The introduction of the national environmental standards later this year will see heightened public awareness of air quality issues. Driven largely by a strong need for action on ambient levels of particles in most parts of the country, the standards lay the foundation for an effective air quality management framework. Atmospheric dispersion modelling is an essential tool in air quality management by providing the link between environmental effects and discharges to air. Its use has grown rapidly in New Zealand over the past 10 years and models are now commonplace in many resource consent applications for discharge permits.
Dispersion modelling is a complex process and, as with all models, the results are only as useful as the model itself and how it is used. Many different approaches to modelling have emerged in New Zealand under the Resource Management Act 1991, and at times models have been used incorrectly, causing problems such as inaccurate data, which can mislead an assessment of environmental effects. These issues often delay the processing of resource consents, and can result in expensive hearings where experts argue over the merits of their preferred models and how they should be used.
In a first step to resolving such issues, this draft guide provides expert and well-debated guidance on dispersion modelling through a series of recommended protocols. To improve consistency and accuracy in modelling, the guide is reasonably prescriptive, but the recommendations are not regulatory requirements so there is flexibility to handle the wide variety of circumstances that occur in New Zealand. Deviations from the recommended approaches can be taken, although these should be clearly explained and justified.
Correct interpretation of modelling results against the national environmental standards and determination of the potential effects of a discharge are as important as accurate modelling results. This guide does not include guidance on interpreting results. Instead, this will be included in a Good Practice Guide for Assessing Discharges to Air (currently under development by the Ministry).