This thesis is concerned with the evaluation and development of models related to the emissions and dispersion of traffic induced road dust. Road traffic is a highly important source of a range of pollutants having adverse health and environmental effects. Development of models describing traffic related emissions and dispersion of pollutants will in the end lead to robust models that well reproduce available air quality measurements. Furthermore, such models will aid the analysis of existing mitigation measures, as well as developing new ones, to reduce the emissions of traffic related pollutants.
The first study deals with the modelling of the dispersion and resulting concentration levels of NOx at distances up to 100 m from major roads in rural areas, using four Gaussian open road line source models developed in Norway, Denmark and Finland. These models were applied to datasets from measurement campaigns conducted in rural areas near major roads in the mentioned countries. NOx was considered since it was measured at all sites, its emissions are well known and since it can be treated as a tracer for the short time scales involved. When comparing the models with the measurements we found that the results are sensitive to the parameterisation of traffic produced turbulence (TPT), especially at distances close to the road in combination with low wind speeds. The Danish model OML-Highway performed best at all sites due to its parameterisation of TPT based on turbulent kinetic energy.
The second study is concerned with the development of a more generalised model framework to describe road dust emissions, i.e. emissions of particulate matter (PM) from the road surface due to road surface wear as well as resuspension of deposited material on the road surface and road shoulders. There is a need for a more generalised road dust emission model since the majority of already developed road dust emission models contain empirical constants or functions related to local air quality measurements. As such, the applicability of these models on other road environments is limited and they cannot be used for analysis of mitigation measures related to road dust emissions. The model concept described in this study accounts for the main processes controlling road dust emissions and does not depend on local measurements; it is based on measurements of road, tyre and brake wear to obtain the relevant emission factors. A mass balance concept is used for describing the variation in dust load on the road surface and shoulders. Furthermore, the model separates the direct emissions and resuspension and treats the road surface and shoulder as two individual sources. When applying the model onto two datasets from measurement campaigns conducted at major roads in and outside Oslo, Norway, during the studded tyre season, it performed well during warm periods and less well for temperatures close to or below 0 ºC in combination with precipitation. As such, refinements of the parameterisations of road surface conditions are needed and measurement campaigns with the aim of understanding the effect of road surface conditions on road dust emissions should be conducted. In spite of the current limitations, the model provides a well described conceptual framework and describes processes that no model has ever done before. The model will in the future provide the potential for good air quality planning.
List of papers presented in this thesis
Berger, J., Walker, S.E., Denby, B., Berkowicz, R., Løfstrøm, P., Ketzel, M., Härkönen, J., Nikmo, J., Karppinen, A., 2010. Evaluation and inter-comparison of open road line source models currently in use in the Nordic countries. Boreal Environmental Research 15: 00-00 (preprint). Will be published 2010.
Berger, J., Denby, B., 2010. A generalised model for traffic induced road dust emissions. Part 1: concept and model description. Submitted to Atmospheric Environment.
Berger, J., Denby, B., 2010. A generalised model for traffic induced road dust emissions. Part 2: model evaluation. Submitted to Atmospheric Environment.