  This paper presents a spectroscopic investigation of 11 HII regions in the
nearby galaxy NGC 2403. The HII regions are observed with a long-slit
spectrograph mounted on the 2.16 m telescope at XingLong station of National
Astronomical Observatories of China. For each of the HII regions, spectra are
extracted at different nebular radii along the slit-coverage. Oxygen abundances
are empirically estimated from the strong-line indices R23, N2O2, O3N2, and N2
for each spectrophotometric unit, with both observation- and model-based
calibrations adopted into the derivation. Radial profiles of these diversely
estimated abundances are drawn for each nebula. In the results, the oxygen
abundances separately estimated with the prescriptions on the basis of
observations and models, albeit from the same spectral index, systematically
deviate from each other; at the same time, the spectral indices R23 and N2O2
are distributed with flat profiles, whereas N2 and O3N2 exhibit apparent
gradients with the nebular radius. Because our study naturally samples various
ionization levels which inherently decline at larger radii within individual
HII regions, the radial distributions indicate not only the robustness of R23
and N2O2 against ionization variations but also the sensitivity of N2 and O3N2
to the ionization parameter. The results in this paper provide observational
corroboration of the theoretical prediction about the deviation in the
empirical abundance diagnostics. Our future work is planned to investigate
metal-poor HII regions with measurable T_e, in an attempt to recalibrate the
strong-line indices and consequently disclose the cause of the discrepancies
between the empirical oxygen abundances.
