Large volumes of sulphur dioxide (SO2), hydrochloric acid (HCL), and ash are thrown into the Earth's stratosphere during an eruption. In most situations, HCL condenses with water vapor and is showered out of the forming volcanic cloud. SO2 reacts with oxygen in the air to form sulfur trioxide (SO3), which then drops back down to earth in the form of a rain or snow shower. The ash that is released into the atmosphere remains there for many years-sometimes even centuries-before being washed out to sea or blown away by wind.
During major eruptions, large amounts of gas are ejected into the atmosphere from cracks and fissures in the volcano's surface. Some of this gas enters the troposphere, the lowest layer of the atmosphere. However not all gases enter the troposphere; some molecules are large enough to remain suspended in the stratosphere. Here they can trigger cooling effects such as dimming the Sun and emitting reflective clouds across large distances. These gases include chlorine, bromine, iodine, fluorine, and astatine.
Minor eruptions may also release gaseous substances into the atmosphere. For example, when lava flows into the ocean, it often contains small amounts of gaseous compounds such as CO2 and H2O that come from the rock itself. Over time, these gases build up in the atmosphere and play a role in changing the Earth's climate.
Sulfur dioxide gas (SO2) is emitted by volcanoes and combines with water in the atmosphere to generate sulfuric acid (H2SO4). When volcanic plumes produce enough H2SO4 to reach the stratosphere, it can form a continuous haze of liquid droplets, reflecting sunlight and cooling the planet for a year or two. This effect was probably not responsible for the extinction of the dinosaurs, but scientists think it may have played a role in the demise of the Neanderthals and other late-surviving species.
Volcanoes can also emit other gases such as carbon dioxide (CO2), nitrogen oxides (NOx), and greenhouse gases like methane (CH4). Volcanoes can emit large amounts of these gases into the atmosphere, changing the environment and having an impact on human activities. For example, large amounts of CO2 released into the atmosphere can cause climate change.
Another type of pollution generated by humans that can come from different sources is air pollution. This term includes anything that causes damage to the environment through chemical reactions or physical changes when exposed to air, such as smoke, chemicals, and so forth. Air pollution can be divided into three main categories: primary, secondary, and tertiary. Primary air pollutants are those that result from the emission of substances into the atmosphere that cannot be removed easily by natural processes, such as dust particles and gaseous compounds.
"Primary" aerosols, such as dust, soot, or sea salt, are emitted directly from the Earth's surface. They are carried into the sky by strong winds, ejected high into the air by erupting volcanoes, or waft away from smokestacks or fires. "Secondary" aerosols, such as bacteria or chemical compounds that are formed when gases from vehicles or factories react with sunlight or rain, are also thrown into the air.
In fact, most of the aerosols in the atmosphere are secondary in origin. When gases emitted into the atmosphere absorb energy from the sun, they become excited and release heat instantly. This rapid heating causes some of the gas molecules to change shape, forming new particles that reflect back some of the sun's light and thus reduce the amount of sunlight reaching the ground. The remaining gas molecules continue on their way toward the earth, carrying with them some newly formed particles that have been illuminated by the sun. These particles remain in the atmosphere for several days or weeks before falling back to earth during precipitation or being removed by wind.
In addition to reflecting light and blocking heat from reaching the ground, aerosols can have a large impact on climate patterns at larger scales as well. Dust clouds often cause local changes in weather by blocking out part of the sun, cooling or warming surrounding areas depending on which direction the dust is moving.
Aerosols eventually fall out of the atmosphere and are the cause of nitrogen and sulfur oxide dry deposition. In the soil, the acids react with aluminum and calcium oxides, producing water-soluble calcium and aluminum salts of nitrate and sulfate anions. These compounds are the source of many nutrients for plants when they are deposited as a result of rainfall or snowmelt.
Some particles will be large enough to remain in the atmosphere for more than a year or two, but most are removed by precipitation or sedimentation before this can happen. Water vapor is also slightly acidic due to the presence of hydroxide ions, so some aerosol particles will be acidified through charge interaction effects. However, most atmospheric aerosols have an alkaline nature and do not contribute to further acidification of the atmosphere.
The reaction between aerosols and hydrogen ion concentrations in clouds and precipitation is one reason why acid rain is harmful to life. If too much acid enters aquatic ecosystems, it can dissolve minerals from the surrounding rock, altering the acidity balance of those waters. This can have severe consequences for organisms that rely on these minerals for structure development or protection against other chemicals. For example, the mineral dolomite (which contains magnesium and calcium carbonates) is soluble in acid rain, which could damage structures built from dolomite (such as coral reefs).
Polar stratospheric clouds (PSCs) play a critical part in the creation of the Antarctic and Arctic ozone holes. These processes produce chlorine free radicals in the stratosphere, which directly damage ozone molecules....
How can volcanic ash aerosols impact the climate? Volcanic ash, dust, and aerosols in the atmosphere increase the quantity of solar energy reflected back into space. As a result, the Earth's lower atmosphere cools. This is because many small particles that reflect light are needed to produce an effect similar to that of large particles like soot or salt spray. The more voluminous the eruption, the more greenhouse gases are emitted, so the greater the cooling effect.
In addition to increasing the amount of sunlight reflected away from the planet, volcanic eruptions can also influence the Earth's weather by altering the composition of the air. For example, a large eruption could cause a thick cloud of sulfur dioxide to spread across the planet which would reduce the amount of oxygen in the air and lead to mass deaths of animals and people. It could also trigger more intense rainstorms or snowfalls if enough SO2 is released into the atmosphere.
Finally, volcanic eruptions can have long-lasting effects on Earth's climate even after the activity has stopped emitting gas and lava, through something called "volcanic winter". As we've seen, volcanoes emit carbon dioxide when they erupt, causing their own extinction mode or "volcanic winter" which would last until natural processes took hold and reduced the CO2 level in the atmosphere again.