We have recently (Gonçalves et al. 2006) shown that previously reported enhanced N abundances in fast, low-ionization emission regions (FLIERs) of PNe, particularly in NGC 7009, may be due to ionization effects. We reached this conclusion through the 3D MOCASSIN photoionization modeling of the bright inner rim of NGC 7009 and its pair of FLIERs, assuming homogeneous elemental abundances throughout the nebula, for N as well as other elements. The (N$^+$/N)/(O$^+$/O) ratio predicted by our models is 0.60 for the rim and 0.72 for the knots, clearly in disagreement with the N$^+$/N=O$^+$/O assumption of the ionization correction factors (ICF) method. Therefore the ICFs are underestimated in both components, rim and knots, but more so in the knots. This effect is partly responsible for the apparent N overabundance claimed for FLIERs.
Based on the above results we are also investigating what would be the effect of the N/O ICF overestimation on the definition of Type I PNe which do not have FLIERs-like structures.
