Fine-scale fluctuations of PM ₁ , PM _2.5 , PM ₁₀ and SO ₂ concentrations caused by a prolonged volcanic eruption (Fagradalsfjall 2021, Iceland)

Abstract. 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 (SO2, PM1, PM2.5, PM10) in populated areas. Despite its relatively small size, the eruption led to statistically-significant increases in PM and SO2 concentrations at distances of at least 300 km. Peak daily-mean concentrations of PM1 (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 PM1 within PM10 increased by ∼50 %. In areas with low background pollution, average PM10 and PM2.5 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 PM1 in Iceland and potentially in other regions exposed to volcanic emissions. Air quality guidelines for PM1 and SO2 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 (<1 km) variability. This suggests disparities in population exposures to volcanic air pollution, even from relatively distal sources, and underscores the importance of a dense monitoring network and effective public communication.