Most abundant initial molecules in their gas and ice phases (prefaced with a ‘g’) for each of the streamline calculations. Apart from molecular hydrogen and helium, the only other gas phase species are CO and CH₄ . The other abundant initial species are all in the ice phase because the streamlines each start near the cold midplane of the PPD. The dashed lines ordered from top to bottom give the elemental abundance of oxygen, carbon, and sulphur (oxygen and carbon are overlapping).