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We have compared the oxygen and nitrogen abundances derived from global emission‐line Sloan Digital Sky Survey (SDSS) spectra of galaxies using (1) the   T  e   method and (2) two recent strong‐line calibrations: the ON and NS calibrations. Using the   T  e   method, anomalously high N/O abundance ratios have been found in some SDSS galaxies. To investigate this, we have Monte Carlo simulated the global spectra of composite nebulae by a mix of spectra of individual components, based on spectra of well‐studied H  ii  regions in nearby galaxies. We found that the   T  e   method results in an underestimated oxygen abundance (and hence in an overestimated nitrogen‐to‐oxygen ratio) if H  ii  regions with different physical properties contribute to the global spectrum of composite nebulae. This effect is somewhat similar to the small‐scale temperature fluctuations in H  ii  regions discussed by Peimbert. Our work thus suggests that the high   T  e  ‐based N/O abundance ratios found in SDSS galaxies may not be real. However, such an effect is not expected to be present in dwarf galaxies since they generally have a uniform chemical composition. The ON and NS calibrations give O and N abundances in composite nebulae which agree with the mean luminosity‐weighted abundances of their components to within  ∼ 0.2 dex.

Abundance determination from global emission‐line SDSS spectra: exploring objects with high N/O ratios