Using the Wasserstein distance to compare fields of pollutants: application to the radionuclide atmospheric dispersion of the Fukushima-Daiichi accident

Theoretical models of the below-cloud scavenging (BCS) of aerosol by rain yield scavenging rates that are 1–2 orders of magnitude smaller than observations and associated empirical schemes for submicron-sized aerosol. Even when augmented with processes which may explain this disparity, such as phoresis and rear capture in the raindrop wake, the theoretical BCS rates remain an order of magnitude less than observations. Despite this disparity, both theoretical and empirical BCS schemes remain in wide use within numerical aerosol models. BCS is an important sink for atmospheric aerosol, in particular for insoluble aerosol such as mineral dust, which is less likely to be scavenged by in-cloud processes than purely soluble aerosol. In this paper, various widely used theoretical and empirical BCS models are detailed and then applied to mineral dust in climate simulations with the Met Office's Unified Model in order the gauge the sensitivity of aerosol removal to the choice of BCS scheme. We show that the simulated accumulation-mode dust lifetime ranges from 5.4 d in using an empirical BCS scheme based on observations to 43.8 d using a theoretical scheme, while the coarse-mode dust lifetime ranges from 0.9 to 4 d, which highlights the high sensitivity of dust concentrations to BCS scheme. We also show that neglecting the processes of rear capture and phoresis may overestimate submicron-sized dust burdens by 83 %, while accounting for modal widths and mode merging in modal aerosol models alongside BCS is important for accurately reproducing observed aerosol size distributions and burdens. This study provides a new parameterisation for the rear capture of aerosol by rain and is the first to explicitly incorporate the rear-capture mechanism in climate model simulations. Additionally, we answer many outstanding questions pertaining to the numerical modelling of BCS of aerosol by rain and provide a computationally inexpensive BCS algorithm that can be readily incorporated into other aerosol models.

A model intercomparison of the atmospheric dispersion of cesium‐137 (¹³⁷Cs) emitted after the Fukushima Daiichi Nuclear Power Plant accident in Japan was conducted to understand the behavior of atmospheric ¹³⁷Cs in greater detail. The same meteorological data with a fine spatiotemporal resolution and an emission inventory were applied to all models to exclude the differences among the models originating from differences in meteorological and emission data. The meteorological data were used for initial, boundary, and nudging data or offline meteorological field. Furthermore, a horizontal grid with the same resolution as that of the meteorological data was adopted for all models. This setup enabled us to focus on model variability originating from the processes included in each model, for example, physical processes. The multimodel ensemble captured 40% of the atmospheric ¹³⁷Cs events observed by measurements, and the figure of merit in space for the total deposition of ¹³⁷Cs exceeded 80. The lower score of the atmospheric ¹³⁷Cs than that of the deposition originated from the difference in timing between observed and simulated atmospheric ¹³⁷Cs. Our analyses indicated that meteorological data were most critical for reproducing the atmospheric ¹³⁷Cs events. The results further revealed that differences in ¹³⁷Cs concentrations among the models originated from deposition and diffusion processes when the meteorological field was simulated reasonably well. The models with small deposition fluxes produced higher scores for atmospheric ¹³⁷Cs, and those with strong diffusion succeeded in capturing the high ¹³⁷Cs concentrations observed; however, they also tended to overestimate the concentrations.

In the case of severe accident with loss of containment in a nuclear plant, radionuclides are released into the atmosphere in the form of both gases and aerosol particles (Baklanov and Sørensen, 2001). The analysis of radioactive aerosol scavenged by rain after the Chernobyl accident highlights certain differences between the modelling studies and the environmental measurements. Part of these discrepancies can probably be attributed to uncertainties in the efficiencies used to calculate aerosol particle collection by raindrops, particularly drops with a diameter larger than one millimetre. In order to address the issue of these uncertainties, an experimental study was performed to close the gaps still existing for this key microphysical parameter. In this paper, attention is first focused on the efficiency with which aerosol particles in the accumulation mode are collected by raindrops with a diameter of 2 mm. The collection efficiencies measured for aerosol particle in the sub-micron range are quantitatively consistent with previous theoretical model developed by Beard (1974) and thus highlight the major role of rear capture in the submicron range.