Modeled absorbing flux in Arp 220 of several a) H₂O, b) OH, c) O, d) NH₃, and e) HCN lines as a function of the depth of the molecular shell measured from the surface of the continuum source, as parameterized by the dust opacity at 50 μm (τ₅₀). Numbers in parenthesis indicate rounded wavelengths in μm. The total column of molecules is proportional to τ₅₀, and we have adopted N_H2O = 2 × 10¹⁸ × τ₅₀ cm^-2, and abundance ratios of OH/H₂O = 0.5, , HCN/H₂O = 0.3, and HCN/NH₃ = 6. The dust temperature is 110 K.