Gas-particle interactions above a Dutch heathland: III. Modelling the influence of the NH₃-HNO₃-NH₄NO₃ equilibrium on size-segregated particle fluxes

Abstract. Micrometeorological measurements of size-segregated particle number fluxes above Dutch heathlands and forests have repeatedly shown simultaneous apparent emission of particles with a diameter (D_p)<0.18 µm and deposition of larger particles when measured with optical particle counters. In order to assess whether this observation may be explained by the equilibrium reaction of ammonia (NH₃), nitric acid (HNO₃) and ammonium (NH₄⁺), a new numerical model is developed to predict the vertical concentration and flux profiles of the different species as modified by the interaction of equilibration and surface/atmosphere exchange processes. In addition to former studies, the new approach explicitly models the height-dependence of the NH₄⁺ and total aerosol size-distribution. Using this model, it is demonstrated that both gas-to-particle conversion (gtpc) and aerosol evaporation can significantly alter the apparent surface exchange fluxes, and evoke the observed bi-directional particle fluxes under certain conditions. Thus, in general, the NH₃-HNO₃-NH₄NO₃ equilibrium needs to be considered when interpreting eddy-covariance particle fluxes. Applied to an extensive dataset of simultaneous flux measurements of particles and gases at Elspeet, NL, the model reproduces the diurnal pattern of the bi-directional exchange well. In agreement with the observation of fast NH₄⁺ deposition, slow nitric acid deposition (both as measured by the aerodynamic gradient method) and small concentration products of NH₃×HNO₃ at this site, this study suggests that NH₄⁺ evaporation at this site significantly alters surface exchange fluxes.