Tephra comminution by vehicles and resuspension of volcanic ash: impact on ambient air quality in urban areas

Tephra fallout is a common feature of different styles of eruptions and is the most widespread of volcanic hazards. Fallouts containing substantial amounts of fine-grained particles pose a concern for human health since exposure to respirable PM (sub-10 μm, i.e., PM10) is associated with adverse health effects. The products of effusive or poorly explosive events (e.g., lava fountains, strombolian eruptions) and their impacts are less researched as they typically generate rather coarse-grained deposits. Yet, reworking of tephra deposits by wind, traffic or other human activities can potentially alter the initial grain size distribution of a deposit and generate finer material. Remobilisation of such reworked deposits can affect ambient air quality (i.e., PM10 levels), thus leading to an increased exposure hazard. In this study, we conducted in situ experiments on the slopes of Etna volcano, Italy, which frequently covers neighbouring urban areas with coarse-grained basaltic tephra. We aimed to understand the changes in tephra grain size distribution and airborne PM10 concentration associated with vehicular activity. For this purpose, we drove a small SUV-type car over an area of a road that we covered with tephra, and investigated the outcomes as a function of 1) the number of car passages (between 10 and 70), 2) the starting thickness of the tephra deposit (between 2 and 10 mm) and 3) vehicle speed (between 20 and 50 km/h). The results show that the grain size of the original tephra deposit decreases with the number of car passages, most notably with higher vehicle velocity (50 km/h) and increasing deposit thickness. Airborne PM10 increased with a higher number of car passages, but also with increased tephra thickness and increased vehicle speed. Our observations have important implications for the management of tephra fallouts in urban areas. We have shown that vehicles will change the grain size distribution of basaltic ash by comminution so local communities can expect that, after an eruption, concentrations of PM10 may increase with time and affect exposures close to roads.