Ammonia deposition near hot spots: processes, models and monitoring methods

Atmospheric reduced nitrogen (NHx) mainly originates from hot spots, which can be considered as intensive area or point sources. A large fraction of the emitted NHx may be recaptured by the surrounding vegetation, hence reducing the contribution of these hot spots to long-range transport of NHx. This paper reviews the processes leading to local recapture of NHx near hot spots as well as existing models and monitoring methods. The existing models range from research models to more operational models that can be coupled with long-range transport model provided the necessary information on emissions is available. Local recapture of NH3 ranges from 2% to 60% within 2 km of a hot-spot and it is sensitive to source height, atmospheric stability, wind speed, structure of the surrounding canopies, as well as stomatal absorption, which mainly depends on green leaf area index and stomatal NH3 compensation point of vegetation, and finally, cuticular deposition, which depends primarily on vegetation wetness. The main uncertainties and limitations on NHx recapture models and monitoring techniques are discussed.