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.