Evidence for changing the critical level for ammonia.

The current critical level for ammonia (CLE(NH3)) in Europe is set at 8mug 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 microg 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 microg 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 microg NH(3) m(-3) is likely to exceed the empirical critical load for N deposition for most forest ecosystems.