Comment on egusphere-2025-937

<strong class="journal-contentHeaderColor">Abstract.</strong> The 2021 Fagradalsfjall fissure eruption was the first of ongoing eruptions in the most densely populated part of Iceland (70 % of population within 50 km). It was monitored by an exceptionally dense reference-grade air quality network (14 stations within 40 km), and the first time that a reference-grade timeseries of PM₁ was collected during an eruption. We used these measurements to identify fine-scale dispersion patterns of volcanic air pollutants (SO₂, PM₁, PM_2.5, PM₁₀) in populated areas. Despite its small size the eruption caused a statistically-significant increase in average and peak PM and SO₂ concentrations in at least 300 km distance. Peak daily-means of PM₁ peak rose to 18&ndash;20 &micro;g/m³from 5&ndash;6 &micro;g/m³; and proportion of PM₁ increased relative to coarser PM fractions(21&ndash;24 % of PM₁₀ compared to 14 % background). Eruption increased PM₁₀ and PM_2.5by ~50 % in populated areas with low background concentrations, but its impact was not measurable in areas with high background sources. This suggests that ash-poor eruptions are one of, or the most, important source of PM₁ in Iceland, and potentially in other areas exposed to volcanic emissions. There were significant fine-scale temporal (&le;1 hour) and spatial (&lt;1 km) fluctuations in volcanic pollutant concentrations. In Reykjav&iacute;k, two stations located &lt;1 km of each other recorded peak hourly-mean concentrations of 480 and 250 &micro;g/m³ SO₂, and 5 and 0 exceedance events, respectively, within a ~12-hour plume advection event. This has implications for population exposures estimates.