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Modélisation du dépôt sec d'ammoniac atmosphérique à proximité des sources

Abstract : Atmospheric ammonia (NH3) originates from agriculture. As a very soluble gas, NH3 is readily deposited to vegetation, through leaf uptake like any other trace-gas, and by adsorption onto plant surfaces, called cuticular uptake. NH3 deposition is now recognised as a major source of pollution to low-nitrogen, sensitive ecosystems. In order to study the variability of short-range dry deposition, in relation to microclimatic conditions and ecosystem types, a mechanistic two-dimensional dispersion and deposition model has been developed. It is based on the coupling of a random walk dispersion sub-model for trace-gases, with a leaf-scale NH3 exchange sub-model. The latest sub-model simulates stomatal absorption, with use of the concept of NH3 stomatal compensation point, and also accounts for cuticular uptake. The model is validated against experimental measurements of NH3 concentration over two contrasting vegetation types; maize and grassland. A specific experiment has been set up to obtain data over maize and is detailed in this thesis. The model shows a good agreement with measurements over grassland, but is biased when compared to data over maize. This bias seems to be related to the wind direction not being in the axis of the measurement masts, and highlights the need for a three-dimensional model when dealing with short-range dispersion. Case-studies shows that cumulated dry deposition within 400 m of a line source can vary between a few tens of percent to up to 20% of the quantity emitted. A sensitivity analysis shows that the main factors influencing local deposition are: (i) the difference between the source height and the canopy height, (ii) the wind speed, and (iii) the thermal stratification of the atmosphere, as well as (iv) the stomatal and cuticular resistance. Further simulations taking into account the links between climatic conditions and canopy growth, shows that under dry and warm conditions, ammonia is mostly deposited through stomatal exchange, whereas under temperate and wet conditions, NH3 is mainly deposited onto the cuticle. A better understanding of the undergoing processes in the cuticular uptake and stomatal compensation point variability, would need to be achieved, in order to decrease the uncertainty in the estimations of NH3 short-range dry deposition.
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https://tel.archives-ouvertes.fr/tel-00003250
Contributor : Benjamin Loubet <>
Submitted on : Thursday, August 7, 2003 - 2:44:19 PM
Last modification on : Friday, April 9, 2021 - 2:02:03 PM
Long-term archiving on: : Wednesday, September 12, 2012 - 10:15:10 AM

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  • HAL Id : tel-00003250, version 1

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Benjamin Loubet. Modélisation du dépôt sec d'ammoniac atmosphérique à proximité des sources. Physique [physics]. Université Paul Sabatier - Toulouse III, 2000. Français. ⟨tel-00003250⟩

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