The method how to find the optimal element abundances in the planetary nebulae (PNe) taking into account both the real probability distribution functions for the line intensities and the small-scale temperature and density fluctuations is present. Results of modelling the PNe spectra and CO abundance determinations are given.

Chemical evolution of the system of galactic planetary nebulae beginning from the early age of the Galaxy is investigated. We calculate radial and vertical abundance gradients of C, N, O, Ne, Ar, Cl and S depending of different ages of the central star of the nebula. It have been founded statistically significant dependence of abundance gradients with the age of central star.