Long term trends in sulphur and nitrogen deposition in Europe and the cause of non-linearities

Emissions of sulphur and oxidized nitrogen compounds in Europe have been reduced following a series of control measures during the last two decades. These changes have taken place during a period in which extensive monitoring of the primary gases and the wet deposition throughout Europe. Since the end of the 1970s, for example land based sulphur emissions declined by between 90% and 70% depending on the region. Over the same period the total deposition of sulphur and its partitioning into wet and dry deposition have declined, but the spatial pattern in the reduction in deposition differs from that of emission. Such non-linearities in the emission – deposition relationship are important to understand as they complicate the process of assessing the effects of emission reduction strategies. Observed non-linearities in terrestrial sulphur emission - deposition patterns have been identified in North West Europe of changes in marine emissions, and are currently slowing the recovery of freshwater ecosystems. Changes in the relative amounts of SO2 and NH3 in air over the last two decades have changed the affinity of terrestrial surfaces for SO2 and have therefore changed the deposition velocity of SO2 over substantial areas. The consequence of this effect has been the very rapid reduction in ambient SO2 concentration in some of the major source areas of Europe. Interactions between the different pollutants, generating non-linearities are now being incorporated in long-range transport models to simulate the effects of historical emission trends and to provide projections into the future. This paper summarizes the current state of knowledge of non-linearity in emission deposition relationships for sulphur and nitrogen compounds in Europe and, using long-rang transport models quantifies the importance in impact assessment.