Tritium dynamics in soils and plants grown under three irrigation regimes at a tritium processing facility in Canada.

The dynamics of tritium released from nuclear facilities as tritiated water (HTO) have been studied extensively with results incorporated into regulatory assessment models. These models typically estimate organically bound tritium (OBT) for calculating public dose as OBT itself is rarely measured. Higher than expected OBT/HTO ratios in plants and soils are an emerging issue that is not well understood. To support the improvement of models, an experimental garden was set up in 2012 at a tritium processing facility in Pembroke, Ontario to characterize the circumstances under which high OBT/HTO ratios may arise. Soils and plants were sampled weekly to coincide with detailed air and stack monitoring. The design included a plot of native grass/soil, contrasted with sod and vegetables grown in barrels with commercial topsoil under natural rain and either low or high tritium irrigation water. Air monitoring indicated that the plume was present infrequently at concentrations of up to about 100 Bq/m(3) (the garden was not in a major wind sector). Mean air concentrations during the day on workdays (HTO 10.3 Bq/m(3), HT 5.8 Bq/m(3)) were higher than at other times (0.7-2.6 Bq/m(3)). Mean Tissue Free Water Tritium (TFWT) in plants and soils and OBT/HTO ratios were only very weakly or not at all correlated with releases on a weekly basis. TFWT was equal in soils and plants and in above and below ground parts of vegetables. OBT/HTO ratios in above ground parts of vegetables were above one when the main source of tritium was from high tritium irrigation water (1.5-1.8). Ratios were below one in below ground parts of vegetables when irrigated with high tritium water (0.4-0.6) and above one in vegetables rain-fed or irrigated with low tritium water (1.3-2.8). In contrast, OBT/HTO ratios were very high (9.0-13.5) when the source of tritium was mainly from the atmosphere. TFWT varied considerably through time as a result of SRBT's operations; OBT/HTO ratios showed no clear temporal pattern in above or below ground plant parts. Native soil after ∼20 years of operations at SRBT had high initial OBT that persisted through the growing season; little OBT formed in garden plot soil during experiments. High OBT in native soil appeared to be a signature of higher past releases at SRBT. This phenomenon was confirmed in soils obtained at another processing facility in Canada with a similar history. The insights into variation in OBT/HTO ratios found here are of regulatory interest and should be incorporated in assessment models to aid in the design of relevant environmental monitoring programs for OBT.

Levels of tritium in soils and vegetation near Canadian nuclear facilities releasing tritium to the atmosphere: implications for environmental models.

Concentrations of organically bound tritium (OBT) and tritiated water (HTO) were measured over two growing seasons in vegetation and soil samples obtained in the vicinity of four nuclear facilities and two background locations in Canada. At the background locations, with few exceptions, OBT concentrations were higher than HTO concentrations: OBT/HTO ratios in vegetation varied between 0.3 and 20 and values in soil varied between 2.7 and 15. In the vicinity of the four nuclear facilities OBT/HTO ratios in vegetation and soils deviated from the expected mean value of 0.7, which is used as a default value in environmental transfer models. Ratios of the OBT activity concentration in plants ([OBT]plant) to the OBT activity concentration in soils ([OBT]soil) appear to be a good indicator of the long-term behaviour of tritium in soil and vegetation. In general, OBT activity concentrations in soils were nearly equal to OBT activity concentrations in plants in the vicinity of the two nuclear power plants. [OBT]plant/[OBT]soil ratios considerably below unity observed at one nuclear processing facility represents historically higher levels of tritium in the environment. The results of our study reflect the dynamic nature of HTO retention and OBT formation in vegetation and soil during the growing season. Our data support the mounting evidence suggesting that some parameters used in environmental transfer models approved for regulatory assessments should be revisited to better account for the behavior of HTO and OBT in the environment and to ensure that modelled estimates (e.g., plant OBT) are appropriately conservative.

Equilibrium morphology of mixed organic/inorganic/aqueous aerosol droplets: investigating the effect of relative humidity and surfactants.

There is considerable uncertainty regarding the phase, morphology, and composition of atmospheric aerosol. In particular, it is important to understand the microphysical structure of mixed inorganic/organic aerosol given that the structure can influence the surface composition, the role of heterogeneous chemistry, gas-particle partitioning of semivolatile organics, and water uptake in the sub- and supersaturated regimes. We present here a thermodynamic model that predicts the equilibrium morphology of mixed inorganic/organic aerosol. The model uses an iterative process to calculate the total surface free energy of all possible morphologies when two immiscible droplets are brought into contact, with the configuration with the lowest total surface free energy representing the final equilibrium structure. Sensitivity tests and validation experiments were performed by investigating the decane/NaCl/aqueous system. The addition of a water-soluble surfactant was found to promote spreading of decane on the aqueous droplet. This was confirmed by laboratory experiments, although the importance of considering the relative volumes of the aqueous and organic phases was found to play a significant role in determining the equilibrium structure. Decreasing the relative humidity (RH) of the surrounding gas phase was found to decrease the spreading of decane on the aqueous droplet, leading to thicker organic lenses on smaller aqueous droplets. We conclude that a core-shell structure is not always predicted to be the thermodynamically favored state of aerosol containing distinct hydrophobic and hydrophilic phases. Gaining a more reliable picture of the microphysical structure of aerosol is crucial to be able to model aerosol behavior and properties in the atmosphere, particularly when aerosol is dominated by internal mixtures of inorganic and organic components and when the organic is present in a (subcooled) liquid state.

Measurements of the equilibrium size of supersaturated aqueous sodium chloride droplets at low relative humidity using aerosol optical tweezers and an electrodynamic balance.

An approach for examining the hygroscopicity of single aerosol particles over a broad range in relative humidity (RH) using aerosol optical tweezers is presented and compared with measurements made using an electrodynamic balance. In particular, benchmark measurements on aqueous sodium chloride aerosol are presented over the RH range 45-75% (293 K), a RH range that had not previously been explored with aerosol optical tweezers. Measurements of the variation in equilibrium wet droplet size with RH are made using cavity-enhanced Raman scattering, with an accuracy of 1 nm in the determination of the wet particle radius. The full range of optical tweezers experimental measurements (including previous dual trapping comparative studies approaching a saturation relative humidity of 100%) are compared with determinations using other experimental techniques and with a range of model treatments. An assessment of the models and all experimental data for estimating the equilibrium size of a sodium chloride droplet suggests that the size can be predicted with an accuracy of better than 0.1% over the RH range 48-100%. Discrepancies between different measurements lead to an increase in uncertainty above 1% below an RH of 48% as efflorescence is approached. The optical tweezers' measurements of equilibrium size consistently agree with model predictions to within an error of 1% (<50 nm for the size range explored here) and mostly with an error of less than +/-0.1%. These data demonstrate the highly accurate nature of measurements of thermodynamic equilibrium size by aerosol optical tweezers and suggest that this approach may be used to investigate the competition between thermodynamic and kinetic factors in governing aerosol particle size over the full RH range.

Role of the aerosol substrate in the heterogeneous ozonation reactions of surface-bound PAHs.

To probe how the aerosol substrate influences heterogeneous polycyclic aromatic hydrocarbon (PAH) oxidation, we investigated the reaction of surface-bound anthracene with gas-phase ozone on phenylsiloxane oil and azelaic acid aerosols under dry conditions in an aerosol flow tube with offline analysis of anthracene. The reaction exhibited pseudo-first-order kinetics for anthracene loss, and the pseudo-first-order rate coefficients displayed a Langmuir-Hinshelwood dependence on the gas-phase ozone concentration on both aerosol substrates. The following parameters were found: for the reaction on phenylsiloxane oil aerosols, K(O3) = (1.0 +/- 0.4) x 10(-13) cm(3) and k(I)(max) = (0.010 +/- 0.003) s(-1); for the reaction on azelaic acid aerosols, K(O3) = (2.2 +/- 0.9) x 10(-15) cm(3) and k(I)(max) = (0.057 +/- 0.009) s(-1), where K(O3) is a parameter that describes the partitioning of ozone to the surface and k(I)(max) is the maximum pseudo-first-order rate coefficient at high ozone concentrations. The K(O3) value for the reaction of surface-bound anthracene and ozone on azelaic acid aerosols is similar to the K(O3) value that we obtained in our previous study for the reaction of surface-bound benzo[a]pyrene and ozone on the same substrate. This finding supports our earlier hypothesis that the substrate influences the partitioning of ozone to the surface irrespective of the organic species (i.e., PAH) adsorbed to it. Preliminary ab initio calculations were performed to investigate whether there is a relationship between the relative binding energies of the ozone-substrate complex and the K(O3) values for the different substrates studied. A comparison between kinetic results obtained on aerosol substrates and thin films is presented.