Ammonia: emission, atmospheric transport and deposition

Abstract

The global emission of ammonia (NH₃) is about 54 Mt N. The major global sources are excreta from domestic animals and fertilizers, but oceans, biomass burning and crops are also important. About 60% of the global NH₃ emission is estimated to come from anthropogenic sources. NH₃-N emissions are of the same order as the NO_x-N emissions on both global and European scales. Emitted NH₃ returns to the surface mainly in the 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 dominate 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 the NH₃ compensation point concentration of the vegetation and the airborne concentration. In general, the compensation point is larger for agricultural than semi-natural plants, and varies with plant growth stage. According to basic thermodynamics the leaf tissue or stomatal compensation point of NH₃ doubles for each increase of 5°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 atmosphere from drying leaf surfaces. Atmospheric transport and deposition models can be used to interpolate NH₃ concentrations and depositions in space and time, to calculate import/export balances and to estimate past or future situations. Adverse effects on sensitive ecosystems caused by high N deposition can be reduced by lowering the emissions and, to a limited extent, also by removing sources close to the ecosystem to be protected.