Volcanic activity can release significant quantities of gas, including sulphur dioxide (SO2), into our atmosphere (Bluth et al., 1993). Volcanic SO2 emissions can have important impacts on atmospheric chemistry and climate, terrestrial and marine environments, and human and animal health (e.g., Oppenheimer et al., 2011; Mather, 2015). In this paper, we focus on the hazard posed to human health: SO2 is an irritant when inhaled and at high concentrations may cause breathing difficulties (Hansell and Oppenheimer, 2004). Those suffering from asthma and chronic lung disease may be especially susceptible to the adverse effects of SO2 and, exposure to high concentrations may provoke attacks of asthma. Based on the findings from experimental studies with exercising asthmatics, the World Health Organization (WHO) has recommended a threshold for short term exposure of 500 µg/m3 over 10 minutes (WHO 2005, 2021). A longer term (24-hour) exposure threshold for SO2 is also used by WHO. Due to its health impacts, SO2 is part of the daily air pollution forecast in many countries, to mitigate the hazard from anthropogenic pollution sources (e.g., DAQI, 2022).
SO2 is a colourless toxic gas with a characteristic odour. Reported odour detection thresholds of SO2 (i.e., the threshold for noticing smell) range from 266 µg/m3 to 12500 µg/m3, with median thresholds between 1766 µg/m3 and 3575 µg/m3 depending on age and gender (Kleinbeck et al., 2011). This implies that at the WHO concentration threshold of 500 µg/m3 most humans are not likely to smell SO2 and will only start smelling it once concentrations reach around a factor between 3 and 7 over the WHO threshold. Consequently, harmful effects may be possible to susceptible individuals before they are aware of their exposure.
Each year several volcanic eruptions release significant amounts of SO2 to the atmosphere. Carn et al. (2016) show that between 2008 and 2014 there were between 1 and 3 eruptions annually that released more than 100 kt of volcanic SO2. Volcanic emissions of SO2 occur over a range of scales through different types of volcanic activity. Persistently degassing volcanoes like Kilauea (Hawaii), passively release gas at the surface and represent a significant source of gases to the global atmospheric budget (Carn et al., 2017). On average, over the past decade, a total of around 60 kilotons (kt) per day of SO2 has been released due to passive degassing from the world’s volcanoes (Carn et al., 2017). Effusive eruptions release larger quantities of gas into the troposphere. For example, the 6-month Holuhraun eruption in Iceland during 2014, released around 60–100 kt/day of SO2 with a total of about 11000 kt, which is more than the anthropogenic amount emitted from Europe in 2011 (Gíslason et al., 2015). Large explosive eruptions can inject vast quantities of SO2 over a short period of time, and high into the troposphere and stratosphere where it can remain for days to weeks. The largest explosive eruption in the last 100 years was the 1991 eruption of Mt Pinatubo, which released 20000 kt of SO2 up to an altitude of more than 30 km (McCormick et al., 1995) and the SO2 cloud was then rapidly advected around the globe. In the last 20 years, the largest explosive emitters of SO2 were Kasatochi (~ 1700 kt) in 2008, Nabro (~ 3500 kt) in 2011 and Raikoke (~ 1500 kt) in 2019 [Carn et al., 2016, 2021]. The volcanic SO2 clouds originating from these eruptions were detected and tracked by several satellite instruments over a period of several weeks (e.g., Theys et al., 2013; de Leeuw et al., 2021).
Some eruptions release SO2 as well as volcanic ash, while some release only one or the other. If both are emitted then, depending on the eruption dynamics and meteorological conditions, they can be co-located and transported together or be separated and travel in different directions as separate clouds (e.g., Schneider et al., 1999; Prata et al., 2017). SO2 is often detected and transported over larger distances compared with volcanic ash [e.g., Prata et al., 2010]. Removal of SO2 from the atmosphere happens by chemical reactions, dry and wet deposition, and uptake on atmospheric aerosols including volcanic ash (Martin et al., 2014, 2018; Zhu et al., 2020). The atmospheric lifetime of SO2 varies from a few days to several weeks and generally increases with altitude [Carn et al., 2016]. The potential for long range transport of volcanic SO2 clouds consequently means they are likely to intersect global air traffic routes.
Between 1980 and 2008 there were at least 15 incidents where commercial aircraft crew reported encounters with volcanic SO2, with many of the reports identifying sulphurous smells in the cabin (Guffanti, Casadevall, and Budding, 2010). When volcanic SO2 enters an aircraft, its odour can cause distress to passengers and crew, and it may represent a significant health hazard if it is present at high enough concentrations. In August 2008, after the eruption of the Kasatochi volcano in Alaska, several pilot reports specified aircraft crew smelling sulphur (Guffanti et al., 2010). The reports were from flights along the west coast of Canada and northern USA where the SO2 cloud from the eruption was located after it had been transported eastwards from the volcano. After the Grímsvötn 2011 eruption in Iceland, pilots reported unpleasant smells of sulphur while flying across the North Atlantic (European Space Agency, 2012). It should be noted that to date, no health consequences from aircraft encounters with volcanic SO2 clouds have been reported in the public literature.
As these reports are from commercial aircraft, no measurements of the encountered SO2 concentrations are available. However, measurements exist for two Icelandic volcanic SO2 clouds that were sampled by research aircraft. Following the Hekla 2000 eruption, in-situ measurements of cloud properties were measured by a research aircraft one and a half days after the eruption (Rose et al., 2003). The aircraft flew in the SO2 cloud for 7–10 minutes, during which time the on-board instrumentation recorded concentrations up to 1 ppm (about six times the WHO concentration threshold, see further details on unit conversion later). The volcanic SO2 cloud from the 2010 Eyjafjallajökull eruption was also sampled by multiple research aircraft at various locations over Europe and measured SO2 values were up to 0.1 ppm (about half the WHO threshold) (Johnson et al., 2012; Schumann et al., 2011).
Schmidt et al. (2014) discuss the hazards to aviation from SO2 emitted by explosive Icelandic eruptions and use model simulations to estimate SO2 concentrations that could occur in European and North Atlantic airspace following real and hypothetical eruptions in Iceland. They concluded that the WHO mass concentration threshold was not exceeded in the far field (≥ 1000 km) for the 2010 Eyjafjallajökull eruption, and that there was a low risk of plume encounters exceeding the WHO threshold for a larger Hekla 2000 type eruption.
It is the responsibility of 9 worldwide Volcanic Ash Advisory Centres (VAACs) to provide operational forecasts of the expected transport and dispersion of volcanic ash clouds (ICAO, 2012), but there are currently no requirements to provide advice to aviation on the presence of volcanic SO2 clouds. There are also no criteria which define when airspace is considered hazardous due to the presence of volcanic SO2 clouds. However, the International Civil Aviation Organization (ICAO) is currently considering the introduction of global SO2 forecast capability to provide advice to the aviation industry, with a focus on identifying and quantifying health risks to aircraft occupants (ICAO, 2018). This includes an assessment of the forecasting requirements for volcanic SO2.
The aim of this paper is to illustrate the implications of applying the WHO SO2 concentration threshold to determine potentially hazardous areas for aviation due to volcanic SO2 clouds. This is achieved by analysing modelled volcanic SO2 clouds from several historic eruptions that span a range of activity and emission scales.
The paper is organised as follows. In Section 2 we discuss the analysis methods including the dispersion model simulations conducted and the eruption cases considered. The results are presented in Section 3 with a detailed focus on the 2008 Kasatochi eruption. In Section 4 we discuss some of the limitations and uncertainties in our results, and a summary and conclusions are given in Section 5.