Big grains go far: understanding the discrepancy between tephrochronology and satellite infrared measurements of volcanic ash
There is a large discrepancy between the size of volcanic ash particles measured on the ground at least 500 km from their source volcano (known as cryptotephra) and those reported by satellite remote sensing (effective radius of 0.5-9 μm; 95% of particles < 17 μm diameter). Here we present new results from the fields of tephrochronology (a dating technique based on volcanic ash layers), dispersion modelling and satellite remote sensing in an attempt to understand why. A literature review and measurements of prehistoric and recent eruptions were used to characterise the size range of cryptotephra grains. Icelandic cryptotephra deposited in NW Europe has lognormal particle size distributions (PSDs) with median lengths of 20-70 μm (geometric standard deviation: 1.40-1.66; 95th percentile length: 42-126 μm). Grain-size range estimates from the literature are similar. We modelled the settling of volcanic ash using measured fall velocities of ash particles, a release height typical of moderate Icelandic eruptions (10 km), and a wind speed typical for NW Europe (10 m s-1), to show that an ash cloud can transport particles up to 80 μm diameter up to 850 km in 24 h. Thus, even moderately sized Icelandic eruptions can be expected to deposit cryptotephra on mainland Europe. Using simulated satellite infrared data for dispersion-model-derived ash clouds, we demonstrate a systematic bias towards small grain sizes in retrievals of volcanic ash clouds that contain large proportions of cryptotephra-sized grains. As the median radius of the simulated PSD increases, fewer ash-containing pixels are correctly identified. Where retrievals are made of simulated clouds with mass median radii larger than ~ 10 μm, the mean retrieved reff plateaus at around 9 μm. Assuming Mie scattering by dense spheres when interpreting satellite infrared brightness temperature difference (BTD) data puts an upper limit on retrieved particle sizes. If larger, irregularly shaped ash grains can also produce a BTD effect, this will result in further underestimation of grain size, e.g. in coarse ash clouds close to a volcano.