Global and regional model simulations of atmospheric ammonia

Ammonia (NH₃) is a basic gas of significant atmospheric interest because of its role in the possible formation of fine particulates and because it is a source of fixed nitrogen in soils and plants. NH₃ processing in the atmosphere has been simulated using two 3-D models: the global chemistry transport model, STOCHEM-CRI and the regional coupled meteorological-chemical model, WRF-Chem-CRI. From analysis of STOCHEM-CRI simulations, NH₃ removal fluxes of dry deposition (24.6 Tg(N)/yr), wet deposition (20.8 Tg(N)/yr), NH₄ ⁺ formation (25.6 Tg(N)/yr) and reaction with OH (1.7 Tg(N)/yr) have been calculated, making a global annual average burden of 0.22 Tg(N) and life-time of 1.1 days. The gas-phase loss by OH, NO₃ and stabilized Criegee intermediates contribute 2.3%, <1% and < 1%, respectively to the total global loss of tropospheric NH₃. The highest concentrations of NH₃ are found to be in the region of South and East Asia, which are associated mostly with agricultural NH₃ emissions. Loss of surface NH₃ by reaction with OH increases by up to 25% along the equator because of the abundances of ozone. Comparison of satellite observations and model results give a better understanding of the temporal and spatial variations of atmospheric NH₃ on a global and regional scales. Using the anthropogenic seasonal NH₃ emission class in the model gives a poor representation of seasonal NH₃. The positive bias in Africa and South America for all seasons is likely due to undetermined sources in the model such as underestimated biomass burning emissions of NH₃ adopted in the model. The regional model results over North-West Europe during summer months are biased low compared with the measurements- suggesting either missing sources, or too efficient loss processes in the region.