The spectra of H II regions vary systematically across the disks of spiral galaxies. We have used computer models of H II regions to study the changes in line intensities resulting from variations in the chemical abundances, the dust/gas ratio, the clumpiness of the gas, and the effective temperatures of the exciting stars. Our results support Searle's contention that both the N II/O II abundance ratio gradient and the O III abundance/H-beta gradient result from variations in chemical abundances. H II regions in the outermost spiral arms are deficient in nitrogen and oxygen, whereas regions in the innermost arms have nitrogen and oxygen abundances larger than solar. Condensations in the nebulae are required for sufficiently strong O II emission.
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Gregory A. Shields (1974) studied this question.