Simultaneous quantification of the contributions of dry, washout and rainout deposition to the total deposition of particle-bound 7Be and 210Pb on an urban catchment area on a monthly scale

• in situ experimental set-up installed to measure dry and wet deposition in urban area.
• Model developed to distinguish washout and rainout components in the wet deposition.
• Natural 7Be and 210Pb used as tracers for deposition of atmospheric aerosol particles.
• Deposition by rainout was the predominant deposition mechanism on monthly scale.
• Distribution of deposition contributions was in agreement with radionuclide origin.

This study deals with the time-evolution of the relative contributions of the three forms of atmospheric particle deposition under dry and wet weather conditions in an urban environment: dry deposition, washout and rainout deposition. An experimental set-up was installed in situ to measure the dry and wet deposition by integrating the processes of deposition over monthly periods. A semi-empirical model (PArticle Washout: PAW) was developed to distinguish the deposition by washout and rainout contained in the wet deposition over these same periods. These two parallel approaches allowed to quantify simultaneously the dry deposition as well as the washout and the rainout deposition of two natural radionuclides of different origin (7Be and 210Pb) on an urban catchment area. The dry deposition on a heterogeneous urban surface was estimated knowing the dry deposition on homogeneous substrates and the distribution of these substrates on the developed surface of the Pin Sec catchment area in Nantes. On average, during the 15 months of experimental monitoring, deposition by rainout was the predominant mechanism contributing to the monthly total deposition of 7Be and 210Pb, to the extent of 67% and 55%, respectively. Dry deposition contributed less to the total deposition, accounting for 21% and 28% of the 7Be and 210Pb, respectively. Lastly, washout was the weakest contributor to total deposition, with 12% for 7Be and 17% for 210Pb. The distribution of the contributions in the case of cosmic 7Be and terrigenous 210Pb is in agreement with the origin of these two radionuclides. By applying this method to other substrates and under different environmental conditions, the calibration of operational models could be refined to obtain a better prediction of the atmospheric particle-bound pollutant deposition.