Position of the N₂H⁺ column density peak in the different models (listed in the lower right corner of each panel) for different CO and N₂ abundances. From left to right and top to bottom: the fiducial models (CH), models with both CO and N₂ binding energies increased (CH-Eb1), models with only CO binding energy increased (CH-Eb2), models with large grains settled to only 80% of small grain scale height (CH-χ0.8), models with a lower cosmic ray ionization rate (1 × 10^-19 s^-1; CH-CR1) and models with a higher cosmic ray ionization rate (5 × 10^-17 s^-1; CH-CR2). Models with N₂/CO ratios <1 are highlighted with blue plus signs. Red circles in the left panels represent the snow surface only models, i.e., N₂H⁺ removed above the CO snow surface. The red lines mark the location of the CO snowline in the models. The gray line indicates the position of the observed emission peak.