Aerosol fluxes and particle growth above managed grassland

• Main Authors: E. Nemitz, J. R. Dorsey, M. J. Flynn, M. W. Gallagher, A. Hensen, J.-W. Erisman, S. M. Owen, U. Dämmgen, M. A. Sutton
• Format: Article
• Language: English
• Published: Copernicus Publications 2009-08-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/6/1627/2009/bg-6-1627-2009.pdf
• ISSN: 1726-4170; 1726-4189

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<sup>−1</sup> to long (0.75 m) and short (0.07 m) grass, respectively. After fertilisation with 108 kg N ha<sup>−1</sup> as calcium ammonium nitrate, sustained apparent upward fluxes of particles were observed. Analysis of concentrations and fluxes of potential precursor gases, including NH<sub>3</sub>, HNO<sub>3</sub>, HCl and selected VOCs, shows that condensation of HNO<sub>3</sub> and NH<sub>3</sub> 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<sup>−1</sup> and 1.68 nm hr<sup>−1</sup> were derived for day and night-time conditions, respectively. This implies total NH<sub>4</sub>NO<sub>3</sub> production rates of 1.11 and 0.44 μg m<sup>−3</sup> h<sup>−1</sup>, respectively. The effect translates into a small error in measured ammonia fluxes (0.06% day, 0.56% night) and a large error in NH<sub>4</sub><sup>+</sup> and NO<sub>3</sub><sup>−</sup> aerosol fluxes of 3.6% and 10%, respectively. By converting rapidly exchanged NH<sub>3</sub> and HNO<sub>3</sub> into slowly depositing NH<sub>4</sub>NO<sub>3</sub>, the reaction modifies the total N budget, though this effect is small (<1% for the 10 days following fertilization), as NH<sub>3</sub> emission dominates the net flux. It is estimated that 3.8% of the fertilizer N was volatilised as NH<sub>3</sub>, of which 0.05% re-condensed to form NH<sub>4</sub>NO<sub>3</sub> particles within the lowest 2 m of the surface layer. This surface induced process would at least scale up to a global NH<sub>4</sub>NO<sub>3</sub> formation of ca. 0.21 kt N yr<sup>−1</sup> from NH<sub>4</sub>NO<sub>3</sub> fertilisers and potentially 45 kt N yr<sup>−1</sup> from NH<sub>3</sub> emissions in general.