Position of the N₂H⁺J = 4−3 (black circles), J = 3−2 (blue plus signs) and J = 1−0 (red crosses) emission peaks for different CO and N₂ abundances in the simple chemical model (model CH; top panels) and the corresponding snow surface only models, i.e., N₂H⁺ removed above the CO snow surface (bottom panels). The emission is convolved with a beam (left panels) or beam (right panels). The red lines mark the location of the CO snowline in the models.