HCNH⁺ abundances as a function of H₂ column density. The colors represent the distances of GMCs to GMC5 which is located at the center of the CMZ of NGC 253. The circles represent HCNH⁺ abundances derived using the LTE approach (see Sect. 3), while the squares represent those obtained from the non-LTE approach (see Sect. 3.3.2). The cyan-shaded region indicates the observed HCNH⁺ abundances of ≲2 × 10⁻⁹ previously reported in the Milky Way, while the brown- shaded region marks the abundances reported toward Sgr B2.