Our atmosphere functions as living ecosystem of chemical reactions. Through the help of the water cycle, chemicals pass through the atmosphere and are eventually taken up by the soil, surface water, or organic materials. Human beings have added enormously to the atmospheric burden of many toxic substances. The most prominent evidence of this is the presence of acid rain: precipitation and particles that have been made acidic by air pollution.
Acid rain is a direct consequence of the atmosphere’s self-cleansing process. The tiny droplets of water that make up clouds continuously capture suspended particles and soluble trace gases. Not all trace gases can be removed by precipitation, sulfur dioxide (SO2) and nitrogen oxides in the atmosphere are chemically converted into forms that are readily added into cloud droplets: sulfuric and nitric acids, the main acids involved in acid rain.
The reaction cycle takes place in the troposphere. It begins when sunlight hits an ozone molecule (O3) the result is a molecule of oxygen (O2) and a single oxygen atom, which combines with a water molecule (H2O) to form two hydroxyl radicals (HO). This sparse but active molecule forms nitrogen dioxide (NO2) into nitric acid (HNO3) which initiates the reactions that transforms sulfur dioxide into sulfuric acid (H2SO4). What this boils down to is, as a result only the amount of pollutant in the air determines how much acid is ultimately produced.
The acid rain may fall hundreds of miles from the pollution source. Once it hits the ground chemical alterations still go on which can reduce the acidity of the water that will reach lakes and streams. Alkaline soils can neutralize acid directly. In acidic soils two other processes can stop the effects of the acidic pH. The acid can be immobilized as the soil vegetation retains sulfate and nitrate ions (from the nitric and sulfuric acids). It can also be buffered through a process that is known as cation exchange. In cation exchange of calcium, magnesium, and other metals found in many soils takes the place of the acid’s hydrogen ions. The source of the metal ions is rock weathering: the dissolving of minerals by precipitation and groundwater containing dissolved carbon dioxide, which gives the positive metal ions with anions of bicarbonate (HCO3 -). Then, when sulfuric acid is added the sulfate (SO4 2-) of the acid can displace calcium and magnesium ions, hydrogen ions responsible for acidity are left behind. The effectiveness of this natural buffer is dependent on the geology of the land.
If air pollution, and specifically acid rain, plays a part in forest decline, it probably does so less as a lethal agent than as a stress. Acid and other pollutants could add to the high level of abiotic stresses, including thin soil, low temperatures, and desiccating winds. Meaning that the actual cause of death for the trees would not be acid rain itself but acid rain may just be the next contributing stress that put the tree over the edge.
What to do about reducing nitric and sulfuric emissions requires going straight to the source of the problem. Many coal-burning power plants have no system of attempted emission purification. There are many new ideas of how to do this, but perhaps the most promising is flue-gas desulfurization (FGD). In which wet limestone is sprayed into the plants hot exhaust where it can absorb and lock up to 90% of sulfur dioxide. Other methods of coal cleansing are being experimented with in an attempt to reduce the formation of oxides of nitrogen. Retrofitting plants with these new techniques could be a solution to further damage to the environment.
The drawbacks to all reform in this field and others similar to it are the slow processes, which everything must go through to gain approval and widespread acceptance and trust. But one thing is for sure, the need for human intervention of the destructive way we have treated our environment need immediate correction.