Comparison between observations and model results for two species that require special abundance profiles: N₂ H⁺ (top) and C₂H (bottom). In both panels, the green symbols represent the observed intensity, previously shown in Fig. 5. In the N₂ H⁺ panel, the dashed red line represents a constant abundance model that fits the intensity at high column densities. The blue dashed line represents the best-fit model, and includes an internal abundance enhancement. In the C₂ H panel, the dashed red line represents a model with no external enhancement, while the blue line represents the externally enhanced best-fit model.