Variability in persistent volcanic SO2 emissions detected by UV satellite measurements

Sulfur dioxide (SO2) is an atmospheric trace gas primarily produced by anthropogenic and volcanic emissions. It is a precursor of sulfate aerosols, which provide a negative feedback to global warming, whilst also being an air pollutant detrimental to human health and the environment. Ultraviolet (UV) satellite measurements from the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite have been used to construct a unique global inventory of volcanic and anthropogenic SO2 emissions since 2005, supplemented by other UV satellite data since 2012 (SNPP/OMPS, S5P/TROPOMI). This inventory provides new insight into decadal trends in volcanic and anthropogenic SO2 emissions. With increasingly effective pollution controls, global anthropogenic SO2 emissions have declined over the last decade, whereas the volcanic SO2 flux has remained relatively stable (~20 Tg/yr); hence the volcanogenic proportion of global SO2 emissions has increased. However, volcanic SO2 emissions since 2005 have been strongly influenced by a few major sources (Ambrym, Kilauea and Nyiragongo) that have all shown reduced emissions in recent years, with significant impacts on the total volcanic SO2 flux (a ~40% reduction since 2018). It remains unclear if this represents only a temporary reduction or if the current, lower level of volcanic SO2 emissions will persist in the coming years. To address this question, we analyze the variation in passive volcanic degassing from the major volcanic SO2 sources in the past decades using UV satellite measurements from OMI, OMPS, TROPOMI and other instruments operational since 1978 (including ERS/GOME [1996-2003] and TOMS [1978-2005]). We also consider the longevity of specific styles of volcanic activity that support high SO2 fluxes (e.g., active lava lakes) based on historical records of volcanism. The observed variability in the volcanic SO2 flux provides strong motivation for continuing and extending satellite-based SO2 emissions inventories into the past and future to track any significant variations in persistent volcanic degassing.