Comparison between averaged line shapes of several lines in Arp 220. HE, LE, PIMS and RIMS mean high-excitation, low-excitation, pure-inversion metastable, and rotation-inversion metastable, respectively. H₂O HE is the average of the 6₁₆ ← 5₀₅ and 3₂₂ ← 2₁₁ lines of H₂O (Paper I); HCN HE is the 18 − 17 line of HCN; OH⁺ HE is the average of the 4₃ − 3₂ and 4₅ − 3₄ lines of OH⁺; H₂O⁺ HE is the average of the 3₂₂ ← 2₁₁ 7/2−5/2, 3₁₃ ← 2₀₂ 5/2 − 3/2, and 4₀₄ ← 3₁₃ 7/2 − 5/2 lines of H₂O⁺; H₃O⁺ PIMS is the average of the pure-inversion, metastable , , , and lines of H₃O⁺; NH₃ RIMS is the average of the rotation-inversion metastable and lines of NH₃; H₂O LE is the average of the 2₁₂ ← 1₀₁ and 2₂₁ ← 1₁₀ lines of H₂O; OH⁺ LE is the average of the 2₂ − 1₂, 2₁ − 1₀, 2₂ − 1₁, and 2₁ − 1₁ lines of OH⁺; H₂O⁺ LE is the average of the 2₁₂ ← 1₀₁ 5/2−3/2 and 3/2−1/2 lines of H₂O⁺; H₃O⁺ 70.8 is the line of H₃O⁺ at 70.8 μm. The dotted and dashed vertical lines indicate the velocity centroids of the averaged HE and LE spectra, respectively.