Emission factors for heavy metals from diesel and petrol used in European vehicles

Abstract

Heavy metals constitute an important group of persistent toxic pollutants occurring in ambient air and other media. One of the suspected sources of these metals in the atmosphere is combustion of transport fuels in road vehicles. However, estimates of the emissions of these metals from road vehicles as reported in national emission inventories show a very high variability in emission factors used. This paper provides high quality data on concentrations of heavy metals in fuels and derives default emission factors from these. The paper discusses these values against the emission estimates presently reported by the Parties to the LRTAP Convention.

The measured concentrations of heavy metals in petrol and diesel fuel show a high variability between different samples taken at gas stations throughout Europe. Metal concentrations in road transport fuels vary over two orders of magnitude, but all remain in the ppb region (a few tenths of a ppb to a few hundred ppb for all metals). The frequency distributions of the measurements could be approximated by lognormal distributions. The emission factors, including 95 percent confidence intervals were derived from a statistical analysis of the survey data. We could not detect a significant difference between samples from different countries.

The fuel based emission factors as derived in this study are complemented with those related to unintentional lubricant oil combustion. This allowed an estimation of total exhaust heavy metal emissions for UNECE Europe, indicating that As, Hg and Se exhaust emissions were dominated by fuel combustion while Cd, Cr, Cu, Ni, Pb, and Zn exhaust emissions were dominated by lubricant oil combustion.

The proposed emission factors were generally lower than previously published emission factors. National emissions of heavy metals from vehicle exhaust, estimated in this study are in many cases considerably lower than those reported by the countries for this source.

Introduction

Heavy metals (HM) are persistent in the environment and subject to bioaccumulation in food-chains. They are known to have adverse effects on the environment and human health (WHO, 2007). Exposure to heavy metals has been linked with developmental retardation, various cancers, kidney damage, and even death in some instances of exposure to very high concentrations. The Protocol for Heavy Metals under the UN-ECE Convention for Long Range Transboundary Air Pollution (LRTAP¹) articulates the importance of heavy metals for air quality (UNECE, 1998). This protocol aims at reducing emissions of heavy metals to the air. A recent European Monitoring and Evaluation Programme (EMEP) report by Ilyin et al. (2011) provides an overview of the current understanding of air pollution by heavy metals in Europe. An important source of information for the parties of the Convention and the Protocol are emission inventories, annually reported by the parties to the LRTAP Convention. Road transport is among the possibly important sources of the emissions of these metals into the atmosphere (Pacyna et al., 2007). There are four main sources for transport HM emissions: three sources of tailpipe emissions: (1) from the fuel, (2) from the lubricant, and (3) from engine wear or after-treatment system abrasion and (4) non-tailpipe emissions from tyre wear, brake wear and possibly from road abrasion (non-tail pipe emissions; e.g., Legret and Pagotto, 1999; Westerlund, 2001; Sanders et al., 2003). This paper concentrates on the first source and briefly reflects on the second.

Parties to the Convention for Long Range Transboundary Air Pollution are obliged to report HM emissions to EMEP annually. The reported emissions are compiled and available from the EMEP Centre on Emission Inventories and Projections (CEIP: http://www.ceip.at/). To compare the reported road transport tail pipe emissions between these countries, so-called implied emission factors can be calculated (the reported emissions divided by the reported total fuel use; Table 1). Given the largely similar vehicle fleets in all countries, one would expect these values to be in a similar range and covering the same HM. The implied emission factors, however, vary over a broad range, sometimes more than two orders of magnitude, between countries and also the HM covered vary considerable (Table 1).

It might be that some countries combined the emission from both tailpipe and non-tailpipe as one entry in the CEIP database, but in that case they have been allocated incorrectly (for details we refer to EMEP CEIP, 2011). However, even if we use this as part of the explanation and join the entries for road transport as a whole, the differences between countries remain significant (data not shown). The broad range of the implied emission factors suggests that the reporting of these emissions from road transport might not be very comparable between the countries.

A compilation of emission factors published since 1990 is shown in Table 2, also indicating a broad range of values in the various papers for all metals. The latest version of the EMEP/EEA Guidebook (EMEP/EEA, 2009; chapter 1.A.3.b, page 87), a source of information to support the Parties to the convention, reflects this situation and only proposes “preliminary” emission factors for heavy metals for these fuels in the sense that updated values are announced. The Guidebook therefore provides only limited guidance.

Tail pipe HM emissions may also be due to combustion of engine lubricants, additives therein and/or engine wear (Rauckyte et al., 2006; Vouitsis et al., 2007a, 2007b). Some countries might have explicitly included these lubricant based emissions in the reported values, while others might not. Denmark (Winther and Slentø, 2010) and the Netherlands (Klein et al., 2011) for instance, have explicitly included these emissions.

This study aims at establishing emission factors for tailpipe emissions, based on heavy metal contents of the fuels used in road vehicles. An earlier study (Denier van der Gon et al., 2007) revealed that the emission factor as generally used for copper from fuel use was too high (Denier van der Gon et al., 2007).

The current study aims to provide high quality measurements of concentrations of the heavy metals as mentioned in the LRTAP Convention and its protocols, in fuels. We will derive default emission factors from these data and discuss these values against the emission estimates presently reported by the Parties to the LRTAP Convention.