Top: brightness temperature profile (black) for the line of sight indicated by the white cross in the middle and bottom panels, the running mean (green), and the corrected brightness temperature (blue) computed as the difference between the original profile and the running mean. Middle: original integrated brightness temperature of ¹³CO(2−1) from APEX, denoted as $W_{{13}_{\text{CO}}}^{21}$. Bottom: integrated brightness temperature of ¹³CO(2−1) after the correction for the baseline, denoted as $W_{{13}_{\text{CO,\hspace{0.17em}corrected}}}^{21}$.