Modeling of particle/gas distribution kinetics of polycyclic aromatic hydrocarbons(PAHs) in the atmosphere: Relevance of mass transfer resistance shifts

A coupled film-intraparticle pore diffusion model was derived to explain the deviations between measured apparent bulk particle/gas distribution coefficients (Log𝐾_{𝑝𝑔,𝑏,𝑎}) and equilibrium values (Log𝐾_{𝑝𝑔,𝑏}) predicted either from octanol-air distribution coefficients (𝐾_𝑜𝑎) or subcooled liquid vapor pressures (P_L^o) of PAHs. The coupled model accounts for both external mass transfer resistance in the bulk air and internal resistance within the intraparticle pore space. For low molecular weight compounds (with small Log𝐾_{𝑝𝑔,𝑏}), mass transfer is dominated by intraparticle pore diffusion, following the square root of time law and the apparent distribution coefficients increase or decrease with the square root of 𝐾_𝑜𝑎 or P_L^o. In contrast, for high molecular weight compounds, external film diffusion becomes the limiting factor, resulting in observed distribution coefficients that appear independent of 𝐾_𝑜𝑎 or P_L^o (slope = 0). Moderate molecular weight compounds fall in between, with the slope transitioning from 1/2 to 0, requiring consideration of both external and internal resistances. The coupled model is strongly influenced by parameters such as intraparticle porosity, airborne particle concentration, grain size, and the contact time between airborne particles and ambient air. High Log𝐾_{𝑝𝑔,𝑏,𝑎} values are associated with fast kinetics, which are enhanced by increased intraparticle porosity, higher airborne particle concentration, smaller particle size, or prolonged contact time (aged particles). The model was validated using three datasets with varying contact times from recent publications. Results for Log𝐾_{𝑝𝑔,𝑏,𝑎} derived from local sources, such as oil combustion tests in the lab and urban data, were well explained by the sorption model. However, data from polar regions required a desorption model with unexpectedly slow solid diffusion rates (𝐷_𝑠 = 10^−18.5 m² s⁻¹). This finding suggests that the properties of aged particles, such as viscosity, change during long-distance transport, leading to more complex mass transfer processes in the remote areas.