Interaction of cesium compounds with abundant inorganic compounds of atmosphere: Effect on cloud formation potential and settling

Abstract

Experiments were conducted in controlled laboratory conditions to determine the size-resolved CCN (Cloud Condensation Nuclei) activity of sub micrometer-sized aerosols containing hazardous nuclear fission products (CsI and CsOH) and abundant ambient inorganic aerosols ammonium sulphates ((NH₄)₂SO₄), ammonium chloride (NH₄Cl), sodium nitrate (NaNO₃), and sodium chloride (NaCl). The presence of these atmospheric-relevant compounds internally mixed with fission product compounds has the potential to affect the capacity of ambient particulates of aerosols to absorb water and function as CCN. Once in the atmosphere, the dynamics of airborne radionuclides and subsequently their fate gets affected by dry and wet deposition processes. The interplay of source and decay terms determines the transit of radioactivity and impacts its local and/or global spread. After being exposed to a sufficiently humid atmosphere, released cesium particles have a high potential to act as CCN. However, the CCN properties of the resulting mixed particles may be altered due to their interaction with atmospheric particles. DMT-CCN counter was used to acquire CCN activation curves with initial dry particle size variation (20 to 300 nm) for mixed particles (ratio 1:1) at 0.1-1% supersaturation (SS). For a variety of particle sizes and mixtures of soluble materials, activation ratio curves were obtained under various SS conditions. From the study of CCN spectra, an estimate of the hygroscopicity parameter (κ) was made, which is sensitive to the chemical composition of aerosols. Under different SS conditions, the CCN activation diameters of mixed aerosols were found to be affected greatly in comparison to pure compounds. For the first time, the spectral efficiency of CCNs and the activation diameters of CsI and CsOH particles combined with important atmospheric aerosols were described at various SS levels. Terminal settling velocities for the mixed particles having a representative diameter as critical activation droplet diameter (wet diameter at particular SS), and varying effective density (based on droplet composition) were obtained and compared with the pure state of particles at different SS levels. The relative difference was significant for some combinations and SS conditions. Any modification in settling velocity ultimately impacts the particle’s lifetime and deposition flux estimations. Hence neglecting the presence of atmospheric salts affects the accuracy of the source term estimates for a postulated nuclear reactor accident scenario. Data on these features is crucial for modelling the behavior of these particles in simulations. In the extremely improbable event of a containment breach occurring under severe nuclear accident conditions, the outcome has the potential to enhance environmental source-term estimations.