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 (Vd) by EC with peak values of 5 to 10 mm s-1 increased with particle diameter (dp) over the range 0.1–0.5 µm, and are faster than predicted by current models. With a mean Vd 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 (Dp<0.18 µm) and deposition of larger particles (Dp>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.