An estimate of the time variation of the abundance gradient from   planetary nebulae III. O, S, Ar, and Ne: A comparison of PN samples
  The time behaviour of the radial abundance gradients in the galactic disk is investigated on the basis of four different samples of planetary nebulae, comprising both smaller, homogeneous sets of data, and larger, albeit non-homogeneous samples. Four different chemical elements are considered, namely, oxygen, sulphur, argon and neon. Our analysis support our earlier conclusions that, on the average, the radial abundance gradients have flattened out in the last 6 to 8 Gyr. 
W. J. Maciel, L. G. Lago, R. D. D. Costa