The curious conundrum regarding sulfur and oxygen abundances in   planetary nebulae
  We carefully consider numerous explanations for the sulfur abundance anomaly in planetary nebulae. No one rationale appears to be satisfactory, and we suggest that the ultimate explanation is likely to be a heretofore unidentified feature of the nebular gas which significantly impacts the sulfur ionization correction factor. 
R.B.C. Henry, A. Speck, A.I. Karakas, and G.J. Ferland
astro-ph.GA