Alberto Caracciolo , Eniko Bali, Eemu Ranta, Sæmundur Ari Halldórsson, Guðmundur H. Guðfinnsson

Exposure to volcanic SO2 can have adverse effects on human health, with severe respiratory disorders documented on short- and long-term timescales. Here, we use melt inclusion and groundmass glass data to calculate potential syneruptive SO2 emissions during the medieval and the recent, 2021–2024, eruptions across the Reykjanes Peninsula,   the most populated area of Iceland, th[AC1] at has recently undergone magmatic reactivation with the 2021–2024 eruptions at Fagradalsfjall and Svartsengi. We target 16 individual eruptions from the medieval volcanic cycle at the Reykjanes Peninsula, the 800–1240 AD Fires, along with the 2021–2023 Fagradalsfjall eruptions and the 2023–2024 eruptions at Sundhnúksgígar. We calculate potential SO2 emissions across the RP for the individual eruptions to be in the range of 0.004–6.3 Mt. These estimates correspond to mean daily SO2 emissions in the range of 1000–111,000 t/day, higher than the mean SO2 measurements of 5240 ± 2700 t/day during the 2021 Fagradalsfjall eruption. By using pre-eruptive sulfur values preserved in undegassed melt inclusions, we develop an empirical approach to calculate best- and worst-case potential SO2 emission scenarios of any past or ongoing Reykjanes Peninsula eruption of known effusion rate. We conclude that the potential sulfur emissions across the RP can be significantly higher than observed during the 2021 Fagradalsfjall eruption, mainly because of the more evolved nature and higher sulfur contents of magmas erupted during the medieval period. Based on dominant NW wind directions on the Reykjanes Peninsula, eruptions in Brennisteinsfjöll pose the greatest health hazard to the capital area. Sulfate aerosol produced during long-term eruptions may impact visibility and air quality in the Keflavík Airport area. Our findings enable assessment of SO2 emission scenarios of future eruptions across the RP and can be used together with gas dispersal models to forecast SO2 pollution at ground level, and its impact on human health.