Critical loads and critical levels for the effects of sulphur and nitrogen compounds

Estimates of the man-induced global annual sulphur flux for the mid-1980s are put at about 93 (± 14) million tonnes sulphur, of which some 80 million tonnes sulphur were emitted in the form of SO2. Of this, 75 per cent results from the combustion of fuel, mainly fossil fuel. Sulphur dioxide may be transformed chemically in the atmosphere and transported hundreds of kilometers before being deposited where harmful effects may result. Thus, solely national efforts to reduce the harmful effects of such deposition by abating emissions are unlikely to be very effective. Transboundary pollution phenomena must be taken into account, and it has proven to be the impetus for evolving regional co-ordinated abatement strategies for SO2 and NOx emissions. The paper presents the approach that is increasingly being adopted as basis for abatement strategies, and discusses the applications of such an approach in the European context. The approach is one in which the acid buffering capacity and rate of a site are being determined. This is employed, along with other site feature, such as the sensitivity if living systems to acid conditions, to set a critical load value for the ecosystem. A similar approach is being presented concerning the direct effects of air pollutant concentrations, such as SO2 or NOx, by setting critical levels. The critical load and critical level concept does provide a relatively firmly based scientific yardstick by which to judge progress towards pollution minimization. Their use as a basis for target load establishment also provides a practical means of effecting progress in pollution abatement agreements that incorporate a cost effective approach. (author). 89 refs, 5 figs, 14 tabs