Observed and calculated R_C I/CO as a function of CO column density N_CO and [C I/CO] abundance (column density) ratio N_C I/N_CO for seven representative ISM conditions, similar to Fig. 8 but based on non-LTE calculations. Upper panels: line ratio R_C I/CO versus the abundance ratio N_C I/N_CO for different N_CO, as indicated by the color. Dashed, dotted and solid lines are for different CO(2−1) [C I](1−0) optical depths as described in Fig. 8 caption. Horizontal thick line is the observed R_C I/CO value. Lower panels: are the contours of R_C I/CO = 0.05, 0.10, 0.20, and 0.50 in the CO column density versus C I/CO abundance ratio plane. Vertical color shading indicates the final abundance ratio N_C I/N_CO that can match both the observed R_C I/CO and the assumed Σ_mol value noted at the top of each panel.