The amount of monitoring required must be assessed on a case-by-case basis. For major projects it is recommended that monitoring be undertaken both before and after the project to ensure the assessment was correct and to quantify the impacts of the proposal on air quality.
The purpose of pre-project monitoring is to establish existing air quality. This is discussed in section 8. The primary purpose of ongoing post-project monitoring is to ensure effects remain within acceptable limits. A secondary purpose may be to ensure the model predictions were correct. In the case of traffic counts, the monitoring would determine the reliability of the original traffic modelling and perhaps form part of a traffic management system or serve as the basis for enforcing a limit on vehicle flows.
Ambient air quality monitoring should determine the concentrations of the critical indicator contaminants (generally CO, NOx and PM10, and possibly benzene). It is recommended that traffic counting, ambient air quality monitoring and meteorological monitoring be undertaken simultaneously to provide adequate data for trend analysis. Comprehensive guidance on setting up ambient air quality monitoring stations is provided in the Guide to Air Quality Monitoring and Data Management (Ministry for the Environment, 2000a).
It is impossible to prescribe exactly when ongoing monitoring may be necessary because many factors are involved, but some likely factors are:
sensitive receptors are in close proximity
the predicted effects are close to the margin
the predictions had a high level of uncertainty, and/or
monitoring forms part of a mitigation measure (eg, flow restrictions)
the scale of the project is such that regional impacts are likely, entailing the need for a review of the regional monitoring network.
Although monitoring needs must be assessed on a case-by-case basis, it is likely that a major project will require at least some monitoring to account for both local and regional effects.
Site selection for pre- or post-project monitoring will also need to be assessed on a case-by-case basis. The purpose of the monitoring should be clearly defined and carefully considered when selecting any monitoring site. Monitoring sites should comply with the Guide to Air Quality Monitoring and Data Management.
Post-project monitoring offers an assurance to the community, the transport developer and the regulatory authorities that the project turned out as predicted and that there have not been any adverse health impacts. Alternatively, monitoring data showing abrupt changes in ambient levels of indicator compounds provide a solid basis for remedial actions.