Chapter III: Quantitative measurements from space
3. Quantitative measurements from space
Volcanic eruptions can have huge impacts on the economy, although they are relatively rare compared to other high-impact natural hazards. For instance, the eruption of the Eyjafjallajökull volcano in 2010 forced the cancellation of about 100 000 flights and generated a EUR 1.4 billion loss for airline operators.
It is actually very difficult for pilots to identify volcanic ash clouds, especially during nighttime, as airplanes are not normally equipped with instruments to detect ash. However, knowledge of the concentrations, vertical extent and particle size of ash is crucial for air traffic security like several incidents in the past have shown.
On 24 June 1982, British Airways Flight 009 en route from London Heathrow to Auckland was flown by the City of Edinburgh, a Boeing 747-236B registered as G-BDXH. The aircraft flew into a cloud of volcanic ash thrown up by the eruption of Mount Galunggung around 110 miles (180 km) south-east of Jakarta, Indonesia, resulting in the failure of all four engines. Partly because the event occurred at night, obscuring the cloud, the reason for the failure was not immediately apparent to the crew or air traffic control. The aircraft was diverted to Jakarta in the hope that enough engines could be restarted to allow it to land there. The aircraft glided out of the ash cloud, and all engines were restarted (although one failed again soon after), allowing the aircraft to land safely at the Halim Perdanakusuma Airport in Jakarta. Source: Wikipedia
If you want to know more about this incident, watch this documentary: https://www.youtube.com/watch?v=YYwN1R8hVsI
As a consequence of this and similar incidents, Volcanic Ash Advisory Centers (VAAC) have been created to monitor the propagation of volcanic ash clouds using satellite data as a primary source of information.
Currently, two types of passive satellite instruments provide quantitative measurements of trace gases such as SO2 emitted by volcanic eruptions. The first category of satellite sensors operates in the UV and visible spectrum (e.g., OMI, GOME-2 and TROPOMI) and the other type measures the infrared spectrum (e.g., IASI and AIRS). The tools best suited for volcanic ash cloud height assessment are active satellite instruments using lidar technology, such as CALIOP.
In the following section you will learn more about the categories of satellite sensors that are used to provide quantitative information on volcanic ash and SO2 clouds.
3.1 Optical absorption spectrometers
Optical spectrometers use reflected short wave solar radiation to monitor the atmospheric composition. One instrument in this category is the Global Ozone Monitoring Experiment-2 (GOME-2) instrument. It is an optical absorption spectroscope onboard the MetOp satellite series that measures reflected and scattered sunlight from the atmosphere and the Earth's surface in visible and ultra-violet bands. Its main purpose is monitoring atmospheric ozone concentration, but it is also able to detect aerosols and atmospheric trace gases like SO2. The horizontal resolution of the instrument is 80 by 40 km, its vertical profiles are representative of the bottom 50 km of the atmosphere, and the instrument takes three days to cover the entire globe.
In the example below, the GOME-2 instrument detected a SO2 plume emitted from Raikoke volcano in June 2019. The AC SAF SO2 product (Figure 21) shows the very small expulsion of SO2 on 21 June 2019.
Figure 21: MetOp-A/B GOME-2 SO2 concentration, 21 June 2019. © AC SAF
The Ozone Monitoring Instrument (OMI) is part of the payload on NASA's Aura satellite, which has a polar sun-synchronous orbit. The instrument is a spectrometer that measures ultraviolet and visible radiation reflected by the atmosphere and the Earth's surface.
OMI distinguishes between aerosol types, such as smoke, dust, and sulfates (Figure 22), and measures cloud pressure and coverage.
Figure 22: OMI/Aura average SO2 column, Eruption of Mount Nyamuragira, 2006. © S. Carn, UC
Another instrument of a similar type is the TROPOMI instrument (TROPOspheric Monitoring Instrument) on the Sentinel-5P satellite. Compared to OMI, TROPOMI has extended scanning capabilities into the near-infrared part of the spectrum. The spectrometer monitors atmospheric trace gases such as ozone, methane and SO2.
As displayed in the image below (Figure 23), TROPOMI scanned the atmosphere next to Mount Sinabung on 19 February 2018 at 07:00 UTC, around five hours after its initial eruption. The volcanic plume drifted in multiple directions at lower levels, possibly due to inconsistent wind directions.
Figure 23: Sentinel-5P TROPOMI SO2 column density, Mount Sinabung eruption on 19 February 2018. © DLR/BIRA/ESA
3.2 Infrared spectrometers
Infrared spectrometers are passive instruments that measure the thermal radiation emitted by the atmosphere to retrieve information on its composition.
An example of this type of instrument is the Infrared Atmospheric Sounding Interferometer (IASI). It is a Fourier Transform Spectrometer based on a Michelson Interferometer coupled with an integrated imaging system that observes and measures infrared radiation in the spectral range of 3.62 to 15.5 µm. This enables the instrument to derive temperature and water vapor profiles for the troposphere and the lower stratosphere, as well as measure quantities of ozone, carbon monoxide, methane and other compounds such as SO2. Besides quantitative information, IASI also provides an estimate of the height of the SO2 plume.
SO2 observations with the IASI instrument are defined by the variation of brightness temperature measurements in the IR window region around 8.5 microns (Figure 24).
Figure 24: The Metop-C IASI image from 28 November 2022 shows the height of the SO2 plume at about 6 km in the immediate vicinity of the volcano (left). On 1 December, over the USA, the plume height was estimated to be at about 8-15 km (right). © ULB/ LATMOS
The Atmospheric InfraRed Sounder (AIRS) is an instrument on NASA's Aqua satellite and is complemented by the Advanced Microwave Sounding Unit (AMSU-A). An advanced, high-resolution spectrometer inside the instrument divides the infrared energy into discrete wavelengths.
AIRS measures incoming infrared brightness from the Earth's surface and the atmosphere. Atmospheric profiles of temperature, moisture and trace gases are derived from this data for climate and weather prediction applications. The sounder measures concentrations of trace greenhouse gases such as ozone, carbon monoxide, carbon dioxide, methane and SO2. It can follow giant plumes of these gases moving across the planet, allowing scientists to better monitor pollution transport patterns (Figure 25).
Figure 25: AIRS SO2 product from the Aqua satellite, Jebel at Tair eruption on 2 October 2007. © F. Prata.
3.3 Space based Lidar instruments
The vertical extent of volcanic ash plumes is an important factor in aviation. Lidar instruments such as CALIOP, which are carried by LEO satellites, are active instruments that provide a vertical profile of clouds and aerosols.
The main purpose of lidar instruments is measuring cloud height from above. While this works well for regular clouds that consist of water droplets or ice crystals, ash clouds can be much more dense, in which case the lower atmospheric levels are shielded by particles in the top layers of the ash plume (Figure 26).
Figure 26: The eruption plume of Mount Kelud on 13 February 2014. © NASA Earth Observatory, Jesse Allen.
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