For the first time in the modern age we have the opportunity to study at first hand the environmental impact of a flood basalt (>1 km3 fissure eruption). Flood basalt eruptions are one of the most hazardous volcanic scenarios in Iceland and have had enormous societal and economic consequences across the northern hemisphere. A flood basalt eruption was included in the UK National Risk Register in 2012 as one of the highest priority risks. The Holuhraun eruption reached the flood basalt size sometime after 20 October 2014. It is now the largest flood basalt in Iceland since the Laki eruption in 1783-84, which caused the deaths of >20% of the Icelandic population by environmental pollution and famine and likely increased European levels of mortality through air pollution by sulfur-bearing gas and aerosol. The pollution from Holuhraun has been intensifying over the last few weeks, reaching a "Dangerous" level for the first time in Iceland on 26 October (as defined by the World Health Organisation). During 18-22 September, SO2 fluxes reached 45 kt/day, a rate of outgassing rarely observed during sustained eruptions, suggesting that the sulfur loading per kg of erupted magma (we estimate >0.35 wt%) exceeds both that of other recent eruptions in Iceland and perhaps also other historic basaltic eruptions globally, raising questions regarding the origin of these prodigious quantities of sulfur. A lack of data concerning conversion rates of SO2 gas into aerosol, the residence times of aerosol in the plume and the dependence of these on meteorological factors is limiting our confidence in the ability of atmospheric models to forecast gas and aerosol concentrations in the near- and far-field from Icelandic flood basalt eruptions. Preliminary study of the erupted products highlights two extraordinary features: (1) matrix glasses contain up to 1000 ppm sulfur (<100 ppm is expected for degassed melt) and are extremely heterogeneous and (2) abundant sulfide liquid globules in the matrix glass are "caught in the act" of breaking down on quenching, suggesting that sulfur is not only supplied by the melt, but also by the breakdown of sulfide liquid during degassing. These observations highlight a previously overlooked but potentially very large reservoir of sulfur that leaves little petrological record. These results might go some way towards understanding the extremely high sulfur yield of this eruption and have implications for assessing the environmental impact. This project combines the expertise of a large group of researchers to understand better the sulfur and chalcophile metal budget of the Holuhraun eruption. We will follow the formation of sulfide liquids, through to their breakdown on degassing, to the outgassing of SO2 gas and conversion to aerosol. The entire pathway is not well understood, particularly given complexities related to the rapid magma ascent rates postulated for the Holuhraun magmas and the lack of ash in the plume, both of which we hypothesise impose kinetic constraints on sulfur processing in different parts of the system. We will carry out detailed petrological, geochemical measurements of lavas and plume chemistry to understand the sulfur budget and to feed into models of plume chemistry and dispersion, which are essential for hazard monitoring.