Aerosol fluxes and particle growth above managed grassland

Abstract. 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.