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

Other literature type English OPEN
Nemitz, E. ; Sutton, M. A. (2004)

Micrometeorological measurements of size-segregated particle number fluxes above Dutch heathlands and forests have repeatedly shown simultaneous apparent emission of particles with a diameter (<i>D<sub>p</sub></i>)&lt;0.18 &micro;m 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 (NH<sub>3</sub>), nitric acid (HNO<sub>3</sub>) and ammonium (NH<sub>4</sub><sup>+</sup>), 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 NH<sub>4</sub><sup>+</sup> 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 NH<sub>3</sub>-HNO<sub>3</sub>-NH<sub>4</sub>NO<sub>3</sub> 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 NH<sub>4</sub><sup>+</sup> deposition, slow nitric acid deposition (both as measured by the aerodynamic gradient method) and small concentration products of NH<sub>3</sub>&times;HNO<sub>3</sub> at this site, this study suggests that NH<sub>4</sub><sup>+</sup> evaporation at this site significantly alters surface exchange fluxes.
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