We have obtained deep optical spectrophotometry of 16 planetary nebulae in M33 , mostly located in the central two kpc of the galaxy , with the Subaru and Keck telescopes .
We have derived electron temperatures and chemical abundances from the detection of the [ O iii ] \lambda 4363 line for the whole sample .
We have found one object with an extreme nitrogen abundance , 12+log ( N/H ) = 9.20 , accompanied by a large helium content .
After combining our data with those available in the literature for PNe and H ii regions , we have examined the behavior of nitrogen , neon , oxygen and argon in relation to each other , and as a function of galactocentric distance .
We confirm the good correlation between Ne/H and O/H for PNe in M33 .
Ar/H is also found to correlate with O/H .
This strengthens the idea that at the metallicity of the bright PNe analyzed in M33 , which is similar to that found in the LMC , these elements have not been significantly modified during the dredge-up processes that take place during the AGB phase of their progenitor stars .
We find no significant oxygen abundance offset between PNe and H ii regions at any given galactocentric distance , despite the fact that these objects represent different age groups in the evolution of the galaxy .
Combining the results from PNe and H ii regions , we obtain a representative slope of the ISM \alpha -element ( O , Ar , Ne ) abundance gradient in M33 of -0.025 \pm 0.006 dex kpc ^ { -1 } .
Both PNe and H ii regions display a large abundance dispersion at any given distance from the galactic center .
We find that the N/O ratio in PNe is enhanced , relative to the H ii regions , by approximately 0.8 dex .