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Primary reference(s)

Baxter, P.J. and C.J. Horwell, 2015. .

Fischer, T.P. and G. Chiodini, 2015. .

Williams-Jones, G. and H. Rymer, 2015. .

Additional scientific description

Volcanic gases can be emitted directly into the atmosphere from magma or by magma interacting with crustal rocks. They can be observed with spectroscopic instruments from ground and space, and their future dispersion can be modelled, allowing forecasts of gas and aerosol concentrations to be made. Volcanic gas composition and concentrations can be modified through interaction with ground or surface waters; gases generated by heating and vaporising groundwater in volcanicgeothermal areas. Volcanic gases can also remain pressurised in the subsurface or within lakes (Oregon State, no date).

Volcanic aerosol sizes range from a few nanometres (nm) to several hundred micrometres (μm). Volcanic aerosol refers to particles formed through condensation of volcanic gases, or through reaction of the gases with the atmosphere and sunlight and is thereby distinct from ‘ash’ or ‘tephra’ that is formed through fragmentation of magma or lava. Aerosols can be in liquid or solid form and evolve between these states with time (Oregon State, no date).

Volcanic gases and aerosols are emitted by almost any type of volcanic activity:

• Emissions from explosive eruptions: Depending on the explosive power, emissions can be injected into the stratosphere or stay in the troposphere and spread around the globe in the most powerful events. Typical emission duration is hours to days (Rose and Durant, 2009).
• Emissions from effusive lava eruptions, open vents and lava lakes: Emission durations can last from days up to several decades or longer. Emissions are typically confined to the troposphere and have been instrumentally detected up to thousands of kilometres from the source (Rose and Durant, 2009).
• Emissions from crater lakes, and volcanic-geothermal systems: These low-energy and relatively low-temperature emissions (typically <100°C) are usually confined to the immediate vicinity of the source. However, large and highly hazardous emissions can occur if gases accumulate in the bottom of a lake and then rapidly release (Schmid et al., 2005).

The chemical composition of volcanic gas and aerosol emissions is highly heterogeneous. The composition changes continuously as the emissions drift away from their source and react with the atmosphere and sunlight. Typically, the most abundant volcanic gas is water vapour (80% or more of the gas mass). Other common gases are carbon dioxide (CO2), sulphur dioxide (SO2), hydrogen sulphide (H2S) and hydrogen halides (hydrogen chloride [HCl] and hydrogen fluoride [HF]). Radon and carbon monoxide (CO) are also emitted in trace amounts (Oregon State, no date).

Aerosol forms by condensation of volcanic gases, both near-instantaneously after emission, and on the timescale of hours to days. Sulphate, a common aerosol component, forms through conversion of SO2 gas. Aerosol contains a variety of trace components, including elements collectively classified as metal pollutants by environmental and health protection agencies (Oregon State, no date).

The abundance of emitted volcanic gases and aerosol varies greatly among eruptions. Recent large eruptions of Holuhraun in Iceland 2014–2015 and Kīlauea Hawaii in 2018, emitted as much SO2 per day as anthropogenic activities in China (50–200 kt/day) over several months (Pfeffer et al., 2018; Kern et al., 2020). A larger-scale emission scenario, which may occur in the coming decades or centuries, includes a ‘Laki-type’ eruption in Iceland which can emit ten times more SO2 than the recent eruptions described above. There are tens, or potentially hundreds, of volcanoes worldwide which emit smaller amounts of SO2 (0.5–5 kt/day) (Carn et al., 2016) but sustain the emissions over years-to-decades (e.g., Mt Etna; Aiuppa et al. 2008).

Volcanic gas and aerosol exposure is listed as the cause of 1% of total volcanic hazard fatalities (2283 people; Brown et al., 2017). This estimate includes only fatalities due to extreme direct exposure and does not include premature mortality caused by long-term air and environmental pollution. It has been estimated that 800 million people live within 100 km of a volcano that has erupted in the last 10,000 years (Auker et al., 2013), a range within which they could be exposed to this hazard.

Metrics and numeric limits

Not available.

Examples of drivers, outcomes and risk management

SO2, particulate matter <2.5 μm in diameter (PM2.5 ) and, in some cases, H2S, are the only volcanic gas and aerosol pollutants that are monitored and forecasted operationally. The monitoring and forecasting capacity is present almost exclusively in high-income countries where the emission source is located (e.g., US, Japan, Italy, Iceland). The impacts of volcanic gas/ aerosol emissions on air quality and human health are challenging to constrain and are generally absent from local hazard assessments and mitigation plans in lower-income countries.

Multiple chemical species in volcanic gases and aerosols may cause a human and/or environmental impact.

• Health impacts: The common effects of volcanic gases, in particular SO2, H2S, HCl and HF are: (i) irritation to the respiratory tract, eyes and skin; (ii) chest tightness, shortness of breath, and headaches; and (iii) asthma aggravation. SO2 is the greatest respiratory hazard, causing health impacts, especially for asthmatics, up to thousands of kilometres from the source. High concentrations of fluoride (from HF) causes damage to teeth and bones; it is especially dangerous to grazing animals. All of the listed gas species, as well as CO2 and CO, can cause death in high concentrations. Volcanic aerosol is typically PM2.5, an air pollutant with no known safe exposure limits (WHO, 2013a). Both acute and chronic exposure to PM2.5 causes respiratory and cardiovascular morbidity and premature mortality (WHO, 2013b). More information on the health hazards and impacts of volcanic gases and aerosols can be found on the International Volcanic Health Hazard Network website (IVHHN, 2020a).
• Environmental impacts: Acid rain is commonly caused by mixing of atmospheric water with volcanic gas and aerosol and leads to degradation of plant health and diversity, crop damage and damage to infrastructure. Metal pollutants can contaminate rainfall and accumulate in soils, surface waters and plants (Bourassa et al., 2012).
• Climate impacts: Large explosive eruptions can form an aerosol blanket in the stratosphere which leads to cooling at the surface of ~0.5°C. The effect may last for about 2 years (Bourassa et al., 2012).

Owing to the multiple impacts of volcanic gases, agencies in Hawaii provided a public dashboard which summarises the various impacts as well as providing access to monitoring and forecasting data (IVHHN, 2020b). The dashboard was accessed more than 50,000 times per week during the 2018 KÄ«lauea volcanic crisis.