Technical Report No. 2
Department of the Environment and Heritage, May 2004
ISBN 0 642 54994 X
The presence of dioxins¹ in car exhaust was first reported in 1978. Since 1986, studies have been conducted in the US and Europe to measure polychlorinated dibenzo-p-dioxins (dioxins or PCDD) and polychlorinated dibenzo-furans (furans or PCDF) emissions from vehicles. Although it is clear that motor vehicles are sources of dioxins and furans, the magnitude of their emissions remains uncertain. There are several reasons for this. First of all, there are very few measured data available internationally on dioxins emissions from road traffic. Relatively few tests on dioxins emissions from vehicles are available considering the variety and numbers of vehicles currently in operation and the range of operational, technical and environmental conditions in which they are operated. Secondly, PCDD/PCDF emissions generally show large variability in emission rates and dioxin profiles. Finally, in many cases, published information on dioxins emission factors is contradictory.
As a result, the emission factors developed through desktop analysis in this report remain uncertain and the only way to reduce this uncertainty would be to conduct more measurements. In addition, it became obvious from the international literature review that a considerable amount of potentially useful information is not available. Gaps in data exist which will lead to added uncertainties in the emission factors developed in this study.
The rationale addressed a number of factors that (may) affect dioxins emissions and, therefore, emission factor development. These factors are:
- vehicle technology type
- fleet age
- situations that result in incomplete combustion
- study type
- measurement methodology
- test protocols
- fuel composition
- ambient temperature
- units and calculation methods
- engine calibration practice.
Consideration of these factors led to a selection of useful data. However, due to the uncertainties associated with dioxins emissions, widely scattered emission factors were derived for the Australian situation. The data did not allow for the development of average emission factors; hence, an emission factor range (minimum to maximum) rather than a point estimate was deemed more appropriate. As a consequence, a range of Australian PCDD/PCDF emission factors have been developed for each vehicle class using Australian fuel economy data.
Total dioxins emissions from Australian road traffic (1998 levels) are estimated to lie between 0.7 and 16.5 g I-TEQ² per year, based on VKT³ data from the Australian Bureau of Statistics (ABS). Vehicles that operated on leaded petrol accounted for about 40-45% of total dioxins emissions. However, leaded petrol was completely removed from sale on 1 January 2002. The phase-out of leaded petrol is, therefore, expected to have already substantially reduced total dioxins emissions. Diesel-fuelled vehicles emit between 35 and 50% of total dioxins emissions, which mainly come from diesel trucks. Despite the fact that vehicles running on unleaded petrol account for 65% of total kilometres travelled, they account for 5 to 20% of total dioxins emissions.
When vehicle activity data are based on fuel consumption data from ABS, similar results are obtained. In this case, total dioxins emissions are estimated to lie between 0.6 and 17.3 g I-TEQ per year. As an independent source of information, fuel consumption data obtained from the Department of Industry, Tourism and Resources (DITR) were used to estimate total dioxins emissions from Australian road traffic. These data resulted in a higher estimate of 0.7-24.3 g I-TEQ per year, but this estimate includes off-road traffic and non-road traffic emissions.
Using total dioxins emission figures for Australia from a previous study (Environment Australia, 1998) as an estimate for 1998 total emission levels, i.e. 150-2,300 g dioxins per annum, road traffic accounts for 0.03 to 16.2% of total dioxins mission levels. These results are in line with emission inventories around the world, ranging from 0.2 to 12% of total annual dioxins emissions. It is noted, however, that these emission inventories are all based on the same limited data that are available from international publications, so these results may not be surprising.
1 The term 'dioxins'. refer to a large group of polychlorinated dibenzo-para-dioxins (PCDDs) and the closely related furans (PCDFs). The terms 'dioxin', 'dioxin/furans', 'PCDDs/PCDFs' are used interchangeably throughout this report.
2 I-TEQ is a unit of measurement. 2,3,7,8-TCDD is considered the most toxic of the dioxins and furan family. By convention it is assigned a toxicity rating (called a Toxic Equivalency Factor, or TEF) of 1.0. The toxicity of a mixture of PCDDs and PCDFs can be expressed by multiplying the concentrations of the congeners present in the mixture by their respective TEFs. The resulting products are called toxicity equivalents (TEQs). TEQs are expressed in the same concentration units as the individual congeners. A number of toxicity rating schemes have been developed (WHO, EADON, NATO/CCMS. Since 1988, the term .international TEFs. or I-TEFs, which is the sum of the toxicity equivalents for all congeners is the preferred unit.
3 VKT is an acronym for vehicle kilometres travelled.