Within the causal chain, the activities and resulting emissions form the basis of the effects and thus the abatement should be focused on the emissions. However, the effects are linked primarily with acid deposition. Therefore, the real progress in reducing the effects should be derived from trends in effects or in acid deposition. Erisman et al. evaluated the progress in acid and nitrogen policies in Europe. Potential acid deposition, the molar sum of SO2, NOx, and NH3, has decreased 2.5% per year between 1980 and 1999. This is mainly due to the reduction in sulfur emissions, which have decreased 59% averaged throughout Europe. The decrease in emissions has resulted in improved environmental quality. Ambient concentrations of SO2 have decreased well below critical levels. NO2 exceedances of critical levels are now only observed near sources and in industrialized areas. Deposition measurements are limited mostly to wet deposition. Trends in measurements show a decrease in wet deposition. However, the decrease is smallest in remote areas, where the most vulnerable ecosystems are located. This is due to nonlinearity in emissions and depositions. Consequently, appreciable emission reductions are still needed to protect ecosystems in remote areas. Although acidity has been decreased to a large extent, nitrogen pollution is still excessive and needs to be reduced to reach critical loads.
In the northeast and mid-Atlantic regions of the United States, where ecosystems are most sensitive to acidic deposition, sulfate levels in precipitation (wet deposition) have declined by up to 25%, mirroring the reductions in SO2 emissions achieved through the implementation of the Acid Rain Program. No distinct regional trends in nitrate have been detected, consistent with NOx emissions, which have remained approximately the same. Trends in dry deposition of sulfur and nitrogen are comparable.
Because sulfur emissions decreased more than nitrogen emissions, the relative contribution of nitrogen deposition compared to sulfur deposition has increased. The focus of policies should therefore be on nitrogen emissions, both oxidized and reduced nitrogen.
The unbalanced reduction in emissions can have unexpected consequences. The large decrease in base cation emissions, which was much faster than the decrease in sulfur emissions, has caused precipitation pH to decrease rapidly in several areas. Another example is the nonlinearity in sulfur and nitrogen emission and the respective deposition in remote areas. Nonlinearity in the relationship between the emission and deposition patterns for sulfur and oxidized nitrogen has been detected, which leads to slower than expected reductions in critical loads exceedance in remote areas. There is evidence that NO3 in rain and wet deposition is increasing at remote, mainly West Coast locations, whereas close to the major sources a small reduction has been detected. This is the result of the nonlinearity.
In relation to acidification, little emphasis is usually placed on the role of deposition of base cations such as Na +, Mg2 +, Ca2 +, and K +. Besides their ability to neutralize acid input, base cations are important nutrient elements for ecosystems. From a variety of measurements of Ca in precipitation in Europe and North America, Hedin et al. detected a long-term decline in 1994. The decline in Ca is due to the abatement of particle emissions from coal combustion and industrial sources. The reduction has resulted in a decrease in the neutralizing capacity of the atmosphere, leading to a decrease in pH. Furthermore, because some abatement of SO2 at industrial sources coincided with a decrease in Ca, the net effect on atmospheric acidity was small; thus, recovery of ecosystems would not proceed as expected. At the time, no data on long-term Ca aerosol concentrations were available. In 1998, Lee et al. reanalyzed previously unreported long-term data sets of the atmospheric aerosol composition for the United Kingdom and showed a clear decline in Ca during the period 1970-1995. Paralleling this decline were trends in scandium and arsenic. Scandium is often used as a tracer for noncombustion sources, whereas arsenic is often used as a tracer for combustion. Declines of both metals clearly show a strong decline in fly-ash from industrial and combustion sources; hence, the decline in Ca can be understood.