Model compounds

PCB-52 and phenanthrene were chosen as model compounds. PCB-52 was selected because it is fairly resistant to photochemical oxidation in the atmosphere (with atmospheric half-life times ranging from 3 to 120 days for all PCB congeners, and 62.5

43 days for PCB-52 specifically, Sinkkonen et al., 2000). Phenanthrene was selected for comparison because of similar physicochemical properties and its photosensitivity, with a much shorter atmospheric half-life of 5.6 hours (Atkinson and Arey, 1994). The relevant physicochemical properties of the two compounds (e.g. thermodynamic properties, half- lives, and diffusion and partitioning coefficients) are summarized in Table S2.1 (Supporting Information S2).

3.2.2 Conceptual model

The conceptual model used to simulate the soil–plant–atmosphere fluxes of SVOCs driven by diurnal temperature changes is shown in Fig. 3.1. It accounts for two distinct soil horizons (high organic carbon topsoil and sandy subsoil), a plant layer, and the atmospheric boundary layer with varying heights. Fig. 3.1 additionally shows how diurnal temperature changes in the atmosphere propagate into the soil over a time period of 1.5 days. For the sake of simplicity, we assumed a sinusoidal temperature change with the minimum and maximum daily temperatures at 3 am and 3 pm, respectively.

Changes in temperature over a day result in changes in the height of the atmospheric boundary layer as well as in vertical mixing. In fact, even under fair weather conditions the height of the atmospheric boundary layer still exhibits strong diurnal changes (Stull, 2000). During daytime, solar radiation leads to surface warming, and hence to the formation of convective thermal eddies that influence mass transport of atmospheric pollutants. Due to turbulence, the atmospheric boundary layer may reach heights of thousands of meters and eddy diffusion is the dominating mass transport process. At night the height of the atmospheric boundary layer decreases due to an inverse temperature field (i.e., increasing temperature with increasing height), caused by radiative cooling of surface air. In the model, the atmospheric boundary layer was simplified with a thickness of 1,000 m during daytime and 100 m at night.

44

Figure 3.1. Conceptual model (not drawn to scale) and processes used to simulate temperature-

driven concentration fluctuations of semi-volatile pollutants in the soil–plant–atmosphere system; changes in the height of the atmospheric boundary layer are simplified by assuming fixed heights during daytime (1,000 m) and at night (100 m); the lower boundary represents the groundwater level. The temperature evolution (bottom) is shown at/above ground surface (z ≥ 0 m) and for two different soil depths: 0.05 m (topsoil), 0.2 m (subsoil).

The plant layer may act as potential source-sink term at ground level for atmospheric concentration fluctuations, the parameterization of which depends on the species, total biomass, leaf area, age, and effective lipid content (or sorption capacity). Root uptake of pollutants from soils was not considered for the short-term diurnal simulations but may play a role for seasonal fluxes of pollutants. The focus was placed on the ground-level plant layer and the influence of the amount of plant layer that, in terms of quantity of biomass per square meter, varies significantly with dependence on the types of land cover, site-specific plant species, season, and plant ages (e.g. Mackay, 2001). Mackay (2001) recommended 1 cm as the equivalent “height” of plant biomass, mainly composed of water (50%), cellulose, starch, ligneous material, and some lipid-like material (1%), the latter of which is responsible for the uptake and re-volatilization of atmospheric pollutants

45 and may change over time as well. In our study, we simplified this ground-level source- sink term as an equivalent volume of 1 L plant layer per square meter land surface (1 cm equivalent height at 10% land coverage). A subset of parameters that describe the compartments and chemical properties are shown in Table 3.1, and extensively in Table S2.1.

Table 3.1. Overview of mass transfer parameters and selected physical properties in soils, plants

and the atmosphere for PCB-52 and phenanthrene (Phe) at 25 °C.

Layer Lipid/organic carbon content Water saturation Sw [-] Porosity n [-] Partition coefficient [L/kg] Effective gas diffusion coefficient Deg [m2/s] PCB-52 Phe PCB-52 Phe Atm. boundary layer NA 0 d _{NA NA NA Variable -} Fig. S2.1 d Variable - Fig. S2.1 d

Plant layer 1.0% a ₀ d _{NA logK}

PA = 5.40 logKPA = 5.14 > 5.2×10-6 d > 6.0×10-6 d Topsoil 2.0% b _{≈ 0.3} e _{0.40 logK} d = 3.56 logKd = 2.44 5.3×10-7 _6.1×10-7 Vadose zone (subsoil) 0.2% c ≈ 0.3 e _{0.33 logKd} = 2.56 logKd = 1.44 4.0×10-7 _4.6×10-7 a _{lipid content in the plant layer (Mackay, 2001)}

b _{data obtained from the European Soil Portal – Soil Data and Information Systems from the website of the European}

Commission http://eusoils.jrc.ec.europa.eu/

c _{10 times lower organic carbon content is assumed in the subsoil than in the topsoil}

d _{implementation of the atmospheric boundary layer following Bao et al. (2015) and Foken (2008). Fig. S2.1 shows}

the vertical profile of effective gas and eddy diffusion of vapor phase PCB-52 in soils and in the atmosphere while the vertical profile of vapor phase phenanthrene is similar, following the same procedure in Bao et al. (2015).

e _{mean water saturation calculated for sandy soils predicted by van Genuchten model (Section S2.3) with 𝛼 = 14.5}

m-1_{, N = 2.68, and l = 0.5 (Carsel and Parrish, 1988)}

Note: NA = not applicable.

As shown in Fig. 3.1, the model is 1-D vertically. For the short-term simulations, the mass of conservative compounds (i.e. PCB-52) in the system is preserved. Photosensitive compounds (here phenanthrene) are photochemically reduced by over 90% mass in the atmosphere during daytime, but this is re-compensated at night, and thus ongoing anthropogenic emissions of PAHs are considered to maintain the typical range of observed atmospheric concentrations. In the subsurface, both compounds (PCB-52 and phenanthrene) partition between the aqueous, the gaseous, and the solid phases and are transported by diffusion (pure diffusion in the aqueous and the gaseous phases). In the atmosphere, the two compounds are present in the vapor phase and partition between

46 plants and air at the temperatures of interest. Partitioning to atmospheric particles was not considered because less than 5% of the two compounds would be involved (assuming e.g. 0.1 mg/m3 _{atmospheric particles and local equilibrium, 76% mass of PCB-52 is} present in the atmosphere, 19% mass in plants, and 2% mass in atmospheric particles; for details refer to Section S2.7). Vertical exchange of pollutants between different compartments is mainly driven by diffusion (or eddy diffusion) and vertical concentration gradients in the gas phase; for partitioning between air, water, soil solids and plant materials local equilibrium was assumed.

Similar to gas exchange across the soil-atmosphere interface, temperature-driven uptake and re-volatilization of atmospheric pollutants by plants can be simulated with a numerical model. McLachlan (1999) proposed a model framework to describe the uptake of SVOCs in plants via one of the following three processes with dependence on the chemical properties of the compounds: equilibrium partitioning into plants, kinetically limited gaseous deposition, and particle-bound deposition. Böhme et al. (1999) experimentally verified the concept (McLachlan, 1999) of uptake of atmospheric pollutants by plants and demonstrated that the uptake for the compounds with logKOA < ~9 may be described by

equilibrium partitioning. Values of logKOA are < ~9 for the two compounds in our study,

PCB-52 (logKOA = 7.73) and phenanthrene (logKOA = 7.60). Terzaghi et al. (2015) also

found that concentrations of PAHs in plants reflect actual concentrations in the atmosphere due to fast equilibration. Thus, for numerical simulations, we implemented a plant layer as an additional compartment in the model assuming local equilibrium between plants and air for PCB-52 and phenanthrene (McLachlan, 1999; Böhme et al., 1999).