View all publications

1 Introduction

This report provides an overview of sources of carbon monoxide (CO), oxides of nitrogen (NOx), sulphur dioxide (SO2), ozone (O3), benzene and benzo(a)pyrene (BaP) in New Zealand.

The assessment of sources of emissions of air contaminants is based on emission inventory studies that have been carried out by regional councils for most of the larger urban areas of New Zealand. An emission inventory provides a quantitative assessment of the amount of emissions of a particular contaminant from selected sources. Estimates of emissions are based on information relating to the frequency and type of activity as well as the use of average emission rates or other emissions information applicable to the activity. Typical sources included in urban area inventories incorporate domestic solid fuel burning, motor vehicles and industrial emissions. In some emission inventory assessments sources such as outdoor burning, lawn mowing, port, marine and rail activities and other activities have also been included. Natural sources are also a contributor to emissions of some contaminants, e.g. NO2, and have been included in some inventory assessments.

The emission inventories carried out in New Zealand have typically included an assessment of emissions of PM10, CO, oxides of sulphur (SOx) as an indicator of SO2, oxides of nitrogen (NOx) as an indicator of NO2, volatile organic compounds (VOC) and carbon dioxide (CO2). Some more recent inventories like Timaru, Nelson and Richmond, have included benzene, while the Timaru inventory also includes an estimate of BaP. Estimates of both benzene and BaP are flagged as indicative only in these inventories as there are high levels of uncertainty regarding emission factors for these sources.

Although no estimates of emissions of ozone are included in the inventories, the VOC and NOx data can be used for assessing potential sources of ozone, as both are precursors to ozone formation. The VOC data is collected specifically for this purpose and should exclude VOCs that do not impact on ozone, although there is some uncertainty surrounding the extent to which all emission factors account for this. Because of the impact of atmospheric chemistry in the formation of ozone, source assessments based on VOC and NOx emissions should be considered indicative only.

In addition to atmospheric chemistry, meteorology and topography also impact on the relationship between estimates of contributions to emissions and actual contributions to concentrations. The relative contribution of different sources to concentrations of contaminants across an area will depend on the temporal and spatial variations in sources and meteorology at different times of the day. It should also be noted that the inventory estimates represent an average across an area. The actual contribution to concentrations at any given point within that area will vary depending on proximity to sources and local meteorology.

To assist in the preparation of quality emission inventories, the Ministry for the Environment's Sustainable Management Fund funded the preparation of a Good Practice Guide for Preparing Emission Inventories in New Zealand in 2001 (Wilton, 2001a). The processes involved in preparing an emission inventory, as described in that report, are shown in Figure 1.1.

Figure 1.1: Overview of the process for preparing an emission inventory (Wilton, 2001a)