The national critical loads and exceedance data sets and maps were developed to provide a national-scale picture of the areas at risk from the adverse effects of acidification and eutrophication. The data and maps are largely derived from national-scale input data sets, appropriate for national-scale assessments; they were not intended for local-scale or site-specific assessments. Whilst the methods applied nationally could also be applied to individual sites, there is rarely sufficient site-specific data to enable this (Hall et al, 2007).
Research has been undertaken to try and identify alternative methods for carrying out site-specific assessments. A project funded by JNCC identified several options for assessing the risks from air pollution (in terms of critical loads and levels) to Sites of Special Scientific Interest (A/SSSIs) in the UK (Hall et al, 2006). Many of the options would require significant resources and/or expertise to apply, such as the collection of soil samples (and subsequent analysis) or identification of lichen species as biomonitors. Therefore the project recommended that the national data still be used as a first-level assessment to “screen” sites potentially at risk. To address this, “site-relevant critical loads” (SRCL) have been developed. For acidity the SRCL are based on national critical loads data and assigned by associating the designated feature habitats for each site to the broad habitat critical loads. For nutrient nitrogen the designated feature habitats have been related to the habitat classes for which empirical critical loads are available (Bobbink & Hettelingh, 2011) and appropriate critical load values applied (Hall et al., 2015). The SRCL for UK Special Areas of Conservation (SACs), Specially Protected Areas (SPAs) and A/SSSIs are available via the UK Air Pollution Information System.
In the next few years further developments in biomonitoring methods may enable field assessments of air pollution impacts to be made.
A non-spatial approach has been developed using Endorsement Theory to relate information on soil types and their associated acidity critical load values, with information on the soils and habitats of the National Vegetation Classification (NVC: Rodwell, 1991)(Wadsworth & Hall, 2007). This approach treats each critical loads class (ie, range of values) as a separate hypothesis and provides an “endorsement” for the likelihood of each class for any terrestrial NVC community. This method could be applied to designated sites (such as SACs) where the designated habitats are described in terms of NVC class(es). An analogous approach has also been developed for nutrient nitrogen using information on abiotic factors such as climate and management to determine where within the published ranges the most appropriate critical load for an NVC community lies (Hall & Wadsworth, 2010).