Atmospheric sulfur dioxide measurements during the 2005 and 2007 eruptions of the Piton de La Fournaise volcano: Implications for human health and environmental changes

Piton de La Fournaise is an active volcano located in the south-eastern part of Réunion Island, a tiny island found in the south-western Indian Ocean (21°S; 55,5°E). This volcano erupts regularly, at least once a year, with notable amount of degassing. Concerning the volcanic eruptions, the regulation of the French Ministry of Environment on the air quality is mainly focussed on the standards applicable for the sulfur dioxide (SO2), in terms of public health and environmental changes. Air samples have been collected continuously every 10 days on passive diffusion tubes of SO2, an atmospheric tracer of volcanic plumes, by ORA (Observatoire Réunionnais de l'Air), at 10 sites which have been located around the volcano since September 2005. The mean SO2 concentrations measured over a 10-day period, in the absence of any volcanic eruption, range between very low values (1–15 μg/m3) for all 10 sites. However, during the December 2005 eruption, notable SO2 concentrations (factor ~ 5–25) have been measured at the locations situated on the north-west flanks of the volcano. The SO2 degassing reached concentrations of 65–379 μg/m3, which is mainly caused by the volcanic activity when coupled with the local-to-regional atmospheric circulations (land/sea breezes, easterlies, etc.). A wide range of atmospheric measurements was performed in the vicinity of Piton de La Fournaise in order to characterize the gaseous and particle emissions, during the April–May 2007 eruption. SO2 concentrations were also measured continuously by SF-2000 analyser, at several coastal locations in the vicinity of the volcano, during and off eruption activities. In the absence of any volcanic eruption, the SO2 concentrations showed day-to-day variations, with hourly-mean concentrations varying between 1 and 20 μg/m3. These values correspond to the background concentrations for this coastal rural location and might as well come in part from non-volcanic origin. The SO2 concentrations however were increased by a factor ~ 10–30 (with a maximum hourly-mean concentration of 2486 μg/m3), during the April–May 2007 eruption. Hereby, several statutory thresholds were exceeded particularly at the Tremblet location. Indeed, the information and recommendation threshold (350 μg/m3/h) as well as the alert threshold (500 μg/m3 on 3 h consecutively) were exceeded 32 and 5 times respectively at this site. These measurements point out that during the times of eruptions with significant amounts of degassing, the threshold values for protecting the ecosystems and the air quality for human health were exceeded at several inhabited locations, with implications for human health and environmental changes, on local-to-regional scale.