Acid rain refers to the rain formed by the combination of sulfur dioxide and nitrogen oxides produced during the combustion of coal, oil and natural gas with water in the atmosphere. Acid rain mainly contains sulfuric acid and nitric acid. The pH value of normal rain is generally around 5.6, but the pH value of acid rain can be reduced to 3 ~ 5 or even as low as 2. 1. Now, acid rain has become one of the most serious environmental problems in the world. The United States, eastern Canada and northern Europe are areas with more acid rain. There are many acid rains in the south of the Yangtze River basin in China, and there is a trend that acid rain areas are connected together.
Among the acid rain components caused by human activities, sulfuric acid accounts for the most, accounting for about 60%-65%, followed by nitric acid, accounting for about 30%, hydrochloric acid accounting for about 5% and organic acids accounting for about 2%. Sulfuric acid is mainly due to sulfur dioxide released by burning fossil fuels, and the biggest emission sources are power plants, steel mills, smelters and so on. And small coal stoves in every household. At present, the world releases about 654.38+600 million tons of sulfur dioxide annually. Nitric acid is formed by nitrogen oxides. Nitrogen oxide gas is mainly produced under high temperature combustion conditions. For example, the combustion chamber of automobile engine and fossil fuel will release nitrogen oxides when burning at high temperature. The artificial source of hydrogen chloride is not only the factory that uses hydrogen chloride, but also the gas will be released when burning garbage (there is a lot of chlorine in plastic products) and burning fossil fuels.
Although the amount of sulfur dioxide and nitrogen oxides caused by human activities is roughly equivalent to that of natural sources (that is, about 50% each), the self-purification ability of nature is limited. It's like a person eating, no matter how big his stomach is, if he is allowed to eat twice as much food, his stomach will go bad.
The process of sulfur oxides and nitrogen oxides forming acid rain in the atmosphere is a very complicated atmospheric chemical and physical process. If there is no cloud and rain when the acid substance is formed, the acid substance will gradually fall to the ground in the form of gravity sedimentation, which is called dry sedimentation, which is different from wet sedimentation such as acid rain and acid snow. When dry sediment meets water on the ground, it will recombine into acid. Because it is not diluted by raindrops with much larger diameters, the acidity of acid clouds is much stronger than that of acid rain. Because there are often clouds on the mountains, the forests on the mountains in acid rain areas suffer the most and often die first.
Acid rain will also affect soil, delay the decomposition of organic matter in soil, destroy soil fertility and reduce the productivity of farmland, forest and grassland. Sulfur and nitrogen are nutrient elements. Weakly acidic precipitation can dissolve underground minerals for plants to absorb. If the acidity is too high and the pH value drops below 5.6, it will cause serious harm. It can directly kill large areas of forests and make crops wither; It will also inhibit the decomposition of organic matter and the fixation of nitrogen in the soil, leach nutrients such as calcium, magnesium and potassium combined with soil ions, and make the soil barren; It can also acidify lakes and rivers, dissolve heavy metals in soil and water sediments into water, and poison fish; Accelerate the corrosion and weathering process of buildings and cultural relics; May endanger human health.
Acid rain will increase the acidity of ponds, lakes and other waters and affect the survival of various organisms in the water. The impact of acid rain is most obvious in Europe and the northeastern United States, and it is widely known, but the threatened areas also include Canada, and perhaps the California Mountains, Loki Mountains and China. In some places, it is occasionally observed that the falling rain is as sour as vinegar. The influence of acid rain is a controversial topic. The harm to aquatic organisms in lakes and rivers was initially the focus of attention, but now people have realized that the harm to buildings, bridges and equipment is another costly consequence of acid rain. The impact of polluted air on human health is the most difficult to determine quantitatively.
Lakes with poor buffering capacity suffer the most. When there is a natural alkaline buffer, the acidic compounds (mainly sulfuric acid, nitric acid and a small amount of organic acids) in acid rain will be neutralized. However, lakes on granite (acidic) strata are vulnerable to direct harm, because the acid in rainwater can dissolve metal ions such as aluminum and manganese. This will reduce the growth of plants and algae, and in some lakes, it will also lead to the decrease or disappearance of fish populations. The damage caused by this form of pollution to plants ranges from harmful effects on leaves to damage to fine roots.