AMMONIA - EMISSION, ATMOSPHERIC TRANSPORT AND DEPOSITION

The global emission of ammonia (NH3) is about 54 Mt N. The major globa l sources are excreta from domestic animals and fertilizers, but ocean s, biomass burning and crops are also important. About 60 % of the glo bal NH3 emission is estimated to come from anthropogenic sources. NH3- N emissions are of the same order as the NOx-N emissions on both globa l and European scales. Emitted NH3 returns to the surface mainly in th e form of dry deposition of NH3 and wet deposition of ammonium (NH4+). In countries with high NH3 emission densities, dry deposition of NH3 from local sources and wet deposition of NH4+ from remote sources domi nate the deposition. In countries with low NH3 emission densities only wet deposition of NH4+ from remote sources dominates the deposition. Surface exchange of NH3 is essentially bi-directional, depending on th e NH3 compensation point concentration of the vegetation and the airbo rne concentration. In general, the compensation point is larger for ag ricultural than semi-natural plants, and varies with plant growth stag e. According to basic thermodynamics the leaf tissue or stomatal compe nsation point of NH3 doubles for each increase of 5 degrees C. However , exchange of NH3 does not only occur through the stomata, but it can also be deposited to leaf surfaces, as well as emitted back to the atm osphere from drying leaf surfaces. Atmospheric transport and depositio n models can be used to interpolate NH3 concentrations and depositions in space and time, to calculate import/export balances and to estimat e past or future situations. Adverse effects on sensitive ecosystems c aused by high N deposition can be reduced by lowering the emissions an d, to a limited extent, also by removing sources close to the ecosyste m to be protected.