Air pollution could have a negative impact on designated sites and hence needs to be investigated by the Conservation Agencies. The concept of a “critical load” and of a “critical level” has been developing since the late 1980’s for use in national and international assessments. The two approaches are very similar and adopt a strict precautionary principle; the critical load is the amount of pollutant that can be deposited without causing harmful effects to sensitive elements (eg, soils, waters, vegetation) of the environment according to present knowledge and the critical level is the concentration of a pollutant in the atmosphere below which there are no known harmful effects.  All the major airborne pollutants have been considered but only three groups; oxides of sulphur, nitrogen compounds and ozone are well enough studied to be suitable for routine and widespread analysis.

 

Any consideration of critical loads and levels must acknowledge that there are considerable uncertainties in the source and magnitude of pollutant emissions, in the location and sensitivity of receptors and in the physiological and biogeochemical processes involved.  These issues are discussed throughout the report with some suggestions as to how to minimise their effects on any conclusions drawn.  Assessment of designated sites can be restricted by; lack of available data, errors and uncertainties in the available data and lack of scientific knowledge (especially field experiments) on the sensitivity of designated features (requiring expert judgement to be used).

 

We have considered a number of options for assessing the risks of airborne pollutants to terrestrial designated sites. Some methods would impose very considerable resource commitments from the agencies. Because air pollution is only one of many potential negative impacts on designated sites and because of realistic resource constraints we recommend that a simple hierarchical or staged approach is adopted. Stage 1 is a centralised automated screening of all sites making use of national data and internationally agreed protocols. In many cases the pollution levels will be either so high or so low as to be able to clearly assess the risk to the sites (in the framework of the CSM). Some sites will be receiving pollutants at a rate where it is unclear if the critical load or level is being exceeded; in those cases we recommend that Stage 2 be performed. In Stage 2 the analysis is repeated using whatever site specific data is available. In many cases the most important single piece of site specific information will be on the soil series relevant the designated feature. Finally we recommend that surveyors visiting the sites as part of the CSM be additionally tasked to record the presence of potential sources of pollutant not likely to be included in national data sets (eg new intensive livestock facilities close to the designated sites) and to record the general condition of the vegetation (to provide qualitative confirmation of the predictions in Stage 1 & Stage 2).