The primary volcanic aerosol emission from Mt Etna: Size-resolved particles with SO 2 and role in plume reactive halogen chemistry

Volcanoes are an important source of aerosols to the troposphere. Within minutes after emission, volcanic plume aerosol
catalyses conversion of co-emitted HBr, HCl into highly reactive halogens (e.g. BrO, OClO) through chemical cycles that
cause substantial ozone depletion in the dispersing downwind plume.
This study quantifies the sub-to-supramicron primary volcanic aerosol emission (0.2–5 lm diameter) and its role in this
process. An in-situ ground-based study at Mt Etna (Italy) during passive degassing co-deployed an optical particle counter
and Multi-Gas SO2 sensors at high time resolution (0.1 Hz) enabling to characterise the aerosol number, size-distribution
and emission flux.
A tri-modal volcanic aerosol size distribution was found, to which lognormal distributions are fitted. Total particle volume
correlates to SO2 (as a plume tracer). The measured particle volume:SO2 ratio equates to a sulfate:SO2 ratio of 1–2% at the
observed meteorological conditions (40% Relative Humidity). A particle mass flux of 0.7 kg s 1 is calculated for the measured
Mt Etna SO2 flux of 1950 tonnes/day.
A numerical plume atmospheric chemistry model is used to simulate the role of the hygroscopic primary aerosol surface
area and its humidity dependence on volcanic plume BrO and OClO chemistry. As well as predicting volcanic BrO formation
and O3 depletion, the model achieves OClO/SO2 in broad quantitative agreement with recently reported Mt Etna observations, with a predicted maximum a few minutes downwind. In addition to humidity – that enhances aerosols surface area
for halogen cycling – background ozone is predicted to be an important control on OClO/SO2. Dependence of BrO/SO2
on ambient humidity is rather low near-to-source but increases further downwind. The model plume chemistry also exhibits
strong across-plume spatial variations between plume edge and centre.