Abstract. Volcanic emissions, specifically from Iceland, pose a pan-European risk and are on the UK National Risk Register due to potential impacts on aviation, public health, agriculture, the environment and the economy, from both effusive and explosive activity. During the 2014–2015 fissure eruption at Holuhraun in Iceland, the UK atmosphere was significantly perturbed. This study focuses one major incursion in September 2014, affecting the surface concentrations of both aerosols and gases across the UK, with sites in Scotland experiencing the highest sulfur dioxide (SO2) concentrations. The perturbation event observed was confirmed to originate from the fissure eruption using satellite data from GOME2B and the chemical transport model, EMEP4UK, which was used to establish the spatial distribution of the plume over the UK during the event of interest. At the two UK European Monitoring and Evaluation Program (EMEP) supersite observatories (Auchencorth Moss, SE Scotland, and Harwell, SE England) significant alterations in sulfate (SO42−) content of PM10 and PM2.5 during this event, concurrently with evidence of an increase in ultrafine aerosol most likely due to nucleation and growth of aerosol within the plume, were observed. At Auchencorth Moss, higher hydrochloric acid (HCl) concentrations during the September event (max = 1.21 µg m−3, cf. annual average 0.12 µg m−3 in 2013), were assessed to be due to acid displacement of chloride (Cl−) from sea salt (NaCl) to form HCl gas rather than due to primary emissions of HCl from Holuhraun. The gas and aerosol partitioning at Auchencorth Moss of inorganic species by thermodynamic modelling confirmed the observed partitioning of HCl. Using the data from the chemical thermodynamic model, ISORROPIA-II, there is evidence that the background aerosol, which is typically basic at this site, became acidic with an estimated pH of 3.8 during the peak of the event.Volcano plume episodes were periodically observed by the majority of the UK air quality monitoring networks during the first 4 months of the eruption (August–December 2014), at both hourly and monthly resolution. In the low-resolution networks, which provide monthly SO2 averages, concentrations were found to be significantly elevated at remote “clean” sites in NE Scotland and SW England, with record-high SO2 concentrations for some sites in September 2014. For sites which are regularly influenced by anthropogenic emissions, taking into account the underlying trends, the eruption led to statistically unremarkable SO2 concentrations (return probabilities > 0.1, ∼ 10 months). However, for a few sites, SO2 concentrations were clearly much higher than has been previously observed (return probability < 0.005, > 3000 months). The Holuhraun Icelandic eruption has resulted in a unique study providing direct evidence of atmospheric chemistry perturbation of both gases and aerosols in the UK background atmosphere. The measurements can be used to both challenge and verify existing atmospheric chemistry of volcano plumes, especially those originating from effusive eruptions, which have been underexplored due to limited observations available in the literature. If all European data sets were collated this would allow improved model verification and risk assessments for future volcanic eruptions of this type.
