Over the last four decades, space-based nadir observations of sulfur dioxide (SO₂) proved to be a key data source for assessing the environmental impacts of volcanic emissions, for monitoring volcanic activity and early signs of eruptions, and ultimately mitigating related hazards on local populations and aviation. Despite its importance, a detailed picture of global SO₂ daily degassing is difficult to produce, notably for lower-tropospheric plumes, due largely to the limited spatial resolution and coverage or lack of sensitivity and selectivity to SO₂ of current (and previous) nadir sensors. We report here the first volcanic SO₂ measurements from the hyperspectral TROPOspheric Monitoring Instrument (TROPOMI) launched in October 2017 onboard the ESA’s Sentinel-5 Precursor platform. Using the operational processing algorithm, we explore the benefit of improved spatial resolution to the monitoring of global volcanic degassing. We find that TROPOMI surpasses any space nadir sensor in its ability to detect weak degassing signals and captures day-to-day changes in SO₂ emissions. The detection limit of TROPOMI to SO₂ emissions is a factor of 4 better than the heritage Aura/Ozone Monitoring Instrument (OMI). Here we show that TROPOMI SO₂ daily observations carry a wealth of information on volcanic activity. Provided with adequate wind speed data, temporally resolved SO₂ fluxes can be obtained at hourly time steps or shorter. We anticipate that TROPOMI SO₂ data will help to monitor global volcanic daily degassing and better understand volcanic processes and impacts.

Volcanic plumes are common and far-reaching manifestations of volcanic activity during and between eruptions. Observations of the rate of emission and composition of volcanic plumes are essential to recognize and, in some cases, predict the state of volcanic activity. Measurements of the size and location of the plumes are important to assess the impact of the emission from sporadic or localized events to persistent or widespread processes of climatic and environmental importance. These observations provide information on volatile budgets on Earth, chemical evolution of magmas, and atmospheric circulation and dynamics. Space-based observations during the last decades have given us a global view of Earth's volcanic emission, particularly of sulfur dioxide (SO₂). Although none of the satellite missions were intended to be used for measurement of volcanic gas emission, specially adapted algorithms have produced time-averaged global emission budgets. These have confirmed that tropospheric plumes, produced from persistent degassing of weak sources, dominate the total emission of volcanic SO₂. Although space-based observations have provided this global insight into some aspects of Earth's volcanism, it still has important limitations. The magnitude and short-term variability of lower-atmosphere emissions, historically less accessible from space, remain largely uncertain. Operational monitoring of volcanic plumes, at scales relevant for adequate surveillance, has been facilitated through the use of ground-based scanning differential optical absorption spectrometer (ScanDOAS) instruments since the beginning of this century, largely due to the coordinated effort of the Network for Observation of Volcanic and Atmospheric Change (NOVAC). In this study, we present a compilation of results of homogenized post-analysis of measurements of SO₂ flux and plume parameters obtained during the period March 2005 to January 2017 of 32 volcanoes in NOVAC. This inventory opens a window into the short-term emission patterns of a diverse set of volcanoes in terms of magma composition, geographical location, magnitude of emission, and style of eruptive activity. We find that passive volcanic degassing is by no means a stationary process in time and that large sub-daily variability is observed in the flux of volcanic gases, which has implications for emission budgets produced using short-term, sporadic observations. The use of a standard evaluation method allows for intercomparison between different volcanoes and between ground- and space-based measurements of the same volcanoes. The emission of several weakly degassing volcanoes, undetected by satellites, is presented for the first time. We also compare our results with those reported in the literature, providing ranges of variability in emission not accessible in the past. The open-access data repository introduced in this article will enable further exploitation of this unique dataset, with a focus on volcanological research, risk assessment, satellite-sensor validation, and improved quantification of the prevalent tropospheric component of global volcanic emission. Datasets for each volcano are made available at https://novac.chalmers.se (last access: 1 October 2020) under the CC-BY 4 license or through the DOI (digital object identifier) links provided in Table 1.

Through integration of multiple data streams to monitor volcanic unrest scientists are able to make more robust eruption forecast and to obtain a more holistic interpretation of volcanic systems. We examined gas emission and gas geochemistry and seismic and petrologic data recorded during the 2015 unrest of Cotopaxi (Ecuador) in order to decipher the origin and temporal evolution of this eruption. Identification of families of similar seismic events and the use of seismic amplitude ratios reveals temporal changes in volcanic processes. SO₂ (300 to 24,000 t/d), BrO/SO₂ (5–10 × 10⁻⁵), SO₂/HCl (5.8 ± 4.8 and 6.6 ± 3.0), and CO₂/SO₂ (0.6 to 2.1) measured throughout the eruption indicate a shallow magmatic source. Bulk ash and glass chemistry indicate a homogenous andesitic (SiO₂ wt % = 56.94 ± 0.25) magma having undergone extensive S‐exsolution and degassing during ascent. These data lead us to interpret this eruption as a magma intrusion and ascend to shallow levels. The intrusion progressively interacted with the hydrothermal system, boiled off water, and produced hydromagmatic explosions. A small volume of this intrusion continued to fragment and produced episodic ash emissions until it was sufficiently degassed and rheologically stiff. Based on the 470 kt of measured SO₂ we estimate that ~65.3 × 10⁶ m³ of magma were required to supply the emitted gases. This volume exceeds the volume of erupted juvenile material by a factor of 50. This result emphasizes the importance of careful monitoring of Cotopaxi to identify the intrusion of a new batch of magma, which could rejuvenate the nonerupted material.

Volcanoes have been found to display periodicities or cyclic trends in a wide range of phenomena. These include the eruptive activity itself, but also in the time series of geophysical and geochemical monitoring data such as volcanic degassing. Here, we test the existence of periodicities of volcanic degassing at 32 volcanoes using the time series of sulfur dioxide (SO₂) emission rates from data of the Network for Observation of Volcanic and Atmospheric Change (NOVAC). We use the Lomb‐Scargle periodogram to analyze the SO₂ data which allows efficient computation of a Fourier‐like power spectrum from unevenly sampled data. We were able to calculate False‐Alarm Probabilities in 28 of the 32 volcanoes, and we identified significant periodicities in the SO₂ emission rates in 17 of the 28 volcanoes. However, we find that most of these periodicities are also present in the plume speeds used to determine SO₂ emission rates. Periodicities at about 30–70, ∼120, and ∼180 days were identified at volcanoes located between 16°N and 16°S and are related to intraseasonality and interseasonality in global trade winds and not volcanic in origin. Periodicities between 30 and 70 days in both plume speed and SO₂ emission rates are associated to the Madden‐Julian Oscillation that is responsible for intraseasonal variability in the tropical atmosphere. Our study highlights the importance of using local wind data for deriving realistic SO₂ emissions and the identification of short‐term periodicity in volcanic behavior.

Changes in the emission rate of volcanic sulphur dioxide (SO${}_2$) are crucial parameters for identifying volcanic unrest and forecasting the eruptive activity. Ground-based ultraviolet (UV) remote sensing provides a near continuous record of the SO${}_2$ emission rate, with Differential Optical Absorption Spectroscopy (DOAS) being the preferred method for quantifying SO${}_2$ absorption from recorded spectra. However, retrieving accurate column amounts of SO${}_2$ using DOAS requires a complex fitting procedure that relies on user expertise for selecting suitable fit parameters and visually inspecting the fit results. We explore an alternative approach that exploits the well-defined spatial frequencies present in sky-scattered UV spectra. We use wavelet coherence to compare UV spectra recorded with calibration cells of known SO${}_2$ concentration in the wavelength–spatial frequency plane. Our findings reveal that the Magnitude-Squared Wavelet Coherence (MSWC) is inversely proportional to the SO${}_2$ concentration, suggesting that this relationship could be used to quantify volcanic SO${}_2$ in natural spectra. To validate this approach, we analyze UV spectra recorded by scanning-DOAS instruments from the Network of Volcanic and Atmospheric Change (NOVAC) at Masaya volcano, Nicaragua, and Soufrière Hills volcano, Montserrat. We observe a favourable comparison between the MSWC values we calculate and the slant column densities (SCDs) of SO${}_2$ obtained using the DOAS and iFit algorithms, respectively. We demonstrate the MSWC to be a robust indicator of SO${}_2$ which may potentially serve as a proxy for differential SCDs of volcanic SO${}_2$. The straightforward computation of the wavelet coherence between spectra offers an efficient means to identify spectra which contain the signature of the volcanic plume and an objective approach to validate results obtained using traditional fitting routines.

Marapi, Sumatra’s most persistently active volcano, exhibits frequent short-lived explosive activity and sporadic prolonged eruptions, often occurring with little to no detectable geophysical precursory signals. Despite advances in monitoring, our understanding of Marapi’s eruptive dynamics remains limited by a lack of information on its gas emissions. Here we present the first chemical dataset of Marapi’s volcanic gases measured using a Multi-component Gas Analyser System (Multi-GAS) from the rim of the active Verbeek crater during inter-eruptive activity in 2014 and 2018. Both datasets reveal gas compositions typical of magma-derived arc gases and a puffing degassing regime, with a periodicity of ~ 26 min in 2014 that decreased to 8–10 min in 2018. Notably, our 2018 measurements—performed just one month before two explosive outbursts on 27 April and 2 May 2018—show comparable SO₂/H₂S molar ratios (3.3 and 2.5) but a marked increase in total sulphur relative to CO ₂ , resulting in a CO ₂ /SO ₂ ratio more than three times lower than in 2014. This chemical shift is consistent with shallower magma degassing preceding the April and May 2018 explosive activity. Our findings underscore the importance of regular, ideally continuous, gas monitoring at Marapi to better resolve magmatic processes that drive its explosive activity. We also report the first CO ₂ /S _T gas ratio measured for a Sumatran volcano, providing further evidence for sulphur-limited degassing along the Sunda Arc compared with other subduction zones.