Steady state critical loads of acidity for sulphur and nitrogen: a multi-receptor, multi-criterion approach.

The critical load approach to emission reductions has become an important element of the United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Transboundary Air Pollution (CLRTAP). The steady state mass balance approach (SSMB) is the most widely used method in Europe for estimating critical loads, typically applied to forest ecosystems on mineral soils in conjunction with a base cation to aluminium (Bc:Al) ratio using a default critical limit of 1.0. The 'typical' approach has been expanded in this paper by: inclusion of a broader range of natural and semi-natural receptor ecosystems; inclusion of multiple chemical criterion for each receptor; and an attempt to include organic soils (peatlands). Critical loads of acidity were estimated for the Republic of Ireland using four receptor ecosystems (coniferous forest, deciduous forest, natural grasslands and moors and heathlands) and seven chemical criteria. The dominant chemical criteria, in the determination of critical loads, were based on a critical pH limit for mineral soils or a critical pH shift in relation to pristine conditions for organic soils. Approximately 68 and 26% of the final distribution of critical loads were estimated using these criteria, respectively. The 5th-percentile critical loads of acidity were more sensitive than those previously estimated for Ireland due to the inclusion of organic soils. Furthermore, coniferous ecosystems had the lowest critical loads due to the high percentage occurring on organic soils, and the removal of base cations through harvesting. The results demonstrate that it may be more appropriate to use multiple criteria and receptors to ensure adequate protection of biological indicators. However, it is important that appropriate critical limits are chosen to protect the biological indicators and receptor ecosystems from long-term damage.

Modelling the spatial distribution of SO2 and NOx emissions in Ireland.

The spatial distributions of sulphur dioxide (SO2) and nitrogen oxides (NOx) emissions are essential inputs to models of atmospheric transport and deposition. Information of this type is required for international negotiations on emission reduction through the critical load approach. High-resolution emission maps for the Republic of Ireland have been created using emission totals and a geographical information system, supported by surrogate statistics and landcover information. Data have been subsequently allocated to the EMEP 50 x 50-km grid, used in long-range transport models for the investigation of transboundary air pollution. Approximately two-thirds of SO2 emissions in Ireland emanate from two grid-squares. Over 50% of total SO2 emissions originate from one grid-square in the west of Ireland, where the largest point sources of SO2 are located. Approximately 15% of the total SO2 emissions originate from the grid-square containing Dublin. SO2 emission densities for the remaining areas are very low, < 1 t km-2 year-1 for most grid-squares. NOx emissions show a very similar distribution pattern. However, NOx emissions are more evenly spread over the country, as about 40% of total NOx emissions originate from road transport.