Atmospheric Chemistry and Physics Review and parameterisation of bi-directional ammonia exchange

Abstract. Current deposition schemes used in atmospheric chemical transport models do not generally account for bi-directional exchange of ammonia (NH3). Bi-directional ex-change schemes, which have so far been applied at the plot scale, can be included in transport models, but need to be pa-rameterised with appropriate values of the ground layer com-pensation point (χg), stomatal compensation point (χ s) and cuticular resistance (Rw). We review existing measurements of χg, χ s as well as Rw and compile a comprehensive dataset from which we then propose generalised parameterisations. χ s is related to 0s, the non-dimensional ratio of [NH+4]apo and [H+]apo in the apoplast, through the temperature depen-dence of the combined Henry and dissociation equilibrium. The meta-analysis suggests that the nitrogen (N) input is the main driver of the apoplastic and bulk leaf concentrations o