Articles | Volume 10, issue 5
Atmos. Meas. Tech., 10, 1875–1892, 2017
Atmos. Meas. Tech., 10, 1875–1892, 2017

Research article 29 May 2017

Research article | 29 May 2017

Ammonia emissions from a grazed field estimated by miniDOAS measurements and inverse dispersion modelling

Michael Bell1, Chris Flechard1, Yannick Fauvel1, Christoph Häni2, Jörg Sintermann3,a, Markus Jocher3, Harald Menzi4, Arjan Hensen5, and Albrecht Neftel3,b Michael Bell et al.
  • 1INRA, Agrocampus Ouest, UMR 1069 SAS, Rennes, France
  • 2School of Agricultural, Forest and Food Sciences, Bern University of Applied Sciences, 3052 Zollikofen, Switzerland
  • 3Agroscope, Institute for Sustainability Science, Zürich, Switzerland
  • 4Federal Research Station Agroscope, Inst. For Livestock Sciences, 1725 Posieux, Switzerland
  • 5Energy research Centre of the Netherlands (ECN), Petten, the Netherlands
  • anow at: AWEL, Zürich, Switzerland
  • bnow at: Neftel Research Expertise, 3033 Wohlen b. Bern, Switzerland

Abstract. Ammonia (NH3) fluxes were estimated from a field being grazed by dairy cattle during spring by applying a backward Lagrangian stochastic model (bLS) model combined with horizontal concentration gradients measured across the field. Continuous concentration measurements at field boundaries were made by open-path miniDOAS (differential optical absorption spectroscopy) instruments while the cattle were present and for 6 subsequent days. The deposition of emitted NH3 to clean patches on the field was also simulated, allowing both net and gross emission estimates, where the dry deposition velocity (vd) was predicted by a canopy resistance (Rc) model developed from local NH3 flux and meteorological measurements. Estimated emissions peaked during grazing and decreased after the cattle had left the field, while control on emissions was observed from covariance with temperature, wind speed and humidity and wetness measurements made on the field, revealing a diurnal emission profile. Large concentration differences were observed between downwind receptors, due to spatially heterogeneous emission patterns. This was likely caused by uneven cattle distribution and a low grazing density, where hotspots of emissions would arise as the cattle grouped in certain areas, such as around the water trough. The spatial complexity was accounted for by separating the model source area into sub-sections and optimising individual source area coefficients to measured concentrations. The background concentration was the greatest source of uncertainty, and based on a sensitivity/uncertainty analysis the overall uncertainty associated with derived emission factors from this study is at least 30–40 %.

Emission factors can be expressed as 6 ± 2 g NH3 cow−1 day−1, or 9 ± 3 % of excreted urine-N emitted as NH3, when deposition is not simulated and 7 ± 2 g NH3 cow−1 day−1, or 10 ± 3 % of excreted urine-N emitted as NH3, when deposition is included in the gross emission model. The results suggest that around 14 ± 4 % of emitted NH3 was deposited to patches within the field that were not affected by urine or dung.

Short summary
This study applies horizontal concentration gradient measurements and inverse dispersion modelling to evaluate ammonia emissions from cattle grazing. The results can contribute to an emission factor for cattle grazing, where emissions where found to be towards the lower end of the range found in the limited number of existing studies. The influences of ammonia deposition, uneven urine patch distribution and climate conditions are discussed.