AbstractAn important limiting factor that hampers accurate determination of the time-weighted average (TWA) concentrations of aqueous micropollutants by passive sampling is unsteady water turbulence, which can cause high variations of the uptake rates of target substances by the samplers during their field exposure. A mobile continuous flow system (MCFS) was developed for the improved utilization of the Chemcatcher passive sampler for time-integrated field monitoring of metals under controlled and reproducible hydrodynamic conditions. The mobile system is based on a specifically designed stirring system integrated in a flow-through cell. This design enables the adjustment of hydrodynamic conditions around the samplers as used in the laboratory calibration experiments. By using this system, the reliability of monitoring data can be improved. A prototype of this mobile continuous flow system was tested during a research cruise in May 2014 (HE422) to collect TWA concentrations for rare earth elements (REEs). The field test covered two areas in the German Bight of the North Sea. The Chemcatcher® results yield TWA concentration levels of REEs between 3.9 ng L−1 and 19.4 ng L−1 for the off-shore (low anthropogenic influence) and between 7.0 ng L−1 and 38.6 ng L−1 for the near-shore (high anthropogenic impact) areas, while the direct measurements of the ultra-trace levels of the REEs in the seawater spot samples were nearly impossible. The successful application of the developed MCFS demonstrated that monitoring with passive samplers on ships is possible with high accuracy under controlled hydrodynamic conditions even at the ultra-trace element concentration levels as typical for off-shore areas.