Dispersion and Transformation of Traffic Exhaust Particles in the Urban Atmosphere

Ultrafine particles, their sources and fate in the atmosphere are currently key issues in atmospheric research owing to concern about their effects on human health and global climate. Traffic emissions are a dominating source of ultrafine particles, especially in urban areas. After release into the atmosphere, the particles are subjected to complex dilution and transformation processes that are often treated in aerosol dynamics and dispersion models to better understand observations and to make predictions or scenario calculations for future situations.

This thesis contributes to the validation and the refinement of existing urban dispersion models and their extension towards a prediction of particle size distributions. By analysing measurements of particle and gaseous pollutants and by application of various dispersion and aerosol dynamics models, the work aims at identifying and exploring the relevant processes that should be included in practical air pollution models for particles.

The relevance of the turbulence produced by the vehicle movements for the dispersion inside a street canyon has been shown in field data and a method to incorporate this effect into results from numerical models and wind tunnel measurements has been developed. Dispersion models of different complexity are applied to real-world situations and compared with field measurements and wind tunnel experiments.

Measurements of ultrafine particle size distribution at kerbside, urban, near-city and rural locations in the Copenhagen area are analysed. They document the temporal and spatial variation of the particle number and mass concentration and can clearly separate the different contributions from the local street traffic, the urban and regional sources. Particle number emission factors per average vehicle and kilometer driven were estimated for typical urban conditions in Denmark.

This thesis further investigates the time scales of various dynamic processes during the evolution of the particle size distribution from its emission from a vehicle exhaust pipe through its dilution at kerbside and urban level. We identify the spatial or temporal scales under which the discussed processes are important. The Multi-plume Aerosol dynamics and Transport (MAT) model has been developed to study the dynamics of the particle size distribution in an urban environment. The model used a new vertical dispersion scheme that is coupled to an existing aerosol dynamics model. The model was tested and applied to measurements in the Copenhagen area.