Evidence for changing the critical level for ammonia

The current critical level for ammonia (CLE_NH3) in Europe is set at 8 μg NH₃ m⁻³ as an annual average concentration. Recent evidence has shown specific effects of ammonia (NH₃) 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 NH₃ 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 NH₃ 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 NH₃ m⁻³ is likely to exceed the empirical critical load for N deposition for most forest ecosystems.