Recent ground-based abundance studies of a large sample of PN and H II regions
by the applicants have revealed that the ratio of C, N and O abundances derived
from optical recombination lines to those from optical and UV collisionally
excited forbidden lines, covers a wide range of 1--15. Since this ratio varies
from object to object the cause has to be sought in the physical conditions in
the nebulae, not in the basic atomic physics. A possible explanation is that
significant temperature and/or density fluctuations exist. The far-IR fine
structure lines from [N III], [O III], [Ne III], [S III] and [Ar III],
observable with the ISO SWS and LWS, provide a unique and powerful tool to
solve this discrepancy in nebular abundance determinations. With excitation
energies of less than 1100 K, ionic abundances derived from these far-IR lines
are insensitive to the electron temperature and to temperature fluctuations and
provide an independent test of the reliability of the abundances derived from
optical recombination lines on the one hand and from optical and UV forbidden
lines on the other. The availability from these far-IR lines of a number of
density-sensitive diagnostic ratios with a wide range of critical densities
will enable us to study quantitatively nebular density inhomogeneity and its
effects on abundance determinations, without complications from temperature
or ionization stratification. ISO LWS01 full grating spectra for about ten PN
suitable for such abundance studies are scheduled in our LWS Post-Main-Sequence
GT programme. SWS01 spectra for a few of them will also be obtained by us, with
most of the remainder being observed by the SWS consortium. Here we propose
to obtain LWS01 and SWS01 full grating spectra from 2.43--196.7 um for an
additional sample of 32 PN, selected based on fresh information from our
recent extensive ground-based abundance survey. A systematic study of a large
enough sample of PN having different morphologies, electron temperatures and
densities, and exciting stars, is essential to achieve a full understanding of
this fundamental discrepancy in nebular abundance determinations from heavy
element optical forbidden and recombination lines, which has a major impact on
our understanding of stellar nucleosynthesis and Galactic chemical evolution.