Evolutionary tracks of the elemental ratios over 3 Myr across all considered scenarios. Ratios are shown in pairs across four key compositional regions: within the H₂O snowline (region 1, top-left), between the H₂O and CO₂ snowlines (region 2, top-right), between the CO₂ and CH₄ snowlines (region 3, bottom-left), and beyond the CH₄ snowline (region 4, bottom-right). Values represent gas-phase elemental ratios, calculated from total elemental abundances averaged over the radial extent of each region and weighted by the surface density of the gas. The colour bars indicate the four chemical scenarios, with time progressing from lighter to darker shades. The three different markers represent the selected grain sizes and are placed along each track every 200 000 yr. The dark marker on each track denotes 1 Myr. Dashed grey lines highlight regions of the parameter space where pairwise comparisons of the elemental ratios provide constraints on specific scenarios or subsets of scenarios.