Column densities of trace neutral species Na I, C I, Ca I, and Fe I vs. N(H_tot) for Galactic sight lines. The stars denote the sight line toward HD 62542: red = main (14 km s⁻¹) component; green = all other components; blue = all components. The solid lines show weighted and unweighted fits to the data (not including the Sco-Oph and Trapezium sight lines); the dotted lines have slope = 2 (as would be expected from considerations of ionization balance). Toward HD 62542, N(Na I) is consistent with the general Galactic relationship, N(C I) is somewhat enhanced, and N(Ca I) and N(Fe I) are deficient, reflecting the specific ionization and depletion behavior of each of those species in this unusual sight line (see Section 4.3.3). The lower abundances of the trace neutral species in most of the Sco-Oph and Trapezium sight lines may be due to stronger than average radiation fields there (e.g., Welty & Hobbs 2001).