Fine-scale fluctuations of PM1, PM2.5, PM10 and SO2 concentrations caused by a prolonged volcanic eruption (Fagradalsfjall 2021, Iceland)

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, PM2.5, 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 18–20 µg/m3 from 5–6 µg/m3; and proportion of PM1 increased relative to coarser PM fractions (21–24 % of PM10 compared to 14 % background). Eruption increased PM10 and PM2.5 by ~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 (≤1 hour) and spatial (<1 km) fluctuations in volcanic pollutant concentrations. In Reykjavík, two stations located <1 km of each other recorded peak hourly-mean concentrations of 480 and 250 µg/m3 SO2, and 5 and 0 exceedance events, respectively, within a ~12-hour plume advection event. This has implications for population exposures estimates.