Critical Levels for Ammonia

The issues involved in assessing and evaluating Critical Levels for atmospheric NH3 and vegetation are discussed, recognizing that measurable effects of exposure to NH3 may be observed in plant species without necessarily any adverse effects on that species. Consequently, impacts of NH3 are best measured in terms of community response, where there is it measurable change in species composition and/or loss of individual species. Such data are currently available only from field measurements, either from measurements close to point sources, where NH3 concentrations have also been measured, or from controlled field fumingation experiments. The relationship between Critical Levels and Critical Loads is explored, and the redundancy of the 1993 Critical Level (8 mu m(-3) as an annual average), when compared with the Critical Loads for N deposition, is demonstrated. Statistical techniques for estimating 'no effect' concentrations from field measurements are presented, and then used to review the available data. For sensitive vegetation types (lichens and bryophytes) field evidence shows changes in species composition at NH3 concentrations (as an annual average) of 1 mu m m(-3), which is proposed as the new Critical Level for annual average NH3 concentrations for sensitive ecosystems (i.e. those in which lichens and bryophytes are important components). There are fewer data for higher plants, but a Critical Level of 3 mu g m(-3) is proposed, as it long-term average NH3 concentration. The existing Critical Level of 23 mu g m(-3) for the monthly average concentration is retained, given the recent changes in seasonal patterns of NH3 emissions from agricultural practices such as slurry spreading.