Dispersion of Fine and Ultrafine Particles Near an Urban Motorway: Combining Modelling and Experiments

Abstract

As ultrafine (diameter<100 nm) particles are concerned, it is important to realise that part of the exhaust particles form only downstream the vehicle pipes in the free atmosphere by condensation. This is an effect which cannot be quantified by dynamometer test stands. Therefore, a combination of atmospheric experiments near real-world traffic sources as well as atmospheric modelling is required in order to capture the full impact of traffic sources on the environment. In this work, we report on particle and traffic sampling experiments and the related small-scale simulations of the particle dispersion downstream an urban motorway. The aim is to derive traffic emission rates for motorway traffic under realistic driving conditions. Ambient particle number size distributions (size range 10−600 nm), NOx concentrations as well as meteorological data were measured at three different sites simultaneously between July and September 2005 next to a six-lane motorway in Berlin, Germany. The traffic volume of about 175 000 vehicles per day was recorded with traffic counters separately for cars, vans and lorries. The average total particle concentration at roadside was 28 000 particles/cm3 (varying 1 200−168 000 particles/cm3). Within distances of 80 and 400 m from the motorway the average particle concentration was observed to decrease to 11 000 and 9 000 particles/cm3, respectively. The average particle number size distribution at roadside showed a clear maximum at particle sizes between 25−60 nm. To characterise atmospheric air flows in the urban obstacle layer surrounding the motorway, a three-dimensional fluid model ASAM (All-Scale Atmospheric Model) was applied over a model domain size of 500×500×100 meters. Generally, the model simulations were able to reproduce the dilution and spatial distribution pattern of traffic-related particles under different wind sectors that was suggested by the experiment. Further analyses will present the derivation of particle emission factors of the vehicles through inverse modelling. We will also present attempts to separately determine emission factors for passenger cars as well as light and heavy duty vehicles.