Gas-particle interactions above a Dutch heathland: II. Concentrations and surface exchange fluxes of atmospheric particles

Abstract. Size-dependent particle number fluxes measured by eddy-covariance (EC) and continuous fluxes of ammonium (NH₄⁺) 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) NH₄⁺ fluxes by AGM are overall in agreement with former measurements and NH₄⁺-N dry deposition amounts to 20% of the dry input of NH₃-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 NH₄⁺ evaporation during deposition: (i) canopy resistances for HNO₃ 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) NH₄⁺ 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 NH₃ and HNO₃ 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 NH₄⁺ of 3−30×10^-6 s^-1 averaged over the 1 km boundary-layer, while NH₃ deposition is underestimated by typically 20 ng m^-2 s^-1 (28%) and flux reversal may occur.