SO 2 loss rates in the plume emitted by Soufrière Hills volcano, Montserrat

Abstract

To improve interpretation of volcanic SO 2 flux data, it is necessary to quantify and understand reactions involving SO 2 in volcanic plumes. SO 2 is lost in volcanic plumes through a number of mechanisms. Here we report SO 2 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 SO 2 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 SO 2 lifetimes. This work is more detailed and precise than previous work and is likely to represent the general case at Montserrat. SO 2 flux measurements made > 2 km downwind from Soufrière Hills volcano significantly underestimate at-source SO 2 emission rates, on the order of 70–146%, when not accounting for the decay rate. Similar SO 2 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 SO 2 emission rate may be underestimated as the estimates are based on measurements taken downwind of volcanoes, by which time significant loss of SO 2 may have taken place. The loss rates calculated here could be used, in conjunction with downwind SO 2 fluxes, to estimate at-source SO 2 emission rates from volcanoes with similar environmental conditions to those at Soufrière Hills volcano.