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Characteristics and Forms of the
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2.8.2 The
Automobile as Pollutant Source
The combustion of petrol or diesel in an automobile motor produces an array of pollutants, of which the following pollutant components stand out in particular measure:
The starting point for the consideration of both emission-side and immission-side exhaust gas levels on roads is the specific pollutant output of motor vehicles, which varies strongly with the respective operating condition. Petrol and diesel motors also exhibit large differences in emissions behavior. The emission rates of pollutant components in automobile exhaust can be calculated by the multiplication of the temporal traffic volume by the so-called "exhaust emissions factors," which are based in turn on the emissions of an individual car or truck in average condition. Motor vehicle emissions depend on the speed and manner of driving, i.e. the so-called "driving models" on the roads. A specific driving model can be assigned to every traffic situation. Table 2/3 gives an overview of various driving models with the indication of the average driving speed. For the determination of the exhaust gas factors, TÜV Rhineland (UBA, 2004) categorized automobiles with various exhaust controls with regards to their exhaust emissions.
Tab. 2/3: Examples of various styles of driving for different types of roads; Source: UBA (2004) These emissions factors refer significantly to
the pollutants benzene, carbon monoxide, nitrogen oxides, lead,
sulfur dioxide, carbon dioxide, hydrocarbons, and particulate
matter. Data for diesel particles were not given directly. The
proportion of diesel particles can be determined from FIGE
(1995) at 60% for cars and 40% for trucks. In these factors, road gradients and cold
engine starts can also be taken into consideration. A cold
engine increases exhaust emissions, as the catalytic converter
is not yet fully functioning. One can choose in the database
from emissions factors for various vehicle categories (e.g. car,
truck, bus, motorcycle) as well as years of reference (up to
2010). Table 2/4 lists examples of emissions factors. Benzene emissions tend to decrease with rising vehicle cruising speeds (not driving speeds!). Remarkably, in the cases of NOx and particulates the emissions factor is roughly 10 times that for trucks as for cars. Various calculation models are used for the evaluation of pollutant levels on roads (see Chapter 4.3). Usually these are the Gauss model, the Lagrange model, and the Box model, the last being used especially for street canyons. Wind tunnel analyses also come into play for the simulation of the exhaust gas propagation on streets and for the quantification of the resulting immission levels (see Chapter 4.2).
Tab. 2/4: Distance-related emissions factors per vehicle in the year 2005 (without cold engine starts); Source: UBA (2004) |
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