Particle deposition velocities (11–3000 nm diameter) measured above grassland by eddy covariance during the EU GRAMINAE experiment in June 2000 averaged 0.24 and 0.03 mm s^−1 to long (0.75 m) and short (0.07 m) grass, respectively. After fertilisation with 108 kg N ha^−1 as calcium ammonium nitrate, sustained apparent upward fluxes of particles were observed. Analysis of concentrations and fluxes of potential precursor gases, including NH₃, HNO₃, HCl and selected VOCs, shows that condensation of HNO₃ and NH₃ on the surface of existing particles is responsible for this effect. A novel approach is developed to derive particle growth rates at the field scale, from a combination of measurements of vertical fluxes and particle size-distributions. For the first 9 days after fertilization, growth rates of 11 nm particles of 7.04 nm hr^−1 and 1.68 nm hr^−1 were derived for day and night-time conditions, respectively. This implies total NH₄NO₃ production rates of 1.11 and 0.44 μg m^−3 h^−1, respectively. The effect translates into a small error in measured ammonia fluxes (0.06% day, 0.56% night) and a large error in NH₄⁺ and NO₃^− aerosol fluxes of 3.6% and 10%, respectively. By converting rapidly exchanged NH₃ and HNO₃ into slowly depositing NH₄NO₃, the reaction modifies the total N budget, though this effect is small (<1% for the 10 days following fertilization), as NH₃ emission dominates the net flux. It is estimated that 3.8% of the fertilizer N was volatilised as NH₃, of which 0.05% re-condensed to form NH₄NO₃ particles within the lowest 2 m of the surface layer. This surface induced process would at least scale up to a global NH₄NO₃ formation of ca. 0.21 kt N yr^−1 from NH₄NO₃ fertilisers and potentially 45 kt N yr^−1 from NH₃ emissions in general.