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

[摘要] Micrometeorological measurements of size-segregated particle number fluxesabove Dutch heathlands and forests have repeatedly shown simultaneousapparent emission of particles with a diameter (D_p)<0.18 µm anddeposition of larger particles when measured with optical particle counters.In order to assess whether this observation may be explained by theequilibrium reaction of ammonia (NH₃), nitric acid (HNO₃) andammonium (NH₄⁺), a new numerical model is developed to predict thevertical concentration and flux profiles of the different species asmodified by the interaction of equilibration and surface/atmosphereexchange processes. In addition to former studies, the new approachexplicitly models the height-dependence of the NH₄⁺ and totalaerosol size-distribution. Using this model, it is demonstrated that bothgas-to-particle conversion (gtpc) and aerosol evaporation can significantlyalter the apparent surface exchange fluxes, and evoke the observedbi-directional particle fluxes under certain conditions. Thus, in general,the NH₃-HNO₃-NH₄NO₃ equilibrium needs to be consideredwhen interpreting eddy-covariance particle fluxes. Applied to an extensivedataset of simultaneous flux measurements of particles and gases at Elspeet,NL, the model reproduces the diurnal pattern of the bi-directional exchangewell. In agreement with the observation of fast NH₄⁺ deposition,slow nitric acid deposition (both as measured by the aerodynamic gradientmethod) and small concentration products of NH₃×HNO₃ atthis site, this study suggests that NH₄⁺ evaporation at this sitesignificantly alters surface exchange fluxes.

[学科分类] 大气科学