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AGB stars are, together with supernovae, the main contributors of stellardust to the interstellar medium (ISM). Dust grains formed by AGB stars arethought to be large. However, as dust nucleation and growth within theiroutflows are still not understood, the dust-grain size distribution (GSD) isunknown. This is an important uncertainty regarding our knowledge of thechemical and physical history of interstellar dust, as AGB dust forms $\sim$70% of the starting point of its evolution. We expand on our chemical kineticsmodel, which uniquely includes a comprehensive dust-gas chemistry. The GSD isnow allowed to deviate from the commonly assumed canonical Mathis et al. (1977)distribution. We find that the specific GSD can significantly influence thedust-gas chemistry within the outflow. Our results show that the level ofdepletion of gas-phase species depends on the average grain surface area of theGSD. Gas-phase abundance profiles and their possible depletions can beretrieved from observations of molecular emission lines when using a range oftransitions. Due to degeneracies within the prescription of GSD, specificparameters cannot be retrieved, only (a lower limit to) the average grainsurface area. Nonetheless, this can discriminate between dust composed ofpredominantly large or small grains. We show that when combined with otherobservables such as the spectral energy distribution and polarised light,depletion levels from molecular gas-phase abundance profiles can constrain theelusive GSD of the dust delivered to the ISM by AGB outflows.

Chemical modelling of dust-gas chemistry within AGB outflows -- II. Effect of the dust-grain size distribution