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

E. Nemitz (1) , M. A. Sutton (1) , G. P. Wyers (2) , R. P. Otjes (2) , M. G. Mennen (3) , E. M. van Putten (3) , M. W. Gallagher (4)

1 CEH - Centre for Ecology and Hydrology [Edinburgh]
2 ECN - Energy Research Centre of the Netherlands
3 RIVM - National Institute for Public Health and the Environment [Bilthoven]
4 UMIST - University of Manchester Institute of Science and Technology

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.

Domaines: Océan, Atmosphère

E. Nemitz, M. A. Sutton, G. P. Wyers, R. P. Otjes, M. G. Mennen, et al.. Gas-particle interactions above a Dutch heathland: II. Concentrations and surface exchange fluxes of atmospheric particles. Atmospheric Chemistry and Physics, 2004, 4 (4), pp.1007-1024. ⟨hal-00295459⟩