This report provides an overview of the sources contributing to emissions of carbon monoxide (CO), nitrogen oxides (NOx), sulphur dioxide (SO2), precursors to ozone formation, benzene and benzo(a)pyrene (BaP) in New Zealand. The assessment 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 a particular contaminant emitted from selected sources. Estimates of emissions are based on information relating to the frequency and type of activity and 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 of 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.
Results of the emission inventories for CO indicate some differences between areas, although in general, motor vehicles and domestic home heating contribute the most CO emissions. Nitrogen oxide emissions primarily occur as a result of emissions from motor vehicles, and sources of SO2 tend to be dominated by industry, or split between motor vehicles and domestic home heating in areas where industry is not prevalent.
Precursors to the formation of ozone include nitrogen oxides and volatile organic compounds. While motor vehicle emissions dominate the former contaminant, sources of VOC emissions are more variable. Domestic heating and motor vehicles tend to be dominant contributors, although in some areas natural vegetation sources and industry are dominant.
Only a limited number of emission inventories have included estimates for sources of benzene and benzo(a)pyrene. In these areas, benzene emissions tend to be dominated by motor vehicles and domestic home heating. Domestic heating is the dominant contributor to BaP emissions in both areas where inventories have included estimates for this contaminant. Similarly, monitoring data for Christchurch suggests that this source is likely to be the dominant contributor.
Limited information is available on trends in sources of emissions. Vehicle emissions projection data from the Ministry of Transport’s NZTER emissions model indicates a decrease in tailpipe emissions of CO and NOx per vehicle with time. The extent to which this may result in a decrease in actual motor vehicle emissions over time, however, will also depend on changes in traffic volume. At this stage it is difficult to assess actual trends in sources of emissions as a second emission inventory assessment has only been carried out in a few locations and methodological differences between these inventories may complicate these comparisons. In the absence of additional regulation, emissions from industry and domestic heating could increase with growth in these sectors.