Ammonia is a key species in the atmosphere playing a crucial role in forming secondary aerosols and, thus, modifying the chemistry of the atmosphere. Mainly from agricultural practices such as livestock excreta and mineral fertilizer use, which vary worldwide, ammonia emission quantification is not straightforward. In addition, the volatilization process depends on environmental variables (temperature, humidity, wind speed) that make their estimation particularly complex. Most global Chemistry Transport Models rely on artificial-seasonal variation inventories, which can lead to uncertainties in the representation of ammonia and its related aerosols. My work takes part in this effort of better estimating ammonia emissions from agricultural practices at the global scale and study their impact on atmospheric chemistry.
Firstly, as a unique approach, I suggest to include a new module (CAMEO; Calculation of AMmonia Emissions in ORCHIDEE) dedicated to agricultural ammonia flow within the global Land Surface Model ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems). More specifically, a representation of livestock feeding, manure management chain, application of fertilizer (mineral and organic) along with soil physical processes for volatilization are developed and parameterized into ORCHIDEE. Global agricultural ammonia emissions from CAMEO reach 44 TgN/yr and the evaluation shows a highly correlated seasonal cycle with IASI derived-emissions (Infrared Atmospheric Sounding Interferometer). It results from the response of ammonia emissions to environmental changes (from land use and meteorology) in CAMEO.
The second step of my work consists in exploiting CAMEO for future Shared Socio-Economic Scenarios (SSPs; designed within the CMIP6 framework). However, livestock densities, a key input data for CAMEO are not available for the future in the literature. Therefore, an original downscaling method is proposed to retrieve future livestock densities at the grid-cell scale from 2015 to 2100 for three different SSPs.
To do so, Integrated Assessment Model (IAM) results for the distribution of grassland and regional livestock production trends are combined. CAMEO demonstrated a possible range of agricultural ammonia emissions situated between 50 to 70 TgN/yr in 2100, highlighting the future intensive use of fertilizer (synthetic and organic). Depending on the scenario and region, increased livestock numbers can have a substantial role, especially in Africa. The sensitivity of the future emissions to climate has been tested and estimated at 15 to 20 %. Finally, the present-day and future ammonia emissions from CAMEO have been prescribed to the global Chemistry Transport Model LMDZ-INCA to assess their impact on the atmospheric composition. Present-day simulated ammonia columns were evaluated against the IASI satellite observations and showed a significant improvement in the spatial and temporal variability compared to a reference simulation. Future CAMEO emissions for a specific SSP (highest emission scenario in 2100) were also tested under different sulfate and nitrate emission conditions (relatively high, low, and present-day levels). Independently from the other emitted species levels in the future, the impact of CAMEO emissions on the nitrate burden is crucial and positive (+50 %). However, among the scenarios, regional patterns are also observed in the surface aerosol formation and the subsequent N deposition fluxes. Knowing the bi-directional property of ammonia at the interface between the atmosphere and biosphere, I highlighted the importance of the coupling between the two compartments around the nitrogen cycle within the IPSL Earth System Model.