Effect of volcanic sulfur dioxide on solar UV irradiance during the 2023 Fagradalsfjall eruption in Reykjavík, Iceland

Sulfur dioxide is an important atmospheric gas that can attenuate solar ultraviolet radiation. It can be found in greater quantities in highly polluted areas and especially in volcanic plumes. During the eruption of the Icelandic volcanic system Fagradalsfjall, which lasted between 10 July and 5 August 2023, the sulfur dioxide plume reached the capital city Reykjavík on 24 July 2023, resulting in almost 50 DU column of this gas, as measured by a B199 MkIII Brewer spectrophotometer. The increased sulfur dioxide concentration resulted in ~25 % reduction in UV Index, compared to only 1 % decrease on 23 July 2023, when up to 3.6 DU sulfur dioxide were recorded. In the UV spectrum, sulfur dioxide affected mostly short wavelengths up to approximately 325 nm, above which the effects were negligible. On 24 July 2023, a 50 DU sulfur dioxide column lead to more than 79 % reduction of UV irradiance at 296 nm. Model simulations showed that a 100 DU column sulfur dioxide would attenuate 96 % of UV irradiance at this wavelength. The effects of sulfur dioxide on vitamin D weighted solar UV irradiance were even greater than on UV Index, as a 50 DU sulfur dioxide column caused a 37 % decline in vitamin D weighted solar UV irradiance, and a potential 100 DU column of this gas would lead to a 58 % reduction of vitamin D production.