Diffusion-advection effects in photon-dissociation regions

Molecular clouds are not static, but species are transported through them by random motions. Therefore, quantifying this transport is essential for understanding the underlying physical conditions in a molecular cloud. We investigated the diffusion-advection effects in the multi-fluid gas of photon-dissociation regions (PDRs). A turbulent mixing-length theory, along with molecular and thermal diffusion, is included in the \kosmatau PDR model. We were able to derive the limits of the total diffusion coefficient and coherence length of turbulent flows as a function of the radius of the cloud. The line intensities from C, C$^+$, C$^{18}$O, CH$^+$, CO, and HCO$^+$ show significant differences for different diffusion-advection scenarios. Hence, C, CO, and other organic molecules can be used as a probe to understand non-stationary chemistry effects measuring changes in the PDR stratification from the diffusion and advection of gas.