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Abstract. Size-dependent particle number fluxes measured by eddy-covariance (EC) and continuous fluxes of ammonium (NH -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.

The results of recent controlled environment and micrometeorological measurements of NH 3 fluxes are presented to highlight the processes controlling NH 3 plant-atmosphere exchange. The presence of NH +4 in leaf tissues results in the existence of an NH 3 ‘compensation point’ concentration for substomatal tissues (x s ), so that both emission and deposition are possible from stomata. In addition, NH 3 may deposit efficiently on to leaf cuticles, short-circuiting any stomatal emission, so that a ‘canopy compensation point’ (X c ) may be defined that is smaller than X s . Ammonia is generally deposited to nitrogen limited ecosystems, indicating a small X s and small leaf cuticle resistance ( R w ). In contrast, fluxes over croplands are typically bidirectional and may reflect a larger X s as a consequence of greater N supply. The paper discusses the processes defining (humidity, acidic pollutants) and X s (plant phenology, species, N nutrition) and proposes a new resistance approach, which integrates X s and R w into one model. Estimating long term bidirectional NH 3 fluxes is still uncertain, though it is now possible to apply a single model concept to a range of ecosystem types and satisfactorily infer NH 3 fluxes over diurnal time scales.

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