Articles | Volume 6, issue 8
Biogeosciences, 6, 1627–1645, 2009

Special issue: Processes controlling the exchange of ammonia between grassland...

Biogeosciences, 6, 1627–1645, 2009

  12 Aug 2009

12 Aug 2009

Aerosol fluxes and particle growth above managed grassland

E. Nemitz1, J. R. Dorsey2, M. J. Flynn2, M. W. Gallagher2, A. Hensen3, J.-W. Erisman3, S. M. Owen1,4, U. Dämmgen5, and M. A. Sutton1 E. Nemitz et al.
  • 1Centre for Ecology and Hydrology (Edinburgh), Bush Estate, Penicuik, Midlothian, EH26 0QB, UK
  • 2School for Earth, Atmospheric and Environmental Sciences, Univ. of Manchester, PO Box 88, Manchester, M60 1QD, UK
  • 3Energy Research Centre for the Netherlands (ECN), 1755-ZG Petten, The Netherlands
  • 4Institute for Environmental and Natural Sciences, University of Lancaster, UK
  • 5Institute for Agroecology, Federal Agricultural Research Centre, Braunschweig, Germany

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.

Final-revised paper