Trucks Driving at Night and Their Effect on Local Air Pollution
L Int Panis, C Beckx, G Cosemans, VITO, BE
Avoiding congestion by driving at night reduces air quality. Using a 1 dimensional Gaussian model and high resolution meteo we demonstrate that identical emissions will cause local impacts 3 times higher at night than during the day.
In January 2007 the Flemish minister of transport proposed to improve the opportunities for loading goods in the port of Antwerp so that trucks can avoid congestion by driving at night. Avoiding congestion is seen as an environmental benefit by many policy makers although the measure would obviously cause negative impacts related to night time noise exposure. Information on the effect of changed timing of the exhaust emissions on their local dispersion was unavailable.
Using a simple 1 dimensional Gaussian plume model, we studied the effect of shifting the timing of the emissions from day to night. Such an effect is easy to demonstrate, but has been overlooked by researchers because the focus of most studies has been either on episodes of severe air pollution or on estimating the annual average concentrations for exposure and analysis of health effects. In this study we have used high resolution meteorological data. Wind speed and atmospheric stability classes at 10 minute time resolution were used to calculate the effect of truck emissions on pollutant concentrations at different distances from the road (10 meters, 100 meters and 1 km). We demonstrate that identical emissions will cause local impacts that are higher at night than during the day. This can be explained by differences in average wind speed and atmospheric stability. The effect is most pronounced at short distances from the road. At a distance of 10 meters, local concentrations will be up to three times higher during the night than during the day although the emissions are the same (assuming equal speed and traffic dynamics). A distance of 10 meters is a typical distance between the center of major roads and the facades of buildings. This has serious implications for the PM air quality targets that prove very hard to comply with in areas affected by residential ribbon development. It has been shown that PM concentrations in 2010 will not meet air quality standards in the vicinity of many important roads. Partly changing the timing of the emissions on these roads from day to night will likely offset whatever benefits are gained from improved traffic flows. The magnitude of this unexpected difference in dispersion is large enough to offset all improvements in European PM emissions standards for heavy duty vehicles between Euro0 and Euro3.
In addition our results also highlights the fact that annual average impacts (e.g. exposure of urban populations) may hide important seasonal differences between summer months when days are long and winter time when rush hours occur either before sunrise or after sunset. Such aspects need to be studied in much more detail before the environmental consequences of specific transport policy measures can be assessed.
Association for European Transport