Estimation of gaseous real-world traffic emissions downstream a motorway

The consequences of air pollution scenarios caused by road traffic or the impact of exhaust gas reduction techniques are estimated by emission models. To ensure the quality of model results, it is necessary to evaluate the used emission factors under real-world conditions. Therefore, the Institut für Meteorologie und Klimaforschung (IMK) of the Forschungszentrum Karlsruhe initiated the field campaign BAB II (BundesAutoBahn, Federal motorway). The campaign was conducted in May 2001 with the objective of measuring the traffic emissions at a motorway section and to compare them to modelled emissions. Based on experiences during a precursor campaign (BAB I, 1997), a symmetric experimental set-up was installed which allowed measurements up- and downwind of a motorway nearby Heidelberg, Germany. This paper focuses on the determination of source intensities and emission factors for CO and NOx, whereas other papers in this issue handle VOC and particulate matter. First the basic approach of BAB II measurements up- and downwind of a motorway was approved, showing that it is possible to detect the plume originating from traffic emissions. A case study during a traffic jam illustrates that driving patterns have a strong impact on the emissions and therefore a detailed traffic census is required to obtain reliable emission calculations. Two different strategies were used: (i) long-time measurements during the whole campaign to obtain vertical profiles each 30 min and (ii) measurements during eight special operation periods (SOP) in a higher temporal resolution of 5 min, using instrumentation in elevators. It could be shown that even at a distance of 60–80 m from the motorway the structure of the plume is still inhomogeneous, and concentration changes within short times. The inhomogeneity of the plume not only affects the temporal scale, the spatial scale is also influenced and frequently concentration maxima in higher altitudes are observed. Mean source intensities of 9.5 kg km−1 h−1 CO and 4.4 kg km−1 h−1 NOx on working days have been calculated. The ratio of CO/NOx was found to be 2.1 on working days and 4.5 on weekends and holidays. Emission factors of 2.62 g km−1 veh−1 CO and 1.08 g km−1 veh−1 NOx have been determined, which agrees well with results derived from similar studies.