Hybrid particulate matter generated by lava-ignited wildfires at the Litli-Hrútur 2023 eruption, Iceland

Lava flows from the Litli-Hrútur 2023 eruption ignited the largest moss wildfires since modern record keeping in Iceland began. Volcanically ignited wildfires present a cascading and more complex hazard than standalone volcanic eruptions and are likely to become more frequent globally due to climate change. Both volcanic eruptions and wildfire events generate well-characterized air pollution hazards through the emission of gas and particulate matter, but the physicochemical consequences of mixing between end-member emission types during compound events remain poorly understood. In this study, we collected samples of end-member volcanic, wildfire, and mixed plume particulate matter during the Litli-Hrútur 2023 eruption and wildfires. Geochemical and morphological analysis showed that wildfire smoke and lava flow outgassing have distinctive chemical signatures and particulate matter (PM) size distributions, but that when mixing occurs between them, either directly at the lava-moss burning interface or during downwind transport, it can result in the formation of hybrid PM. This hybrid PM may be formed through mechanical interactions via well-established processes such as agglomeration and particle scavenging, although these interactions are unique in the context of a compound volcanic-wildfire event as they occur between emissions from two different sources. We demonstrate that the formation of hybrid PM via these mechanisms may result in altered physicochemical characteristics, and suggest that this may have consequences for depositional processes and atmospheric and environmental transport pathways of key species when compared to stand-alone volcanic eruptions.