Rotational lines of Na³⁵Cl in IRC +10216 as observed with the IRAM 30-m telescope (black histograms) and as calculated with the radiative transfer model (green lines). Continuous lines refer to the best model, using the abundance profile shown in Fig. 13, while dashed lines result from a model without the abundance decrease in the 1 − 3 R_∗ region. Note that dashed lines overestimate the intensity at the center of high-J lines.