Evidence for changing the critical level for ammonia

Abstract

The current critical level for ammonia (CLE NH3) in Europe is set at 8 μg NH 3 m −3 as an annual average concentration. Recent evidence has shown specific effects of ammonia (NH 3) on plant community composition (a true ecological effect) at much smaller concentrations. The methods used in setting a CLE NH3 are reviewed, and the available evidence collated, in proposing a new CLE NH3 for different types of vegetation. For lichens and bryophytes, we propose a new CLE NH3 of 1 μg NH 3 m −3 as a long-term (several year) average concentration; for higher plants, there is less evidence, but we propose a CLE NH3 of 3 ± 1 μg NH 3 m −3 for herbaceous species. There is insufficient evidence to provide a separate CLE NH3 for forest trees, but the value of 3 ± 1 μg NH 3 m −3 is likely to exceed the empirical critical load for N deposition for most forest ecosystems.