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Executive Summary

The purpose of this guide is to provide good-practice protocols for modelling the dispersion of discharges to air from industrial complexes in New Zealand. Guidance is provided for all modellers, from relative newcomers to experts. The guideline provides recommendations which direct modellers towards adopting a best practice approach. The recommendations are somewhat prescriptive, but allow flexibility. They are consistent with current practice in Australia and the USA, with some adaptation for New Zealand-specific conditions. The practitioner should always justify the methods used, whichever modelling approach is taken.

For convenience, dispersion model types are divided broadly into steady-state Gaussian-plume models and 'advanced' models. This is a differentiation on roughly historical grounds: plume models have been in common use for decades, while advanced models are beginning to be used more widely for regulatory applications. From a practical standpoint, the greatest difference between model types is in the requirements of meteorological information and computer resources. However, some 'steady-state' models are highly sophisticated and not necessarily 'Gaussian', so the distinction can be blurred. Although the guide encourages modellers to move towards advanced models - because in principle they are more realistic - it does discuss the advantages and limitations of all model types. The use of an 'advanced' model need not be the best option.

This guide provides useful guidance for the modeller by discussing specific models currently in use in New Zealand. The list includes AUSPLUME, ISCST3, AERMOD, CTDMPLUS, CALPUFF and TAPM. Model configuration, data requirements, model applicability, physical and chemical formulations and the interpretation of results are discussed for these models.

Much of the guide is devoted to practical advice and provides recommendations on the aspects of dispersion modelling essential to a realistic assessment using a dispersion model. These aspects are the choice of input parameters, the specification of emissions and meteorology, and the analysis of results.

A chapter on model configuration discusses model domain size and receptor distribution, dispersion parameters, stability class specification, the use of turbulence measurements, settings for plume rise and inversion penetration, land-use variations and averaging times. It also describes how the different models simulate emissions from different source types, and provides guidance on emission factor databases and on accounting for time-varying emissions. It further describes how the models simulate the interaction between pollutant plumes from different sources within an industrial complex, in terms of building wake effects and enhanced plume buoyancy.

The simulation of terrain effects on pollutant dispersion is examined in detail, including a description of methods used by the main models.

There is some discussion on atmospheric chemistry - a common requirement is the determination of NO2 concentrations, given emissions of NOx. A couple of empirical methods for this are described, although the guide does not favour one over the other.

A complete chapter of this guide is devoted to the meteorological aspects of dispersion modelling. The complex terrain of New Zealand, and the coastal location of most settlements, can lead to highly complicated meteorological features in the vicinity of many pollutant sources. These include land-sea breezes, slope-valley flows and internal boundary layers (with associated fumigation effects), which may cause complex patterns of pollution dispersion.

A fundamental difference between steady-state and advanced models is in their meteorological data requirements. The development of single-site meteorological data for steady-state dispersion models is discussed, including screening data sets, the treatment of calms, missing data, and the derivation of parameters such as stability class and mixing height. The development of three-dimensional time-dependent meteorological data sets for advanced dispersion models using prognostic and diagnostic models is also discussed in detail. The advantages and limitations of all approaches are examined.

Guidance on the analysis of model results is given, to ensure that results are realistic and credible. This includes model validation, assessment of uncertainties and sensitivity tests. Advice is given on the presentation of statistical summaries, tables, graphs and contour plots at the reporting stage. There is also guidance on the incorporation of background concentrations and the assessment of environmental and health effects.

The good practice guide focuses mainly on discharges from industrial sources, but there is some discussion on other specialised applications, such as airshed modelling, dispersion from roadways, regional and long-range transport, accidental releases, steam effects and visibility. Many of the recommendations regarding industrial discharges apply equally to these other cases.

The guide attempts to be forward thinking by acknowledging that dispersion modelling requirements (that is, new applications) and the models themselves are changing, and by providing guidance on the use of the latest, state-of-the-science dispersion models.