Fractional abundances as a function of height, z, above the midplane for models B and C. In model B, the sharp decrease in abundance of CO and CH₄ at ﹩z\sim 60{\mbox{--}} 70﹩ AU and of H₂CO and isotopomers at ﹩z\sim 120﹩ AU is due to the onset of thermal desorption. The carbon atoms produced in the gas phase when CO is released and then broken up are reaccreted onto the grains between 60 and 140 AU where they form hydrocarbon ices. At ﹩z> 140﹩ AU, most accreted carbon atoms are desorbed before they can react, and hence, the abundance of hydrocarbons in the ice falls off. In model C photodesorption ensures that all molecules are removed from the grains by ﹩z=80{\mbox{--}} 100﹩ AU.