Abundance correction Δ_{3D − ⟨ 3D ⟩} , computed according to Eqs. (A.1)–(A.4), versus the difference between ionization and excitation potential, E_ion − χ. Each curve corresponds to a different value of E_ion. The thermodynamic variables θ(x,y) and p_e(x,y) were taken from the 3D model at monochromatic optical depth log τ₈₅₀ = −0.48, where the mean temperature is 3360 ± 140 K. The temperature dependence of the partition functions U₀ and U₁ has been neglected.