Fig. 3.

Schematic view of the processes governing the flux of a virion towards the ACE2 receptor in the presence of sorbing PM2.5 up to the stage of (a) cell recognition with the ACE2 receptor, and (b) the preliminary stage of endocytosis. The concentration of the free virion, c_V, is sketched by the solid red line and the average concentration of V-PM entities, c_V‐PM, is indicated by the dashed red line. In the region where V-PM maintains equilibrium with V, the change in c_V‐PM follows the change in c_V. For clarity arbitrary thicknesses are shown for the operational reaction layer at the host cell/medium interface ($\overline{\lambda }$) and the mean solution diffusion layer for V and V-PM ($\overline{\delta }$). The various $\overline{f}$ terms denote the interaction forces between V-PM and the cell surface, ${\overline{f}}_{\text{cell},\mathrm{V}‐\mathrm{PM}}$, between free V and the cell surface, ${\overline{f}}_{\text{cell},\mathrm{V}}$, and between free V and PM, ${\overline{f}}_{\mathrm{V},\mathrm{PM}}$. The diffusion coefficients for free V and V-PM are denoted by D_V and D_V-PM, respectively, and K_H,PM is the coefficient for sorption of V onto PM in the Henry regime.