International audience ; Size-dependent particle number fluxes measured by eddy-covariance (EC) and continuous fluxes of ammonium (NH 4 + ) measured with the aerodynamic gradient method (AGM) are reported for a Dutch heathland. Daytime deposition velocities ( V d ) by EC with peak values of 5 to 10 mm s -1 increased with particle diameter ( d p ) over the range 0.1?0.5 µm, and are faster than predicted by current models. With a mean V d of 2.0 mm s -1 (daytime: 2.7; night-time 0.8 mm s -1 ) NH 4 + fluxes by AGM are overall in agreement with former measurements and NH 4 + -N dry deposition amounts to 20% of the dry input of NH 3 -N over the measurement period. These surface exchange fluxes are analyzed together with simultaneous gas-phase flux measurements for indications of gas-particle interactions. On warm afternoons the apparent fluxes of acids and aerosol above the heathland showed several coinciding anomalies, all of which are consistent with NH 4 + evaporation during deposition: (i) canopy resistances for HNO 3 and HCl of up to 100 s m -1 , (ii) simultaneous particle emission of small particles ( D p <0.18 µm) and deposition of larger particles ( D p >0.18 µm), (iii) NH 4 + deposition faster than derived from size-distributions and size-segregated EC particle fluxes. These observations coincide with the observations of (i) surface concentration products of NH 3 and HNO 3 well below the thermodynamic equilibrium value and (ii) Damköhler numbers that indicate chemical conversion to be sufficiently fast to modify exchange fluxes. The measurements imply a removal rate of volatile NH 4 + of 3?30×10 -6 s -1 averaged over the 1 km boundary-layer, while NH 3 deposition is underestimated by typically 20 ng m -2 s -1 (28%) and flux reversal may occur.