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Hydrodynamical (or 3D) and non-local thermodynamic equilibrium (NLTE) effectsare known to affect abundance analyses. However, there are very fewobservational abundance testsof 3D and NLTE models. We developed a new way oftesting the abundance predictions of 3D and NLTE models, taking advantage oflarge spectroscopic survey data. We use a line-by-line analysis of the ApachePoint Observatory Galactic Evolution Experiment (APOGEE) spectra (H band) withthe Brussels Automatic Code for Characterizing High accUracy Spectra (BACCHUS).We compute line-by-line abundances of Mg, Si, Ca, and Fe for a large number ofglobular cluster K giants in the APOGEE survey. We compare this line-by-lineanalysis against NLTE and 3D predictions. While the 1D-NLTE models providecorrections in the right direction, there are quantitative discrepanciesbetween different models. We observe a better agreement with the data for themodels including reliable collisional cross-sections. The agreement betweendata and models is not always satisfactory when the 3D spectra are computed inLTE. However, we note that for a fair comparison, 3D corrections should becomputed with self-consistently derived stellar parameters, and not on 1Dmodels with identical stellar parameters. Finally, we focus on 3D and NLTEeffects on Fe lines in the H band, where we observe a systematic difference inabundance relative to the value from the optical. Our results suggest that themetallicities obtained from the H band are more accurate in metal-poor giants.More atomic data and extended self-consistent 3D-NLTE computations need to bemade. The method we have developed for testing 3D and NLTE models could beextended to other lines and elements, and is particularly suited for largespectroscopic surveys.

Probing 3D and NLTE models using APOGEE observations of globular cluster stars