Fine-scale fluctuations of PM 1 , PM 2.5 , PM 10 and SO 2 concentrations caused by a prolonged volcanic eruption (Fagradalsfjall 2021, Iceland)

The 2021 Fagradalsfjall eruption marked the first in a series of ongoing eruptions in a densely populated region of Iceland (>260 000 residents within 50 km distance). This eruption was monitored by an exceptionally dense regulatory air quality network, providing a unique opportunity to examine fine-scale dispersion patterns of volcanic air pollutants (SO 2 , PM 1 , PM 2.5 , PM 10 ) in populated areas. Despite its relatively small size, the eruption led to statistically-significant increases in PM and SO 2 concentrations at distances of at least 300 km. Peak daily-mean concentrations of PM 1 (measured in the capital area, 25–35 km distance from the source) rose from 5–6 to 18–20 µg m −3 , and the proportion of PM 1 within PM 10 increased by ∼50 %. In areas with low background pollution, average PM 10 and PM 2.5 levels increased by ∼50 % but in places with high background sources, the eruption's impact was not detectable. These findings suggest that ash-poor eruptions are a major source of PM 1 in Iceland and potentially in other regions exposed to volcanic emissions. Air quality guidelines for PM 1 and SO 2 were exceeded more frequently during the eruption than under background conditions. This suggests the potential for an increase in adverse health effects. Moreover, pollutant concentrations exhibited strong fine-scale temporal (≤1 h) and spatial (