Edinburgh, no stranger to an occasional haze, experienced an unprecedented atmospheric event on 31 May, unlike any seen in over 30 years. While sea haar from the North Sea often blankets Scotland’s capital, especially in late spring and summer, the haze observed that Friday seemed distinctively different. Could it be air pollution? And if so, from where? Northerly winds were bringing unseasonably cold conditions to the UK, but these are usually associated with clean polar air. UKCEH scientists from the Atmospheric chemistry and effects group Dr Massimo Vieno, Dr Marsailidh Twigg and Dr Eiko Nemitz explain more.

On 29 May 2024 at 12:45 GMT (13:45 BST), a new fissure eruption started on the Reykjanes Peninsula, northeast from Sýlingarfell in southwest Iceland. This marked the fifth eruption in a series that began in December 2023 near the town of Grindavik. Initially deemed a local concern due to its non-explosive nature, the eruption’s impact on air travel and UK airspace remained minimal. However, due an unusual meteorological configuration, sulphur dioxide (SO2) levels in Scotland rocketed to levels not witnessed since the 1970s on the morning of 31 May. 

The Scottish Environment Protection Agency’s national volcanic emissions network first detected an increase in SO2 on the Isle of Lewis on the evening of 30 May, originating from this eruption. During the early hours, the plume moved southward, peaking in Scotland’s Central Belt by 6am on 31 May. St Leonards in Edinburgh reported a maximum concentration of 1161 µg m-3.  

We investigated the source using a combination of observations and modelling data, making it highly likely that the high SO2 levels could be attributed to the Icelandic volcano. The UKCEH’s EMEP4UK atmospheric chemistry transport computer model was used to calculate in near real-time the plume of SO2 from the eruption and simulate the concentration at the measurement sites.  

The EMEP4UK model confirmed the sequence of events that led to the high-concentration episode. Had the eruption happened 100 km to the north or happened a few hours later, the SO2 emitted would have missed the UK, instead travelling north to the Arctic region. 

What distinguished this event is that concentrations of SO2 were much higher than those previously reported in the UK, surpassing those of previous Icelandic eruptions in recent years. This anomaly can be attributed to meteorological circumstances, which resulted in high concentrations as the plume moved across the UK (Twigg et al. 2016). The 2010 Eyjafjallajökull eruption, for example, was an explosive eruption, so the plume was aloft and not much was seen at ground level in the UK apart from large ash particles that rapidly fell to the earth.  

Alongside high SO2, the volcanic plume comprises a mixture of other gases.  At UKCEH’s Atmospheric Observatory Auchencorth Moss, we are now investigating the chemical composition of this plume and if the haze observed was due to this plume. 

Sulphur dioxide – an air pollutant from the past  

Sulphur dioxide, once a prominent pollutant due to industrial and domestic emissions, especially of coal, was a key component of the urban air pollution that earned Edinburgh the nickname “the Auld Reekie”, as well as of the London smog of the 1950s. Later, it was also identified as a key cause of the acid rain that damaged lakes and forests in the 1980s. The former was addressed with national air pollution agreements; the latter with international agreements. These initiatives have been highly successful in reducing SO2 emissions, for example through the addition of desulphurisation to coal-fired powerplants and the introduction of ultralow sulphur fuels. As a result, UK SO2 emissions have decreased by 98% since 1970. Normal ambient concentrations are barely measurable, and farmers have started to compensate for the lack of atmospheric sulphur deposition by adding sulphur to their fertiliser mix for optimum nutrient balance.
Whilst some previous Icelandic volcanic eruptions led to measurable SO2 increases across the UK measurement network, we have not observed concentrations of this magnitude since the 1970s.

Should we be concerned?

While this event, exceeded air quality objectives for 10 hours in Edinburgh, it did not breach workplace exposure limits. Nor did it pose a significant health risk. The current air quality objective for SO2 is 350 µg m-3 (hourly average), and a maximum concentration of 1161 µg m-3was reported from the AURN network at Leonard St. Concentrations stayed well below the workplace exposure limit for an 8-hour shift average of 1300 µg m-3 and for 15-minute exposure of 2700 µg m-3 (https://www.hse.gov.uk/pubns/priced/eh40.pdf). Our modelling effort helped predict that this plume would pass rapidly over the UK. Through chemical reactions, sulphur dioxide can contribute to the formation of small airborne particles (PM2.5) that are harmful to human health. Measurement and model results indicate that PM2.5 concentrations stayed well below levels of concern during this event. 

Ecosystems are vulnerable to sulphur dioxide. Very high concentrations of sulphur dioxide can result in immediate direct damage to vegetation. However, due to the shortlived nature of the plume this is unlikely to occur but may have made a small contribution to raising the long-term exposure. Deposition of sulphur dioxide over time weakens plants and damages ecosystems. However, because of the fleeting nature of this event, it is likely that the ramifications for nature are minimal.

The scientific analysis of this event provides valuable insights into how well we can predict the impacts of volcanic eruptions on human health and our environment. SEPA’s volcanic emission network combined with UKCEH’s modelling and analysis means we are well equipped to provide early warnings and risk assessment. 

About UKCEH’s air quality work

UKCEH undertakes both forecast air quality modelling and near real time observations of air pollutants. As atmospheric events occur, UKCEH has the capability to respond at short notice. 

The EMEP4UK is an off-line atmospheric chemistry transport model (ACTM) based on the EMEP MSC-W model which has been developed by the co-operative programme for monitoring and evaluation of the long-range transmission of air pollutants in Europe. The model EMEP4UK is capable of representing the UK hourly atmospheric composition at a horizontal scale ranging from 100 km to 1 km. The Weather Research Forecast (WRF) model was used as the main meteorological driver. Further details of the air quality forecasts from the model can be found here. Though emission rates at the time were unknown the EMEP4UK model previous use in responding to volcanic fissure eruptions from Iceland, we were able to replicate the observations across the UK and forecast how long the plume would impact UK air quality. The magnitude of the SO2 emission source added to the model was estimated from previous volcanic activities in Iceland, and were injected into the atmosphere at an altitude varying form 2 km to 1 km. At the start of the eruption the emissions were set to 15000 kg/s of SO2 for the first few hours, reduced to 5000 kg/s SO2 afterwards. The emissions used in this simulation were set to match the observed SO2 in the UK.

About Auchencorth Moss

UKCEH Auchencorth Moss Atmospheric Observatory is the largest instrumented site in Scotland for monitoring air quality.  The site was established in 1994 to study methane fluxes and now monitors over 300 chemical and physical aspects of the atmosphere. The site has the capability to report near real time information on composition of our air to provide information to assess air pollution events.