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.