Reassessment of critical levels for atmospheric ammonia

Abstract

The existing Critical Levels (CLEs) for NH3 require revision in the light of new experimental evidence from field-based experiments and surveys. The existing annual CLE (8 mg NH3 m-3), when expressed as an equivalent deposition of N to an ecosystem, is less protective than the current Critical Load for most, if not all, European ecosystems and habitats. Field-based evidence relating effects on vegetation to NH3 concentrations measured over one year or longer show that the current annual CLE is set too high. A new long-term CLE for the most sensitive vegetation type (lichens and bryo-phytes) is proposed, based on observed changes to species composition in the field. Most of the evidence comes from studies in the UK, but there is corroborative evidence from Switzerland, Italy and Portugal. The proposed long-term CLE for NH3 for ecosystems in which lichens and bryo-phytes are important is 1 mg NH3 m-3. There is less evidence for long-term effects of NH3 on species changes in communities of higher plants; on the basis of expert judgement we propose a long-term CLE for higher plants of 3 mg NH3 m-3. No assumptions have been made on the mechanism by which NH3 exposure leads to changes in species composition. Several recommendations are made, to address uncertainties relating to the lack of observational data and long-term NH3 concentration measurement, particularly in southern and eastern Europe. There is also need for better understanding of the mechanisms whereby NH3 affects plants, so that predictive models can be constructed for extrapolation to other types of vegetation and land use in different climatic zones. © 2009 Springer Netherlands.