The 1987 Montreal Protocol is an international agreement under which substances that deplete the ozone layer are being phased out.

The ozone layer, which sits about 15-30 kilometres above the Earth, reduces the amount of dangerous ultraviolet light that reaches the Earth from the Sun. Too much ultraviolet light can cause skin cancer and cataracts in people. It also distorts plant growth, damages the marine environment and leads to the breakdown of materials such as plastics.

New Zealand has ratified the Montreal Protocol and implemented its objectives through the Ozone Layer Protection Act 1996 and the Ozone Layer Protection Regulations 1996. This legislation controls ozone-depleting substances to prevent their release to the atmosphere through bans on their import and use, etc. Site-specific assessments of the effects of releasing such substances to the atmosphere should therefore not be necessary. The full list of ozone-depleting substances can be found in the Regulations.

8.4.6 Comparing model results with air quality criteria

Once all the above have been considered, the final step in the assessment is to compare the predicted results with the selected criteria. As noted in section 5, it is important to ascertain both the short-term and long-term impacts of discharges to air. For most assessments this will necessitate the use of both the national ambient air quality standards (one-hour and 24-hour standards) and the national ambient air quality guidelines (annual guidelines). In all cases, as noted in section 5, the selection of air quality criteria should be justified. In doing so the purpose of the standard or guideline should be clearly stated. None of the criteria provided in section 5 are levels that may be 'polluted up to'. It is also important to recognise the limitations of dispersion modelling when predicting impacts.

For relevant averaging times the model results for maximum, 99.9th percentile and 99.5th percentile concentrations should be given. As a rule of thumb, modelling using less than a 24‑hour average (eg, one hour) should present maximum and percentile concentrations, whereas averaging times of 24 hours or more should only show the maximum concentration levels.

The recommended procedure in the Good Practice Guide for Atmospheric Dispersion Modelling requires consideration of meteorology when assessing worst-case impacts. (This is reproduced in full below.)

Source: Ministry for the Environment, 2004a.

The above recommendation applies to one-hour time averages only. Following are a few examples.

• For sulphur dioxide, nitrogen dioxide and ozone, compare the highest 99.9 percentile predicted concentrations for all receptors with the one-hour national ambient air quality standards. A number of other percentile values (eg, 99.5th and 99th percentiles) should also be reviewed. The maximum predicted concentrations for all receptors should then be compared with the relevant eight-hour, 24-hour or annual average guidelines.

• For carbon monoxide, compare the maximum predicted concentrations for all receptors with the national ambient air quality standard of 10 mg/m³ as an eight-hour average.

• For PM₁₀, compare the maximum predicted concentrations for all receptors with the national ambient air quality standard of 50 µg/m³ as a 24-hour average. Similarly, compare the maximum predicted annual concentration for all receptors (if more than one year of meteorological data are used) with the national ambient air quality guideline of 20 µg/m³ as an annual average.

The review of other percentile values is very important because it furthers our understanding of how meteorology (and other factors) affect the maximum downwind concentrations.

Use monitoring data instead?

In the UK, modelled results may be compared with monitoring data. The guideline describes the uncertainties in model inputs as potential causes of disagreements between model results and long-term monitoring data. The use of multiple years of meteorological data for the model runs is recommended. It is also recommended that source information, and in particular emission estimates, be reviewed when monitoring results do not agree with model predictions.

The US EPA points out the difficulty in precisely modelling concentrations at an exact location for a specific time. The uncertainties in both the source and meteorological data limit the use of these event comparisons in identifying biases in the dispersion model. The US EPA Guideline on Air Quality Models permits the use of monitoring data for existing facilities if model estimates are not available. In such cases, the applicant is required to demonstrate with at least one year of valid monitoring data that model results are not applicable. Decisions on the number and locations of the monitors are made on a case-by-case basis.

8.5 When a health risk assessment is required

In some situations it may be necessary to undertake a more comprehensive air pollution health risk assessment as part of a detailed study. This would include determining exposure and dose via a number of different pathways (inhalation, dermal, ingestion, etc), assessment of dose-response data, and characterisation of the health risks from the exposure and dose assessments.

The air quality criteria discussed in section 5 are all designed to protect public health. In most circumstances it is appropriate to assess the potential health effects of discharges to air from industry by comparing model predictions with these criteria. An air pollution health risk assessment is typically only required if the national ambient air quality standards or guidelines are breached, or are close to being breached.

Health risk assessment should not be confused with health impact assessment, which may be required for significant projects or strategies. Health impact assessment is a formal approach used to predict the potential health effects of a policy or project, with particular attention paid to impacts on health inequalities. Guidance on undertaking health impact assessments for policy development is available from the Public Health Advisory Committee (www.nhc.govt.nz/PHAC/phac_pubs.html).

Circumstances in which a more comprehensive air pollution health risk assessment is recommended include when:

• there is a significant discharge of contaminants with no clear threshold for adverse effects

• community consultation outcomes and/or plan provisions require it

• there is a significant discharge and/or background concentration of contaminants that are toxic, carcinogenic, teratogenic, mutagenic or bioaccumulative

• background ambient air concentrations already approach or exceed national ambient air quality standards or guidelines, or regional objective levels.

The circumstance identified in the last bullet point needs special consideration. In this case it may be necessary to ensure that air quality is not further degraded as a result of the proposal (by implementing offsets, for example).

Health risk assessments are specialised tasks and are typically only undertaken for large, or particularly toxic, discharges. It is recommended that expert assistance be sought for any full health risk assessment.

8.6 Accuracy

Air quality assessment is a highly technical process. By way of illustration, it relies on:

• monitoring ambient air quality

• monitoring meteorology

• numeric prediction of meteorology by prognostic and diagnostic models

• monitoring stack gas and other source emissions

• estimating emissions (combustion calculations, simulations, etc)

• numeric prediction of plume dispersion in the atmosphere.

Accuracy is improved by adopting good practice techniques and equipment. The areas identified above are largely covered in the Ministry for the Environment good practice guides on ambient monitoring, emissions monitoring (Ministry for the Environment, 1998) and dispersion modelling (Ministry for the Environment, 2004a) (see Figure 1.2 in section 1).

Standard quality control techniques, such as the use of calculation checking and aspects as basic as checking that model data are correctly input, are extremely important and should be used. It is also recommended that within an assessment report each aspect of an assessment be auditable and repeatable so that the approach, assumptions and calculations can be independently reviewed.

In summary, an assessment report should include, or be supported by, supplementary reports or data including:

• emissions testing reports

• ambient monitoring reports

• equipment specifications

• model output files

• all input data

• copies of spreadsheets and calculations

• electronic model data input files, etc.

When using dispersion models in situations where there is a reasonable degree of uncertainty about any input parameters, sensitivity analysis should be undertaken. Model runs should be carried out simulating the higher and lower boundaries of expected input parameters (such as emissions estimates or stack temperature), as well as the best estimate. Such sensitivity analysis can improve the confidence in an assessment. This is particularly important where there is vulnerability to adverse effects at the upper boundary of input estimates.

For example, a process might be new and emission levels based on simulation. Sensitivity analysis might be undertaken by modelling both the expected and the more unlikely upper estimate emissions. Where such analysis predicts ambient concentrations to be above the assessment criteria for the upper case estimate only, a management response would be expected. The management response might require continuous emissions monitoring under a consent condition to verify the best estimate of emissions, with an associated emission management response agreed in the event that the upper emission estimate is subsequently found to be accurate.

8.7 Reporting a Tier 3 assessment

The results of a Tier 3 assessment should be included in any assessment of environmental effects. The report should summarise the findings of the assessment, including the basis for process information, air quality information, any assumptions and their justification. Section 4.4 describes the recommended content of an assessment report. The size and nature of the report will depend on the project, but for any Tier 3-type assessments each of the sections described should be included.

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