Advection of NH3 over a pasture field and its effect on gradient flux measurements

Other literature type, Article English OPEN
Loubet, B. ; Milford, C. ; Hensen, A. ; Daemmgen, U. ; Erisman, J.-W. ; Cellier, P. ; Sutton, M. A. (2009)
  • Journal: (issn: 1726-4189, eissn: 1726-4189)
  • Related identifiers: doi: 10.5194/bg-6-1295-2009
  • Subject: EXPERIMENTATION GRAMINAE | gaz trace | advection | micrométéorologie | fertilisation azotée | ammoniac | expérimentation au champ | dépôt | couvert végétal | prairie | modèle | allemagne | émission | méthode de mesure | pollution atmosphérique | flux | mesure physique | dispersion | pâturage | europe

Deposition of atmospheric ammonia (NH<sub>3</sub>) to semi-natural ecosystems leads to serious adverse effects, such as acidification and eutrophication. A step in quantifying such effects is the measurement of NH<sub>3</sub> fluxes over semi-natural and agricultural land. However, measurement of NH<sub>3</sub> fluxes over vegetation in the vicinity of strong NH<sub>3</sub> sources is challenging, since NH<sub>3</sub> emissions are highly heterogeneous. Indeed, under such conditions, local advection errors may alter the measured fluxes. In this study, local advection errors (Δ<i>F</i><sub>z,adv</sub>) 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 Δ<i>F</i><sub>z,adv</sub> was determined up to 810 m downwind from farm buildings emitting between 6.2 and 9.9 kg NH<sub>3</sub> day<sup>&minus;1</sup>. The GRAMINAE experiment provided a unique opportunity to compare two methods of estimating Δ<i>F</i><sub>z,adv</sub>: one inference method based on measurements of horizontal concentration gradients, and one based on inverse dispersion modelling with a two-dimensional model.<br> <br> Two sources of local advection were clearly identified: the farm NH<sub>3</sub> emissions leading to positive Δ<i>F</i><sub>z,adv</sub> ("bias towards emissions") and field NH<sub>3</sub> emissions, which led to a negative Δ<i>F</i><sub>z,adv</sub> ("bias towards deposition"). The local advection flux from the farm was in the range 0 to 27 ng NH<sub>3</sub> m<sup>&minus;2</sup> s<sup>&minus;1</sup> at 610 m from the farm, whereas Δ<i>F</i><sub>z,adv</sub> due to field emission was proportional to the local flux, and ranged between &minus;209 and 13 ng NH<sub>3</sub> m<sup>&minus;2</sup> s<sup>&minus;1</sup>. The local advection flux Δ<i>F</i><sub>z,adv</sub> 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.
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