Units: µg m-3
Criteria used to set critical level: Direct effects including leaf discoloration, loss and growth reduction.
Resolution: By Habitat
Established by: Working Group on Effects of the UNECE Convention on Long Range Transboundary Air Pollution.
Additional Info:
Critical levels for NH3 are assigned for all vegetation types, and have been estimated according to the time period of exposure.
Annual, long-term, critical levels have been set to 1 µg NH3 m-3 for lichens and bryophytes, and 3 µg NH3 m-3 for higher plants (with an uncertainty of 2-4 µg m-3). Ecosystems where sensitive lichens and bryophytes are an important part of the ecosystem integrity are set at 1 µg NH3 -3. Critical levels have also been set for hourly, daily, monthly and annual periods, and these are: hourly: 3300 µg m-3, daily: 270 µg m-3 , monthly: 23 µg m-3.
The ammonia concentrations across the UK provided in APIS are annual estimates, and therefore the annual critical level is applied.
References:
UN ECE Convention on Long-range Transboundary Air Pollution (2007): Report on the Workshop on Atmospheric Ammonia: Detecting Emission Changes and Environmental Impacts
Ashmore, M.R. and Wilson R.B. (Eds.) (1994): Critical levels of air pollutants for Europe. Report of the Egham workshop. Air Quality Division, Department of the Environment, London.