title: Evidence for changing the critical level for ammonia.
abstract: 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.
authors.affiliation: Centre for Ecology & Hydrology, Edinburgh Research Station, Bush Estate, Penicuik, EH26 0QB, UK.
authors.forename: J N
authors.initials: JN
authors.lastname: Cape
authors.forename: L J
authors.initials: LJ
authors.lastname: van der Eerden
authors.forename: L J
authors.initials: LJ
authors.lastname: Sheppard
authors.forename: I D
authors.initials: ID
authors.lastname: Leith
authors.forename: M A
authors.initials: MA
authors.lastname: Sutton
mesh_terms: D000393:Air Pollutants / Q000633:toxicity*; D000641:Ammonia / Q000633:toxicity*; D044002:Bryophyta; D004463:Ecology / Q000379:methods / Q000592:standards; D017753:Ecosystem; D005060:Europe; D008011:Lichens; D008452:Maximum Allowable Concentration; D010944:Plants; D014197:Trees
chemical_list: D000393:Air Pollutants; D000641:Ammonia