Investigating the Impact of Changes in Traffic Emission Structures on Urban Air Pollution with a Nested Model System

In relation to the health damage potential of atmospheric trace gases air quality is one main indicator for quality of life in an industrial conurbation. In the last years, several administrative attempts were made to reduce the emissions of air pollutants, but these steps are partly opposed by an increasing demand for energy. In particular, reduction of ozone precursors is an important objective due to their potential of causing photochemical smog. An outstanding problem in this context is the individual traffic. According to a predicted increasing traffic volume, the environmental effects of motor vehicles intensify in spite of improved, emission reducing automotive technology.

The spatially and temporally highly variable character of emissions, and the fact that many of the emitted species undergo chemical reaction processes during their transport through the atmosphere inhibits an exclusively observational quantification of traffic emissions. For that reason the effect of air pollutants can more profitably be estimated by a combination of emission inventories, models of atmospheric dynamics and air chemistry.

Hence, to describe the complex relationship between release and transmission of gaseous pollutants in the atmosphere the model system CARLOS (Chemical and Atmospheric transport on Regional and LOcal Scale) was developed. This model system has multiple-nesting capability which allows to account for large scale features while simulating local scale structures in nested regions. On the regional scale the meteorological model MM5 is used as input to the chemistry transport model (CTM2). The results in return serve as boundary conditions for the small scale high resolution models (FOOT3D/CTM2F), which are capable to simulate flow and pollution pattern e.g. on the scale of urban districts. To study the impact of traffic based emissions on urban air quality and the potential benefit of reduction strategies emission inventories of a high resolution dynamic traffic model are used within the CARLOS system. By simulating individual routes for single vehicles in a road network the dynamic model pays regard to e.g. interactions of the road user and even traffic jams. The design of this model also allows studies about the impact of local administrative regulations or municipal development planning on traffic flow and the related emissions.

Presently on local scale special emphasis is given to nitric oxides (NO_x) and hydrocarbons (HC) emitted by vehicles in context with summertime ozone (O₃) production. Therefore, the model was applied to simulate a period of August 1997 with high ozone mixing ratios in the state North-Rhine-Westphalia in Germany. This base case scenario which is reproduced well by the model system acts as starting point for further investigations with fictitious but realistic traffic scenarios in the Cologne area. One of these traffic scenarios simulates e.g. the impact of the intended motorway expansion around Cologne on the emission structure and possible consequences for air quality within the city.