The causal chain represents the sequence of emissions into the atmosphere, transport and chemical reactions, deposition to the earth surface, and cycling within the ecosystems until eventually the components are fixed or transformed in a form having no effects. This causal chain is shown in Fig. 2. After the causal chain is understood and parameterized in models, it can be used to assess the possibilities for reducing the effects.
The use of fossil fuels for energy production is one of the main causes of acid deposition. Fossil fuels
FIGURE 2 Emission, transport, and deposition of acidifying pollutants.
contain sulfur compounds, which are emitted into the atmosphere as SO2 after combustion. Oxidized nitrogen has several sources but is formed mainly by combustion processes in which fuel N is oxidized or atmospheric N2 is oxidized at high temperatures. These processes are relevant for industries, traffic, and energy production. Among the other sources of oxidized nitrogen, soils are most important. Ammonia is mainly emitted from agricultural sources. Fossil fuels seem less important for ammonia than for the other two pollutants. Through the use of fertilizer, which is produced from natural gas, there is a link with energy. Indirectly, the increased energy use from fossil fuels to produce fertilizer has increased nitrogen emissions from agriculture and contributed to acidification. Energy use first increased mobility. Food is therefore not necessarily produced in the area where it is consumed. Furthermore, relatively cheap nutrient resources (animal feed) are transported from one continent to another, where they are concentrated in intensive livestock breeding areas. Also, fossil fuels, first in the form of coal but now mainly from natural gas, provide the basis to produce fertilizers, which are used to produce the food necessary to feed the ever-growing population throughout the world and to fulfill the need for the growing demands for more luxurious food.
Base cations play an important role because they can neutralize acids in the atmosphere and the soil, limiting the effect of acid deposition. On the other hand, base cations contribute to the particle concentrations in the atmosphere, affecting human health when inhaled. Sources of base cations are energy production, agricultural practices, and road dust.