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We investigate the chemical abundances of NGC3603 in the Milky Way, of30Doradus in the Large Magellanic Cloud, and of N66 in the Small MagellanicCloud. Mid-infrared observations with the Infrared Spectrograph onboard theSpitzer Space Telescope allow us to probe the properties of distinct physicalregions within each object: the central ionizing cluster, the surroundingionized gas, photodissociation regions, and buried stellar clusters. We detect[SIII], [SIV], [ArIII], [NeII], [NeIII], [FeII], and [FeIII] lines and derivethe ionic abundances. Based on the ionic abundance ratio (NeIII/H)/(SIII/H), wefind that the gas observed in the MIR is characterized by a higher degree ofionization than the gas observed in the optical spectra. We compute theelemental abundances of Ne, S, Ar, and Fe. We find that the alpha-elements Ne,S, and Ar scale with each other. Our determinations agree well with theabundances derived from the optical. The Ne/S ratio is higher than the solarvalue in the three giant HII regions and points toward a moderate depletion ofsulfur on dust grains. We find that the neon and sulfur abundances display aremarkably small dispersion (0.11dex in 15 positions in 30Doradus), suggestinga relatively homogeneous ISM, even though small-scale mixing cannot be ruledout.

Chemical composition and mixing in giant HII regions: NGC3603, 30Doradus, and N66