Advection of NH3 over a pasture field and its effect on gradient flux measurements 1Institut National de la Recherche Agronomique (INRA), UMR Environnement et Grandes Cultures, 78850 Thiverval-Grignon, France
2Centre for Ecology and Hydrology (Edinburgh Research Station), Bush Estate, Penicuik, Midlothian, EH26 0QB, UK
3Energy research Centre of the Netherlands (ECN), Postbus 1, 1755 ZG Petten, The Netherlands
4Inst. für Agrarökologie, Bundesforschungsanstalt für Landwirtschaft (FAL), Bundesallee 50, 38116 Braunschweig, Germany
*now at: Institute of Earth Sciences "Jaume Almera", CSIC, Lluis Solé i Sabarís, 08028 Barcelona, Spain
Received: 07 Oct 2008 – Published in Biogeosciences Discuss.: 06 Jan 2009 – Published: 28 Jul 2009Abstract. Deposition of atmospheric ammonia (NH3) to semi-natural ecosystems
leads to serious adverse effects, such as acidification and eutrophication.
A step in quantifying such effects is the measurement of NH3 fluxes
over semi-natural and agricultural land. However, measurement of NH3
fluxes over vegetation in the vicinity of strong NH3 sources is
challenging, since NH3 emissions are highly heterogeneous. Indeed,
under such conditions, local advection errors may alter the measured fluxes.
In this study, local advection errors (ΔFz,adv) were estimated
over a 14 ha grassland field, which was successively cut and fertilised, as
part of the GRAMINAE integrated Braunschweig experiment. The magnitude of
ΔFz,adv was determined up to 810 m downwind from farm buildings
emitting between 6.2 and 9.9 kg NH3 day−1. The GRAMINAE
experiment provided a unique opportunity to compare two methods of
estimating ΔFz,adv: one inference method based on measurements
of horizontal concentration gradients, and one based on inverse dispersion
modelling with a two-dimensional model.
Two sources of local advection were clearly identified: the farm NH3
emissions leading to positive ΔFz,adv ("bias towards
emissions") and field NH3 emissions, which led to a negative ΔFz,adv ("bias towards deposition").
The local advection flux from the
farm was in the range 0 to 27 ng NH3 m−2 s−1 at 610 m
from the farm, whereas ΔFz,adv due to field emission was
proportional to the local flux, and ranged between
−209 and 13 ng NH3 m−2 s−1. The local advection flux
ΔFz,adv was either positive or negative depending on the
magnitude of these two contributions. The modelled and inferred advection
errors agreed well. The inferred advection errors, relative to the vertical
flux at 1 m height, were 52% on average, before the field was cut, and
less than 2.1% when the field was fertilised. The variability of the
advection errors in response to changes in micrometeorological conditions is
also studied. The limits of the 2-D modelling approach are discussed.
Citation: Loubet, B., Milford, C., Hensen, A., Daemmgen, U., Erisman, J.-W., Cellier, P., and Sutton, M. A.: Advection of NH3 over a pasture field and its effect on gradient flux measurements, Biogeosciences, 6, 1295-1309, doi:10.5194/bg-6-1295-2009, 2009.