Abstract Number: 13 Working Group: Remote/Regional Atmospheric Aerosol
Abstract In the context of accidental release of radioactive particles into the atmosphere, knowing atmospheric particle dry deposition velocities on ground surfaces is essential to assess risks both on populations and environment. The dry deposition velocity (m s-1): (1) is the ratio of deposition flux (# m-2 s-1) over atmospheric concentration (# m-3); (2) has a general applicability as it does not depend on #-entity; (3) depends on the deposition process integrating time-step. The dry deposition velocity depends on the aerosol particle diameter. Currently, there is no in situ dry deposition velocity available in the literature for nanoparticles, whereas it is required to predict deposition of radionuclides as, for examples, ruthenium-106 and iodine-129,131 (reactive gas). In this framework, one of the main challenges is to find a suitable tracer to follow atmospheric nanoparticle deposition at short time scales. In this study, we tested the capability of the naturally-occurring radionuclide bismuth-214 (214Bi, beta emitter, T1/2 = 19.7 min) to trace the deposition flux of the atmospheric nanoparticles at hour scale over a grassland (ABBA project, June 2017). 214Bi is a decay product of radon-222 (222Rn, T1/2 = 3.8 days) emitted by terrestrial natural radionuclides. 214Bi activity was assumed to be the same as its daughter polonium-214 (alpha emitter, T1/2 = 1.6 10-4 s) that was quickly counted by a multichamber alpha spectrometer (Canberra, model 7401) located in a mobile lab. Vertical fluxes were calculated using the activity concentration gradient method from samplings on diffusive grids located at 0.7 and 2.7 m above grass. Next, atmospheric activity concentrations were calculated from grid activities using the Jenkings equations. Results showed a significant dry deposition velocity normalized by the shear velocity of 0.06 ± 0.03.