SO2 loss rates in the plume emitted by Soufrière Hills volcano, Montserrat

To improve interpretation of volcanic SO2 flux data, it is necessary to quantify and understand reactions involving SO2 in volcanic plumes. SO2 is lost in volcanic plumes through a number of mechanisms. Here we report SO2 measurements made with miniature ultraviolet spectrometers at Soufrière Hills volcano, Montserrat; a low altitude volcano (∼1000 m above sea level) whose plume entrains humid marine air in the planetary boundary layer. Traverses very near (<400 m) beneath the ash-free plume were made at various distances from the source (from ∼2 km to ∼16 km), thereby spanning plume ages of about 6 to 35 min with minimal attenuation. We find average SO2 loss rates of ∼10-4 s-1 (e-folding time of ∼2.78 h), slightly lower than estimated previously for Soufrière Hills. These are in the fast end of the range of loss rates measured at other volcanoes (10-3-10-7 s-1, e-folding times of 0.28-2778 h), indicating that Montserrat plumes have short SO2 lifetimes. This work is more detailed and precise than previous work and is likely to represent the general case at Montserrat. SO2 flux measurements made >2 km downwind from Soufrière Hills volcano significantly underestimate at-source SO2 emission rates, on the order of 70-146%, when not accounting for the decay rate. Similar SO2 loss is likely to occur in plumes from other tropical low altitude volcanoes under conditions of high relative humidity (∼20% of active volcanoes worldwide). These results suggest that the global volcanic SO2 emission rate may be underestimated as the estimates are based on measurements taken downwind of volcanoes, by which time significant loss of SO2 may have taken place. The loss rates calculated here could be used, in conjunction with downwind SO2 fluxes, to estimate at-source SO2 emission rates from volcanoes with similar environmental conditions to those at Soufrière Hills volcano.