Transport and formation processes for fine airborne ash from three recent volcanic eruptions in Alaska: Implications for detection methods and tracking models

Airborne fine volcanic ash was collected during the eruptions of Augustine Volcano in 2006, Pavlof Volcano in 2007, and Redoubt Volcano in 2009 using Davis Rotating Unit for Measurement (DRUM) cascade impactors to observe atmospheric processes acting on ash as an atmospheric particle. During the Redoubt eruption, samples were also collected by Beta Attenuation Mass (BAM-1020) and Environmental Beta Attenuation Mass (EBAM) monitors. BAM-1020s and EBAMs provided real-time mass concentration data; DRUM samplers provided samples for post-eruptive analysis. DRUM samples were retrospectively analyzed for time-resolved mass concentration and chemistry. EBAM and BAM-1020s reported near real-time, time-resolved mass concentrations. Scanning Electron Microscopy with Energy Dispersive Spectroscopy was conducted to determine particle size, shape, and composition. Image processing methods were developed to determine particle size distributions and shape factors.;Ash occurred as single grains, ash aggregates, and hybrid aggregates. Ash aggregates occurred in plumes from pyroclastic flows and were found in a discrete aerodynamic size range (2.5-1.15 microm). Hybrid ash was common in all samples and likely formed when downward mixing ash mingled with upward mixing sea salt and non-sea salt sulfate. The mass concentration of sulfate did not vary systematically with ash which indicated that the source of sulfate was not necessarily volcanic. Ash size distributions were log-normal. Size distribution plots of ash collected from the same plume at different transport distances showed that longer atmospheric residence times allowed for more aggregation to occur which led to larger but fewer particles in the plume the longer it was transported. Ash transport and dispersion models forecasted ash fall over a broad area, but ash fall was only observed in areas unaffected by topographic barriers. PM10 (particulates ≤ 10 microm in aerodynamic diameter or OA) ash was detected closer to the volcano when no PM2.5 (particulates ≤ 2.5 microm O A) ash was observed. Further downwind, PM2.5 ash was collected which indicated that the settling rates of PM10 and PM2.5 influenced their removal rates. Diurnal variations in ash mass concentrations were controlled by air masses rising due to solar heating which transported ash from the sampling site, or descending due to radiative cooling which brought ash to the sampling site. Respirable (PM2.5) ash was collected when there were no satellite ash detections which underscored the importance of ash transport and dispersion models for forecasting the presence of ash when mass concentrations are below satellite detection limits.