Interstellar Extinction and Elemental Abundances: Individual Sight Lines
  While it is well recognized that both the Galactic interstellar extinction
curves and the gas-phase abundances of dust-forming elements exhibit
considerable variations from one sight line to another, as yet most of the dust
extinction modeling efforts have been directed to the Galactic average
extinction curve, which is obtained by averaging over many clouds of different
gas and dust properties. Therefore, any details concerning the relationship
between the dust properties and the interstellar environments are lost. Here we
utilize the wealth of extinction and elemental abundance data obtained by space
telescopes and explore the dust properties of a large number of individual
sight lines. We model the observed extinction curve of each sight line and
derive the abundances of the major dust-forming elements (i.e., C, O, Si, Mg
and Fe) required to be tied up in dust (i.e., dust depletion). We then confront
the derived dust depletions with the observed gas-phase abundances of these
elements and investigate the environmental effects on the dust properties and
elemental depletions. It is found that for the majority of the sight lines the
interstellar oxygen atoms are fully accommodated by gas and dust and therefore
there does not appear to be a "missing oxygen" problem. For those sight lines
with an extinction-to-hydrogen column density $A_V/N_H$ $>$
4.8$\times$10$^{-22}$ mag cm$^2$/H there are shortages of C, Si, Mg and Fe
elements for making dust to account for the observed extinction, even if the
interstellar C/H, Si/H, Mg/H and Fe/H abundances are assumed to be proto-solar
abundances augmented by Galactic chemical evolution.
