Evidence for changing the Critical Level for ammonia

The current Critical Level for ammonia (CLENH3) 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 CLENH3 are reviewed, and the available evidence collated, in proposing a new CLENH3 for different types of vegetation. For lichens and bryophytes, we propose a new CLENH3 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 CLENH3 of 3 ± 1 µg NH3 m-3 for herbaceous species. There is insufficient evidence to provide a separate CLENH3 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.