Contribution of fine ash to the atmosphere from plumes associated with pyroclastic density currents

A co-pyroclastic density current (co-PDC) plume forms as a mixture of fine-grained (<90 μm) particles and hot gas lofts from the top of a pyroclastic density current. Such plumes can rise tens of kilometers and inject substantial volumes of fine ash into the atmosphere with significant implications for airspace disruption, populations, livestock, and agriculture in downwind areas. Co-PDC deposits have a remarkably consistent grain size that remains constant with distance from source, regardless of eruption style, highlighting the complex sedimentation mechanisms that control deposition of co-PDC ash due to its fine grain size. Observations and numerical simulations of co-PDC onset emphasize the role played by the dynamics of PDCs in the development of co-PDC columns and plumes. The key differences between co-PDC and vent-derived plume source conditions and dispersion dynamics have important implications for application of remote sensing and numerical modeling methods.