Potential for the Further Development and Application of Critical Levels to Assess the Environmental Impacts of Ammonia

This paper provides the background that originally argued the case to review the ammonia critical level. These arguments were originally developed in 2003, although the case for reviewing the ammonia critical level was already clearly shown by Burkhardt et al. (1998). They demonstrated that the UNECE ammonia critical level established at Egham in 1992 was much less precautionary than parallel values of critical loads for nitrogen deposition. A differing ‘protection timescale’ for critical loads and critical levels could be seen partly to explain this. Simple extrapolation of the critical level relationship with averaging time showed that a long-term ammonia critical level (20–30 years) would be of the order of 2.5 μg m⁻³, a factor of 3 smaller than the Egham annual critical level. Similarly, new evidence from studies of epiphytic lichen communities was emerging, suggesting that the long-term critical level for ammonia could be in the range 0.6–3 μg m⁻³.

In addition to these arguments, there was a practical case emerging, which highlighted the need to set realistic values of the critical level. The dependence on critical loads requires estimates of nitrogen deposition, which are typically uncertain and difficult to measure in a local or regulatory context. By comparison, it is much easier to measure ammonia concentrations. Therefore, the ammonia critical level has an important role to play as a practical indicator threshold, for example, that can be used to assess local air pollution impacts (e.g. “Natura 2000”) and to inform the setting of air quality targets for effects of ammonia on ecosystems.