Using Multiple Chemical Criteria for Critical Loads of Acidity in Maritime Regions

Critical loads have become a well-establishedpart of the work programme of the UnitedNations Economic Commission for Europe (UNECE) Conventionon Long-Range Transboundary Air Pollution (CLRTAP). Thelinking of ecosystem response to deposition level is thecentral principal of the critical loads approach. Foreach ecosystem, a biological indicator is chosen, asuitable chemical criterion selected and a criticalchemical limit assigned. The Bc:Al ratio is the mostwidely used chemical criterion for setting criticalloads. However, critical loads based on this criterionare very sensitive to marine deposition. In regions whichreceive high depositions of marine-derived base cations,such as the Republic of Ireland (ROI) and the UnitedKingdom (UK), critical loads based on the Bc:Al criterionwill inevitably be high. Therefore, it is proposed thatcritical loads are estimated using multiple chemicalcriteria with appropriate critical limits to protect thechosen biological indicators. The range of publishedchemical criteria have been applied to the ROI and theUK. The chemical criterion corresponding to the mostsensitive critical load have been mapped and thecontribution of each to the final maps investigated. Thesimulations indicate that the most sensitive criteria forsetting critical loads are based on specifying critical Hor Al concentrations. However, the choice of critical limits andmodel parameters will ultimately effect the criticalloads. Therefore, it is important that appropriate criticallimits are chosen to protect the biological indicator andreceptor ecosystem from long-term damage.