Acid rain is the phenomenon in which acidic compounds like nitrogen oxide (NOx) and sulfur dioxide (SO2) emitted from cars or factories react in the atmosphere to form nitric and sulfuric acids. The acidic compounds present in the air have stronger acidity and mix readily with water, oxygen, and other chemical compounds to produce acidification of rainwater.
The pH scale is the index used to measure the level of acidity of acid rain. Since the advent of the acid rain phenomenon, when the pH of rainwater fell to a level of less than 5.6, it was considered to be acid rain. Acid rain is believed to be acidified artificially. Normal rain is slightly acidic at about pH 5.6 because carbon dioxide (CO2) in the atmosphere dissolves in it and creates equilibrium. However, since substances which acidify rainwater naturally other than CO2 are discovered, at the present, many countries including Japan and the U.S. provide a rough indication of acid rain as a pH level of about 5.0 or less precipitation.
Acid deposition has two parts. First is wet deposition, which refers to acidic rain, fog, and snow. Rain and snowfall are not distinguished meteorologically. The combination of rain and snowfall is called precipitation. Furthermore, wet deposition occurs when pollutants are deposited in combination with precipitation, predominantly by rain and snow, but also by clouds and fog. The other part is dry deposition. When precipitation does not fall, air pollutants descend on the earth, water, or substance surfaces through acidic gases and particles. When the wet and dry deposition processes are combined, the repercussion is acid deposition or acid rain.
There are two types of mechanisms which acidify precipitation. One process is the acidification of cloud water by acid substances generated in the atmosphere. It is called “rainout”. Whereas, when precipitation already starts, the raindrop or snowflake takes in acid substances in the atmosphere during falling. It is called “washout”. Rainout helps pollutants travel long distances, while washout reflects the amount of regional pollution.
Substances which cause acid rain are carried away about 1000 km, hence, international cooperation is needed for measures to prevent or lessen the spread of the occurrence of acid rain. The international cooperative system, including acid rain surveillance, has already been established in North America and Europe. In the East Asia region the concern is focused on the future rise of air pollutant concentration and the influence of acid rain in this region. Therefore, under the initiative of Japan, the Acid Deposition Monitoring Network in East Asia (EANET) is organized. Preceded by a trial operation of approximately 2 1/2 years since April 1998, full operation has begun in January 2001. Under the participation of 13 nations, a common surveillance system is established, and international cooperation concerning the acid rain problem among participating nations is promoted based on the data.
Acid rain damages the forest ecosystem like deterioration of trees, as well as its effects on buildings and cultural properties. Coniferous trees, such as spruce and Japanese cedar, or some deciduous trees have been degenerating in the mountainous areas and suburbs all over the world. It is a wide suspicion that acid rain causes such damages. The mechanism of its destructive effect on trees is emphasized more on the indirect route via the roots due to soil acidification rather than the direct process through the absorption by the leaves. In addition, outdoor cultural properties in Greece, China, and Japan have been seemingly damaged by acid rain.
Precipitation in the acid rain is measured comparatively easy. An automatic acid fog sampler is taken out as soon as precipitation falls. The samples are collected after precipitation stops. A precipitation detector opens and closes the lid in order to recover the samples. The main substance from the sample is analyzed by using ion chromatography, and simultaneously the pH level is measured. In contrast, the assessment of dry deposition is complicated. The reasons are as follows: 1) The deposition speed depends on forests and water surfaces. 2) The evaluation of dry deposition is difficult because of various air pollutants. However, the techniques for scientific measurement and assessment are currently being established gradually.
In Northern Europe, lakes or forests are treated with lime in order to neutralize acid rain. This is an emergency evacuation measure only. When choosing a measure with a long-term influence on the ecosystem, the technique is not necessarily the recommended one. However, a specifically effective technical measure against acid rain does not exist as of yet. The major point to consider is the control of the substances causing acid rain particularly air pollutants, such as nitrogen oxide (NOx) and sulfur dioxide (SO2). The exhaust control of sulfur dioxide is performed by combustion management using Fluidized Bed Combustion Method, which burns coal with limestone, or Fuel Gas Desulfurization Method, which produces a neutralization reaction by using basic material. Nitrogen oxide from cars uses the three-way catalyst. As regards nitrogen oxide from factories, its emission is controlled by Flue Gas Denitrification Method.