Winther, M. & Slentø, E. 2010: Heavy Metal Emissions for Danish Road Transport. National Environmental Research Institute, Aarhus University, Denmark. 99 pp. – NERI Technical Report no. 780.

Summary

Introduction
This report presents new heavy metal emission factors for cars, vans, trucks, buses, mopeds and motorcycles, for each of the emission sources fuel consumption, engine oil, tyre wear, brake wear and road abrasion. The emission components covered are Arsenic (As), Cadmium (Cd), Chromium (Cr), Copper (Cu), Mercury (Hg), Nickel (Ni), Lead (Pb), Selenium (Se) and Zinc (Zn), all of them relevant for emission reporting to the UNECE CLRTAP (United Nations Economic Commission for Europe Convention on Long Range Transboundary Pollutants) convention. The report also presents a new Danish inventory for the year 2007.

Method
The heavy metal contents in fuel and engine oil used in the present study are based on measurement data from the European research organisation CONCAWE for fuel and information from key experts for engine oil. The fuel consumption pr km comes from the existing Danish emission inventory, and functions for engine oil consumption are established based on key expert information. The product of fuel/engine oil consumption pr km and the associated heavy metal content values make up the heavy metal emission factors. For each vehicle category total emissions are calculated as the product of emission factors and total mileage.

For tyre wear the present survey mainly uses information of wear rates pr vehicle type provided by the Danish Tyre Trade Environmental Foundation. This information has been combined with literature values for airborne fractions of worn particulate matter and heavy metal content in tyre material in order to estimate kilometre related emission factors for heavy metals. For each vehicle category total emissions are calculated as the product of emission factors and total mileage.

For brake wear and road abrasion pr vehicle type the kilometre based emission factors for airborne particulate matter come from the existing Danish non exhaust emission inventory. The product of these emission factors and literature values for heavy metal content in brake lining/road asphalt material make up the kilometre related emission factors for heavy metals. For each vehicle category total emissions are calculated as the product of emission factors and total mileage.

Results
For the list of heavy metal components, the following emissions in total TSP (in brackets) are calculated for the year 2007: As (8 kg), Cd (48 kg), Cr (197 kg), Cu (51 779 kg), Hg (28 kg), Ni (158 kg), Pb (6 989 kg), Se (33 kg) and Zn (28 556 kg).

The specific vehicle category activity/emission source category combinations and the related emission factors determine the total emissions for each vehicle type/emission source.

Table ES1 Total heavy metal emissions (kg) pr vehicle type for Denmark in 2007 calculated in the present study. 

Vehicle	Cars	Vans	Trucks	Trucks	Trucks	Trucks
Emission component			Gasoline	3.5-7.5 t.	7.5-16 t.	16-32 t.
As	3.845	2.018	0.005	0.146	0.098	0.637
Cd	28.753	9.946	0.018	0.589	0.390	2.764
Cr	109.389	37.879	0.131	3.472	2.378	16.271
Cu	31 259.785	18 321.180	3.045	104.072	67.594	412.694
Hg	17.003	4.936	0.016	0.149	0.169	1.742
Ni	79.464	34.839	0.084	2.729	1.853	12.681
Pb	4 301.144	2 462.325	0.315	9.085	6.152	41.757
Se	14.755	7.929	0.014	0.469	0.369	2.937
Zn	14 347.945	6 807.679	8.479	350.512	273.864	2 224.187
Continued
Vehicle	Trucks	Urban buses	Coaches	Mopeds	Motorcycles	Total
Emission component	>32t.
As	0.700	0.335	0.175	0.019	0.060	8.038
Cd	2.973	1.163	0.730	0.024	0.281	47.629
Cr	18.270	5.026	2.895	0.223	1.242	197.177
Cu	436.501	402.757	207.143	154.802	409.404	51 778.976
Hg	2.480	0.793	0.456	0.038	0.250	28.030
Ni	13.902	7.290	4.019	0.274	1.171	158.306
Pb	45.691	29.336	15.891	20.744	56.640	6 989.079
Se	3.500	1.352	0.739	0.098	0.408	32.570
Zn	2 632.965	1 021.562	582.966	57.048	248.947	28 556.153

Pr vehicle type cars are the most important source of emission for all heavy metal species, followed by vans, trucks, buses and 2-wheelers. However, the car emissions shares that range between 62 % (Pb) and 46 % (Se), are somewhat smaller than the share of total mileage. Trucks and buses use more fuel and engine oil pr kilometre driven, and have higher emission factors for brake, tyre and road wear, with Pb and Cu as an exception for brake wear.

Table ES2 Total heavy metal emissions (kg) pr source category for Denmark in 2007 calculated in the present study.

Emission component	Fuel	Engine oil	Tyre wear	Brake wear	Road abrasion	Total
As	0.8	-	0.8	6.5	-	8.0
Cd	0.5	39.4	2.5	5.2	0.1	47.6
Cr	31.6	65.0	3.4	74.5	22.7	197.2
Cu	21.7	106.5	14.8	51 624.6	11.4	51 779.0
Hg	28.0	-	-	-	0.1	28.0
Ni	4.5	39.4	24.2	72.1	18.2	158.3
Pb	3.3	173.5	76.3	6 682.2	53.8	6 989.1
Se	0.6	-	18.9	13.0	-	32.6
Zn	101.0	7 876.2	9 491.7	11 000.9	86.5	28 556.2

Almost all Hg emissions (100 % as a rounded share) come from fuel usage, and for Cr and As the emission shares are 16 and 9 %, respectively. For fuel all other emission shares are insignificant (between 0 % and 3 %).

For engine oil the largest emission shares are noted for Cd (83 %) and engine oil also has substantial emission shares of Cr (33 %), Zn (28 %) and Ni (25 %).

For tyre wear the most important emissions are Se (58 %), Zn (33 %) and Ni (15 %).

Brake wear is the most important source of emissions for Cu (100 %), Pb (96 %), As (82 %), Ni (46 %), Zn (39 %) and Cr (37 %). For Se the brake wear emission share is 40 %.

For road abrasion the most important emission species are Cr (12 %) and Ni (11 %). The road abrasion emission shares of other emission components are between 0 % and 1 %.

Conclusion
Set in relation to a revised Danish emission budget, road transport (emission shares in brackets) is a key source for Cu (95 %), Zn (54 %) and Pb (53 %), and of some relevance for Cr (15 %). For the remaining emission components, road transport is only a small source of emission.

The road transport emissions of Cu and Pb almost solely originate from brake wear, also being the most dominating source for Cr. For Zn, brake and tyre wear are almost equally important sources. Consequently, brake wear, and secondly tyre wear, are the most relevant road transport sources to address, in order to reduce the Danish grand totals.

Important outcomes of the present project are the proposed heavy metal emission factors and calculated 2007 emission estimates pr fuel type and vehicle type, for each of the five emission sources; fuel and engine oil consumption, vehicle tyre and brake wear and road abrasion. The gathered information of heavy metal content pr unit of consumption/emission has been essential for the establishment of emission factors for each of the five sources of emissions.

For the exhaust based emissions related to fuel and engine oil consumption it is a big improvement to have two separate sets of emission factors based on new information of heavy metal content. Until now, bulk emission factors related to the total fuel consumption have been used. By treating the sources separately updates of emission factors and calculated totals become easier, if new emission knowledge become available.

As regards the wear related emissions, the wear rate and airborne fraction of the worn material, and the associated heavy metal content determine the resulting heavy metal emission factors. For tyre wear, the new factors for raw particulate emissions are regarded as being more precise than the ones used in the previous Danish non-exhaust emission inventory, since weight, wear percentage and tyre life times are provided by Danish experts in the tyre business.

The wear rates and airborne fractions of worn material for brakes, especially for heavy duty vehicles and for road abrasion in general, are regarded as uncertain. However, the outcome of the literature study did not bring any new information, which justifies any update of the particulate emission factors from the existing Danish inventory. These latter factors and the associated heavy metal content of worn material determine the heavy metal emission factors proposed in the present report.

Most importantly for the Cu and Pb metal content of brake material used by trucks and buses, there is a big difference between the figures used in the present study and averages based on other literature values. The metal content values used in this report for trucks and buses are very low, and rely on scarce data. The data differences point out the need for data updates, when new information become available.

By using the detailed emission factors and inventory calculation methods established in the present project, estimates of heavy metal emissions can be made for other years than 2007. The emission factors are independent from inventory year and the emissions for each source/fuel/-vehicle type combination are calculated as the product of the specific emission factor and the relevant inventory year specific activity data; fuel or engine oil consumption or total mileage.

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