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 minutes with minimal attenuation. We find average SO 2 loss rates of ~10 -4 s -1 (e-folding time of ~2.78 hours), 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 hr), 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 atsource 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 atsource SO 2 emission rates from volcanoes with similar environmental conditions to those at Soufrière Hills volcano.3

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The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a useful tool for detecting low quantities of sulfur dioxide at passively degassing volcanoes such as Lascar volcano, Chile. Two mini-UV spectrometers (MUSes) were used to make transects of Lascar volcano's sulfur dioxide plume on December 7, 2004, during a coordinated overpass of ASTER. SO 2 burdens were retrieved using the thermal infrared channels of the acquired ASTER image. This allowed for a direct comparison between the two methods in order to validate the ASTER SO 2 retrieval. The results were extremely encouraging with ASTER deriving SO 2 fluxes within the range of fluxes obtained by the MUSe.

Volcano monitoring seeks to better understand volcanic systems, in order to be able to predict their activity. The monitoring techniques include seismic, deformation, gas, hydrologic, and visual monitoring, among others. Gas monitoring is important, among other reasons, because it provides information about the dynamics and evolution of magmatic and hydrothermal systems. Measurements can be conducted directly or by remote sensing methods (groundbased or satellite-based). Here we focus on the main ground-based remote sensing methods, which are currently used at volcanoes. These include the correlation spectrometer (COSPEC), the DOAS, the ultraviolet camera, and the Fourier Transform Infrared spectrometer (FTIR). Each technique has its advantages and disadvantages, and to understand them, we describe here the general aspects of each one, including methods to conduct the measurements, the sources of error, and some examples of case studies. Keywords: Sulfur dioxide, DOAS, COSPEC, UV camera, FTIR, volcanic plumes. RESUMEN:El monitoreo volcánico busca entender mejor los sistemas volcánicos, para poder realizar predicciones de su actividad. Las técnicas de monitoreo incluyen el sísmico, de deformación, de gases, hidrológico, y visual, entre otros. El monitoreo de gases es importante, entre otras razones, porque nos provee información sobre la dinámica y evolución de los sistemas magmáticos e hidrotermales. Las medidas se pueden hacer de forma directa o por métodos de percepción remota (en superficie o satelital). Aquí nos enfocamos en los principales métodos de percepción remota en superficie, que se usan actualmente en volcanes. Estos incluyen el espectrómetro de correlación (COSPEC), el DOAS,