Volcanic Eruptions: The Unseen Air Pollution Culprit

Volcanic eruptions are a significant source of air pollution, emitting a range of harmful substances into the atmosphere. These include volcanic gases such as sulphur dioxide, hydrogen fluoride, and carbon dioxide, as well as ash and lava. The impact of volcanic eruptions on air quality can be felt thousands of kilometres away and can last for days, decades, or even centuries. While the duration of volcanic eruptions is usually limited to a few weeks or months, the pollutants they release can have long-lasting effects on the environment and human health. Sulphur dioxide, for example, has been linked to respiratory issues such as asthma and bronchitis, as well as cardiovascular and lung disease. The eruption of Mount St. Helens in 1980 released approximately 10 million tons of carbon dioxide in just 9 hours, causing an international pollution event. With over a billion people living within 100 km of an active volcano, understanding and mitigating the impact of volcanic eruptions on air quality is crucial for public health and safety.

Sulphur dioxide gas

Sulphur dioxide (SO2) is one of the most common gases released during volcanic eruptions. It is a colourless gas with a pungent odour that irritates the skin, eyes, nose and throat. It is also non-flammable, not explosive and relatively stable.

SO2 is hazardous to human health, and high concentrations of the gas have been recorded downwind of many volcanoes. For example, in 1996, ambient concentrations of SO2 in a tourist car park near Kilauea, Hawaii, rose to 4.0 ppm during an episodic increase in activity, which was nearly ten times higher than the USA 3-hour concentration guideline. In 1991, sulphur dioxide was also detected in fumes from Cerro Hudson, Chile, which caused some inhabitants of the Huemules valley to fall sick.

SO2 is of concern on a global scale due to its potential to influence climate change. When injected into the stratosphere during large volcanic eruptions, SO2 is converted to sulphate aerosols, which reflect sunlight and have a cooling effect on the Earth's climate. The conversion of SO2 to sulphuric acid has the most significant impact on climate. Several eruptions during the past century have caused a decline in the average temperature at the Earth's surface of up to half a degree (Fahrenheit scale) for periods of one to three years. For example, the 1991 eruption of Mount Pinatubo injected more than 250 megatons of gas into the upper atmosphere in a single day, and the 1823-1824 Laki fissure eruption in Iceland released approximately 120 million tons of SO2, causing regional cooling of Europe and North America.

On a local scale, SO2 also contributes to air pollution. It can cause acid rain, and when combined with other gases, it forms volcanic smog (VOG), which causes persistent health problems for downwind populations.

Ash and lava

Volcanic eruptions can cause air pollution in several ways, and ash and lava play a significant role in this process. When a volcano erupts, it releases a mixture of gases, lava, small rocks, and steam onto the Earth's surface. The ash produced during an eruption can have both short-term and long-term impacts on air quality and human health.

Fresh volcanic ash is known to be gritty, abrasive, and sometimes corrosive. While ash is not highly toxic, it can irritate the eyes, skin, and airways, especially for infants, the elderly, and individuals with respiratory conditions. Volcanic ash can travel hundreds to thousands of miles downwind from a volcano, affecting areas far from the eruption site. The weight of ash during heavy ashfall can also cause structural damage to buildings, limiting the time people can remain indoors during and after an eruption.

Volcanic ash can contain sulphuric and other acids adsorbed from the gases in the plume, increasing the irritancy of the ash on the respiratory system. Studies have shown that exposure to volcanic ash can lead to respiratory issues, with cardiorespiratory hospital admissions in Iceland increasing on days with high volcanic ash concentrations. Additionally, volcanic ash can impair the surrounding biosphere when it settles on the Earth's surface. For example, ash particles laden with hydrogen fluoride can detrimentally affect the local ecosystem.

Lava flows, along with violent volcanic eruptions, can release volcanic gases such as sulfur dioxide, carbon dioxide, and hydrogen fluoride. These gases contribute to air pollution and can lead to acid rain, affecting both the environment and human health. Sulfur dioxide, in particular, has been associated with regional cooling due to its impact on the atmosphere. While carbon dioxide released by volcanoes has not been directly linked to global warming, it is a greenhouse gas and can remain close to the Earth's surface, potentially causing detrimental effects on the biosphere.

Carbon dioxide emissions

Carbon dioxide (CO2) is a greenhouse gas and is the primary gas blamed for climate change. During volcanic eruptions, huge amounts of volcanic gas, including carbon dioxide, are injected into the stratosphere. While carbon dioxide released in contemporary volcanic eruptions has never caused detectable global warming of the atmosphere, it has the potential to promote global warming.

Volcanoes emit carbon dioxide in two ways: during eruptions and through underground magma. Carbon dioxide from underground magma is released through vents, porous rocks and soils, and water that feeds volcanic lakes and hot springs. Much of the carbon dioxide released by volcanoes is emitted by the degassing of subterranean magma when the volcano is not erupting.

Published estimates of the global CO2 emission rate for all degassing subaerial (on land) and submarine volcanoes lie in a range from 0.13 gigaton to 0.44 gigaton per year. In 2010, human activities were responsible for a projected 35 billion metric tons (gigatons) of CO2 emissions, about 80 to 270 times larger than the respective maximum and minimum annual global volcanic CO2 emission estimates. While large, violent eruptions may match the rate of human emissions for the few hours that they last, they are too rare and fleeting to rival humanity’s annual emissions.

The eruption of Mount St. Helens in 1980 vented approximately 10 million tons of CO2 into the atmosphere in only 9 hours. However, it currently takes humanity only 2.5 hours to emit the same amount. The total annual CO2 emissions from human activities are akin to one or more Yellowstone-sized super eruptions occurring every year.

Health impacts

Volcanic eruptions can have severe health impacts on humans, animals, and plant life. The respiratory system of humans and animals can be extremely damaged when exposed to volcanic gases like sulphur dioxide and hydrogen fluoride. Inhalation of volcanic ash and gases can cause mild symptoms such as eye irritation and more severe symptoms such as breathing difficulties, unconsciousness, and even death. People with asthma or other respiratory conditions are particularly vulnerable, and can experience worsened symptoms. Infants, the elderly, and those with respiratory ailments may also be troubled by volcanic ash.

Volcanic ash can travel hundreds to thousands of miles downwind from a volcano. Fresh volcanic ash is often gritty, abrasive, corrosive, and unpleasant. Ash can get into the eyes, especially in windy conditions, and cause scratches. It can also be hazardous to grazing livestock and damage drinking water and wastewater treatment facilities by clogging or damaging equipment.

The gases and dust particles thrown into the atmosphere during large volcanic eruptions can also have a cooling effect on the climate, which can last for months to years. Sulphur dioxide, for instance, moves into the stratosphere and combines with water to form sulphuric acid aerosols. These aerosols create a haze of tiny droplets in the stratosphere that reflects incoming solar radiation, causing a cooling effect on the Earth's surface.

The eruption of Mount Kilauea in Hawaii in 1986 released about 2,000 tonnes of SO2 into the lower troposphere. This resulted in acid rain and air pollution downwind from the volcano, causing persistent health problems for those in the affected region. Similarly, the 1883 eruption of Krakatau in Indonesia released aerosol particulates that dispersed as far as New York, causing conflagrations in New Haven, NY, 13 days after the eruption.

Climate change

Volcanic eruptions are often discussed in the context of climate change as they release carbon dioxide (CO2) and other gases into the Earth's atmosphere. However, the impact of human activities on the carbon cycle is far greater than that of volcanic activity. For instance, human activities release an amount of CO2 equivalent to a Mount St. Helens-sized eruption every 2.5 hours and a Mount Pinatubo-sized eruption twice daily.

Volcanoes can influence climate change by injecting huge amounts of volcanic gas, aerosol droplets, and ash into the stratosphere during major eruptions. While the injected ash falls rapidly from the stratosphere and has little impact on climate change, volcanic gases like sulfur dioxide can cause global cooling. Sulfur dioxide moves into the stratosphere and combines with water to form sulfuric acid aerosols, which reflect incoming solar radiation, causing cooling of the Earth's surface. This cooling effect can last for months to years, depending on the eruption. The aerosols can remain in the stratosphere for up to three years, causing significant cooling worldwide.

Volcanoes also release greenhouse gases such as water vapor and carbon dioxide, which contribute to global warming. While volcanic eruptions have been responsible for periods of global warming during Earth's history, current volcanic activity is not occurring on the same massive scale. Even large eruptions have not been shown to measurably warm the globe, as the amount of CO2 released is not significant on the scale of our atmosphere.

It is important to note that the warming caused by human activities, primarily from burning fossil fuels, will persist for much longer than the temporary cooling effects of volcanic eruptions. While volcanic eruptions can provide insights into short periods of cooling in Earth's history, the impact of human-induced climate change is far more significant and long-lasting.

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