Reassessment of Critical Levels for Atmospheric Ammonia

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 mu g 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 NH, 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 bryophytes) 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 bryophytes are important is 1 mu g NH3 m(-3). There is less evidence for long-term effects of NH; 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 mu g NH3 m(-3).