Air quality monitoring throughout New Zealand indicates that concentrations of suspended particles (PM10) in the air exceed the ambient air quality guideline values in many urban areas during the winter months. In order to manage these emissions it is important to have a good understanding of the sources of particle pollution.
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 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.
This technical report comprises a review of emission inventory studies into sources of PM10 in urban and some rural areas of New Zealand.
Emission inventories have been successfully used in New Zealand as an air quality management tool to develop strategies to reduce ambient air concentrations of PM10. Examples of the application of an emission inventory to air quality management include Christchurch (Wilton, 2001a) and Nelson (Wilton, 2002).
To assist in the preparation of quality emission inventories, the Ministry for the Environment's Sustainable Management Fund funded the preparation of the Good Practice Guide for Preparing Emission Inventories in New Zealand (Wilton, 2001b). The process involved in preparing an emission inventory, as described in that report are shown in Figure 1.1.
One of the key aspects to preparing a useful emission inventory is the design phase of the inventory and the consideration of existing air quality issues. Because concentrations of particles have been identified as the main ambient air quality issue in many urban areas, emission inventories for many urban areas of New Zealand have been designed with a focus on sources of PM10.
When considering the results of emission inventory studies, it is important to note the difference between estimating contributions to emissions and determining the contribution to concentrations. There are two points that are important in this distinction. Firstly, the emission inventory estimates emissions only. The relative contribution of different sources to PM10 concentrations across an area will depend on the impact of meteorology, and in particular temporal and spatial variations in sources and meteorology at different times of the day. The second point is 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.
A further limitation to the use of emission inventories as an air quality management tool is that reliable techniques for the estimation of emissions from natural sources such as sea spray and wind blown dusts are yet to be developed. In some areas, these sources may be significant contributors to PM10 concentrations. In such instances, alternative methods of source determination such as receptor modelling methods may be more appropriate.