Establishing the link between ammonia emission control and measurements of reduced nitrogen concentrations and deposition

In the context of international efforts to reduce the impactsof atmospheric NH₃ and NH₄ ⁺ (collectively, NH_x), it is important to establish the link between NH₃emissions and monitoring of NH_x concentrations and deposition. This is equally relevant to situations where NH₃emissions changes are certain (e.g. due to changed source sectoractivity), as to cases where NH₃ abatement technologies havebeen implemented. Correct interpretation of adequate atmosphericmeasurements is essential, since monitoring data provide the onlymeans to evaluate trends in regional NH₃ emissions.These issues have been reviewed using available measurements and modelling from nine countries. In addition to historic datasets,the analysis here considers countries where NH₃ source sector activity changed (both increases and decreases) and countries where NH₃ abatement policies have been implemented.In The Netherlands an `ammonia gap' was identified between the expected reduction and results of monitoring, and was attributedinitially to ineffectiveness of the abatement measures. The analysis here for a range of countries shows that atmospheric interactions complicate the expected changes, particularly sinceSO₂ emissions have decreased at the same time, while at manysites the few years of available data show substantial inter-annual variation. It is concluded that networks need to beestablished that speciate between NH₃ and aerosol NH₄ ⁺, in addition to providing wet deposition, and sample at sufficient sites for robust regional estimates to be established. Such measurements will be essential to monitor compliance of the international agreements on NH₃ emission abatement.