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Abstract

In urban areas, road traffic is a dominant source of nitrogen oxides (NOx = NO + NO2). Although the emissions from individual vehicles are regulated by the European emission standards, real driving emissions often exceed these limits. In this study, two MAX-DOAS instruments on opposite sides of the motorway were used to measure the NO2 absorption caused by road traffic at the A60 motorway close to Mainz, Germany. In combination with wind data, the total NOx emissions for the occurring traffic volume can be estimated. We show that the measured emissions exceed the maximum expected emissions calculated from the European emission standards by a factor of 11 ± 7. One major advantage of the method used here is that from MAX-DOAS measurements the integrated NO2 concentration over the lowermost 2 to 3 km is determined. Thus, all emitted NO2 molecules are detected independent from their altitude and therefore the whole emission plume originating from the nearby motorway is captured by these measurements which is a key advantage compared to other approaches such as in-situ measurements.