Chemistry Reference
In-Depth Information
Table 4 Emission factors reported for non-exhaust traffic emissions
Brake wear
(mg/km/veh.)
Resuspension
(mg/km/veh.)
Country
Traffic situation
Reference
9
a
5
a
Germany
Motorway, free flow,
20% heavy duty
[
44
,
64
]
3
a
(1.6
b
/9
c
)
48
a
(28
b
/160
c
)
Switzerland
Motorway, free flow,
15% heavy duty
[
65
]
15
a
(8
b
/81
c
)
27
a
(1
b
/262
c
)
Urban street canyon
a
Total fleet
b
Low duty vehicles
c
Heavy duty vehicles
to frequent exceedances of the daily PM limit value. Even the annual PM limit
cannot be attained at a few locations (e.g. Stuttgart, Fig.
4
).
PM from traffic comprises compounds which originate from the combustion
exhaust (primary and secondary carbonaceous aerosol, secondary nitrate) and from
non-exhaust processes (brake, clutch and tyre wear and road dust re-suspension).
As a “rule-of-thumb”, the ratio of exhaust to non-exhaust contributions for the
urban environment was found to be 60:40 in Germany [
59
]; however, the ratio is
highly variable for different traffic situations, fleet compositions and road condi-
tions. Based on the various studies carried out in Germany and neighbouring
countries during the last decade generalised procedures to calculate non-exhaust
PM emission factors for various traffic situations have been derived [
60
]. The range
of emission factors, which will be imbedded into the handbook on traffic emission
factors (HBEFA [
61
]), is 26-45 mg/km for passenger cars/light duty vehicles and
100-1,200 mg/km for heavy duty vehicles.
Still, considerable uncertainties exist with regard to the different kinds of non-
exhaust PM emissions. In recent studies tyre abrasion, initially estimated to make up
to 10% of total traffic PM10 contribution, has been shown with a new method for
analysing specific rubber components to have much lower impact [
62
]. Accordingly,
most tyre particles have aerodynamic diameters above 10
m
m and their contribution
to PM10 is 0.5 mass% at maximum.
Similarly, different results have been found for the impact of brake wear
particles. While unequivocally a strong enrichment of brake-related chemical
trace elements (Sb, Ba, Cu) is found at trafficked sites [
63
], the quantification of
overall PM contribution from brake wear is associated with uncertainties. An even
wider range of emission factors was found for re-suspension of road dust
(cf. Table
4
). It should be noted that re-suspension may be a strong source of PM
during wintertime when de-icing salt is spread out. For a traffic site in southern
Germany exceedance of the daily PM limit value could be tracked back to road
salting in 12 of 43 cases [
66
].
