Background air quality is the level of contaminant across the airshed from all sources. This includes contributions from natural sources (eg volcanoes, forest fires, wind-blown dust, etc) and from human-generated sources (eg industry, domestic heating and ‘remote’ roads).
Background air quality is used to assess the cumulative impact of a discharge to air. In the case of a roading project, the air pollution coming from the road being improved (the relative impact) needs to be added to the background concentration of air pollution (ie background air quality excluding nearby roads) to allow the cumulative (or absolute) impact of the project to be calculated.
The Transport Agency has developed default background air quality values by census area unit. This information can be used as an input into an air pollution dispersion model, like the Agency’s Air Quality Screening ModelAir Quality Screening Model to estimate the cumulative impact. These default values are provided in the background air quality map (below) and in the background air quality data spreadsheet (a full version of the spreadsheet is available upon request from environment@nzta.govt.nz ). Information on how these values were derived is available in the Background air quality for NZ Transport Agency state highway assessments technical report.
The Transport Agency has released a draft Background air quality guide on applying these default values to project assessments and how to assess whether predicted air quality impact from a road project, combined with background air quality, is likely to result in air quality standards or guidelines being exceeded.
In order to reduce uncertainty and comprehensively assess the air quality effects of a roading project, the default background air quality values may need to be supplemented with more site-specific information. Suitable data may be available from regional councils, the Transport Agency or research organisations. However, projects with a high air quality risk, typically large projects in major urban centres, may require a dedicated monitoring campaign to to determine background air quality.