  Seyfert narrow line regions (NLR) have emission line ratios which are
remarkably uniform, displaying only ~0.5 dex variation between different
galaxies. Existing models have been unable to explain these observations
without the introduction of ad hoc assumptions, geometrical restrictions or new
parameters. Here we introduce a new model: dusty radiation pressure dominated
photoionization, which provides a natural self-regulating characteristic
leading to an invariance of the spectrum over a very wide range (>100) of
ionization parameter. The dusty model is able to reproduce both the range and
the absolute value of the observational line ratios not only in the standard
optical diagnostic diagrams but also in UV diagnostic plots, providing an
explanation to the problem in NLR observations.
