Dispersion of traffic-related exhaust particles near the Berlin urban motorway - Estimation of fleet emission factors

Atmospheric particle number size distributions of airborne particles (diameter range 10-500 nm) were collected over ten weeks at three sites in the vicinity of the A100 urban motorway in Berlin, Germany. The A100 carries about 180000 vehicles on a weekday. The roadside particle distributions showed a number maximum between 20nm and 60nm clearly related to the motorway emissions. The average total number concentration at roadside was 28000 per cubic cm with a total range of 1200-168000 per cubic cm. At distances of 80 and 400 m from the motorway the concentrations decreased to mean levels of 11000 and 9000 per cubic cm, respectively. An obstacle-resolving dispersion model was applied to simulate the 3-D flow field and traffic tracer transport in the urban environment around the motorway. By inverse modelling, vehicle emission factors were derived that are representative of a fleet with a relative share of 6% lorry-like vehicles, and driving at a speed of 80 km/h. Three different calculation approaches were compared, which differ in the choice of the experimental winds driving the flow simulation. The average emission factor per vehicle was 2.1 (plus/minus 0.2) x 10^(14) per km for particle number and 0.077 (plus/minus 0.01) x 10^(14) cubic cm per km for particle volume. Regression analysis suggested that lorry-like vehicles emit 123 (plus/minus 28) times more particle number than passenger car-like vehicles, and lorry-like vehicles account for about 91% of particulate number emissions on weekdays. Our work highlights the increasing applicability of 3-D flow models in urban microscale environments and their usefulness for determining traffic emission factors.

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