Units: µg m-3
Criteria used to set critical level: : Adverse ecophysiological effects on vegetation.
Resolution: By Habitat
Established by: Working Group on Effects of the UNECE Convention on Long Range Transboundary Air Pollution.
Additional Info:
The critical Level for NOX is based on the sum of the NO and NO2 concentrations (there is insufficient knowledge to establish separate critical levels for the two pollutants). Since the proportions of NO and NO2 contributing to NOX is often unknown, the critical level is expressed in terms of equivalent NO2 concentrations.
Since the type of response varies from a fertiliser effect to toxicity, depending on concentration, all effects were considered to be adverse. However, increased growth of certain species was of greatest concern for natural and semi-natural vegetation due to the likelihood of changes in competition between species (UNECE CLRTAP, 1996). Furthermore, NOX is not sufficiently toxic alone, and the critical level for NOX should only be applied where levels of SO2 and O3 are close to their critical levels.
References:
UN ECE Convention on Long-range Transboundary Air Pollution (1996): Manual on methodologies and criteria for mapping critical levels/loads and geographical areas where they are exceeded. Federal Environmental Agency (Umweltbundesamt) Berlin, Germany.
NEGTAP (National Expert Group on Transboundary Air Pollution) 2001: Transboundary Air Pollution: Acidification, Eutrophication and Ground-Level Ozone in the UK. ISBN 1 870393 61 9. Available online.