Volcanic Emissions

During the Mt. Kelud Feb 2014 eruption the ash cloud was detectable on 13-14 Feb in the infrared with the reverse absorption technique by, for example, the Advanced Very High Resolution Radiometer (AVHRR). The Infrared Atmospheric Sounding Interferometer (IASI) observed the ash cloud also on 15 Feb when AVHRR did not detect any ash signal. The differences between ash detection with AVHRR and IASI are discussed and the reasons for the differences  supported with radiative transfer modelling. The effect of conccurent ice clouds on the ash detection and the ash signal in the IASI measurements is demonstrated. Specifically, a radiative transfer model is used to simulate IASI spectra with ash only, with ice cloud only and with both ash and ice clouds. It is shown that modelled IASI spectra with ash and ice clouds better reproduce the measured IASI spectra than ash only or ice only modelled spectra. The ash and ice modelled spectra that best reproduce the IASI spectra contain less ash than the ash only spectra that come closest to reproducing the measured spectra. Implications for remote sensing of volcanic ash clouds are discussed.

Measurements of volcanic sulphur dioxide (SO₂) emissions can offer critical insight into the current and near-future activity of volcanoes, however, the majority of active volcanoes lack regular ground-based monitoring. Different satellite spectrometers are now used to observe SO₂ emissions from space. One such instrument, the Infrared Atmospheric Sounding Interferometer (IASI) has been used to successfully quantify SO₂ emissions and altitudes from several eruptions since 2007.

In this study, we use a rapid linear retrieval algorithm as a global survey tool to show that IASI observations detect SO₂ emissions from volcanic eruptions, certain persistently degassing volcanoes, and anthropogenic sources over the IASI time series.  An iterative retrieval is used over Ecuador and Northern Kamchatka to explore how trends in SO₂ emissions observed by IASI compared to reported changes in the level of volcanic activity.

The IASI retrieval is able to quantify tropospheric SO₂ from multiple volcanoes in both regions across the period sampled. Over Ecuador, Tungurahua showed the most persistent signal, with a strong correlation (up to 0.95) in relative emission rates between quiescent and active periods when comparing IASI with ground-based and OMI datasets. Over Kamchatka, close proximity of volcanoes coupled with large explosive plumes made it challenging to attribute SO₂ to individual volcanoes, but nonetheless, IASI detected clear peaks in SO₂ emissions coincident with reports of elevated volcanic activity. The Kamchatka example is the first time a long-term satellite survey has been conducted in the region, and highlights the value of infrared satellite spectrometers such as IASI in regions where short wavelength observations are limited.

Due to the large emission of gases and particles into the atmosphere, the volcanic eruptions are one the most important sources of natural pollution. The detection and retrievals of volcanic clouds is important because of their effects on the environment, climate, public health and in particular to aviation. About this latter, it is well known that the volcanic ash clouds represent a severe threat for aviation safety because they can cause abrasion of turbine blades, windscreens, fuselage, loss of power and in extreme cases the failure of the turbine engines. The volcanic activity is monitored worldwide by using both satellite and ground-based instruments with advantages and drawbacks. Because today doesn’t exist a single system able to give a comprehensive description of a particular phenomenon, a multi-sensor approach is needed.

The Multi-platform volcanic Ash Cloud Estimation (MACE) procedure exploit the complementarity between geostationary, polar satellite sensors and ground based measurements to improve the ash detection and retrieval and to fully characterize the volcanic ash clouds from the source to the atmosphere. The proposed method integrates in a novel manner the volcanic ash retrievals at the space–time scale of typical geostationary observations using both the polar satellite estimations and in-situ measurements. The typical ash thermal infrared (TIR) retrieval will be integrated by using a wider spectral range from visible (VIS) to microwave (MW) and the ash detection will be extended also in case of cloudy atmosphere or steam plumes. MACE has been developed during the FP7 APhoRISM project aimed to develop innovative products to support the management and mitigation of the volcanic and the seismic crisis.

The MACE approach has been tested on different recent eruptions, representative of different eruption styles in different clear or cloudy sky conditions.

MACE will be also suitable to be implemented in the next generation of ESA Sentinels satellite missions.

The global stratospheric aerosol layer (‘Junge layer’) is an important component influencing the radiation budget of the earth’s atmosphere. Its modulation by strong volcanic eruptions has a  direct influence on climate by increasing the albedo in the visible spectral range. Furthermore, also during periods of low volcanic activity, a moderate increase of the background aerosol layer is discussed as part of the explanation for a slowdown in the rise of global temperatures (the so-called global warming hiatus) since the turn of the millennium. This negative radiative forcing of stratospheric aerosols is the reason for proposals to artificially manipulation of the Junge layer by injection of sulfur into the stratosphere. Thus, a comprehensive understanding of the sulfur cycle is important for analyses and predictions on basis of climate model calculations.

As baseline for model evaluation and process studies, it is essential to obtain information about the global distribution and temporal evolution of the key components involved, namely sulfur dioxide (SO₂), carbonyl sulfide (COS) and the mass of sulfate aerosols. Measurements of these parameters in the upper troposphere and stratosphere are, however, sparse.

Due to their observation geometry in combination with a broad spectral coverage and a high spectral resolution, FTIR limb-sounding measurements are especially suited to obtain altitude resolved information of a large variety of atmospheric trace gases and particles.  Thus, using the observations by MIPAS on Envisat it is possible to derive global distributions of SO₂, COS and aerosol mass. In case of SO₂, for the first time a global picture of the vertically resolved distribution of SO₂ between 15 and 45 km altitude has been obtained. MIPAS SO₂ measurements after volcanic eruptions allow to calculate vertically resolved lifetimes of this gas and thereby evaluate the results derived from satellite based nadir measurements in the infrared and UV/Vis spectral range. MIPAS COS observations allow to analyse its global source and sink processes which are not well understood due to the extremely sparse global distribution of ground-based observations. Finally, the simultaneous determination of aerosol mass together with SO₂ and COS from MIPAS spectra will allow to close the budget between source gases and particulate matter.

Photogrammetric methods can improve and complement the volcanic ash cloud top height (ACTH) estimates provided by other techniques currently used to retrieve the cloud top height from space-based measurements. The multiangular sensors on board some satellites in principle could also be used for photogrammetrical measurement, but since volcanic ash clouds can move with high velocities (over several m s^-1) these instruments are unsuitable for accurate ACTH estimation. Therefore we propose a novel application of a method based on the parallax between data acquired by SEVIRI and MVIRI instruments on board MSG and METEOSAT7 geostationary platforms respectively. This pair of sensors provides optimal viewing geometry to apply the parallax method. We used a combination of MSG SEVIRI HRV (1000 m nadir spatial resolution, 5 min temporal resolution) and MVIRI VIS (2500 m nadir spatial resolution, 30 min temporal resolution) bands. Since MVIRI and SEVIRI acquisitions are not simultaneous, we accounted for the wind advection by considering two sequential SEVIRI images (i.e. the SEVIRI images collected immediately before and after each MVIRI image) and computing the ash cloud position at the time of MVIRI image acquisition by interpolation. We chose to use the visible channels for this pilot study to maximise the spatial resolution and achieve the best ACTH estimation accuracy (~2 km) based on available volcanic eruption data of GEO sensors pairs. However, the new forthcoming meteorological sensors in the geostationary orbits will have finer spatial and spectral resolution and better acquisition repetition cycle than the currently operational systems. Expected relevant improvements will also apply to the thermal infrared channels commonly used for volcanic ash cloud detection and retrieval. Moreover, the GEO observation capability in the VIS/IR will rapidly increase in the near future allowing continuous global coverage and optimal viewing geometry for the application of the proposed parallax-based ACTH estimation. The method has been applied to the Mt. Etna volcano (Sicily, Italy) lava fountain event occurred on November 23, 2013 at resulting in an estimated ash cloud-top height of ~8 km.