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2021-04-02

Stratospheric aerosols originate for the most part from volcanic eruptions and have a large influence on the Earth’s atmosphere and climate. BIRA-IASB is involved in the monitoring of these aerosols, delivering high quality data to centralised databases like the Copernicus programme, to be integrated in climatological models, among other things.

Meet Mt. Pinatubo

For half a millennium, the Pinatubo volcano - located in the western part of the Philippines - had been lying dormant, peacefully allowing forest growth to cover its flanks. However, 30 years ago, on April 2nd 1991, the volcano suddenly awoke from its torpor. The mounting magmatic pressures inside Earth’s crust could no longer be contained. After months of regular earthquakes, the northern flank of the mountain finally cracked open when rising magma came into contact with subsurface water, causing it to instantly turn to steam, causing a violent explosion.

Pinatubo crater
Aerial view to the south of Pinatubo crater (2.5 km wide) showing the start of a small
explosion on August 1. Credit: T.J. Casadevall, U.S. Geological Survey

It was only a first, relatively small eruption though with respect to what had to come. A few weeks later, scientists arrived to monitor the new activity more closely. They registered the continuing earthquakes, and in May, measurements of sulphur dioxide gas (SO2) escaping from the fissure began. The SO2 emissions were quickly increasing until the end of May, from 500 metric tons to 5000 metric tons, and then substantially decreased again, suggesting that something was blocking the degassing process and causing a build-up deep of gas underground.

In June, small eruptions started occurring again, building up to the true destructive potential of the Mt. Pinatubo’s eruption, which the volcano displayed on June 15, 1991. The gas bubbles that had been rising and accumulating within the magma chamber and column for the past weeks exploded, forcing about 5 km3 of matter into the stratosphere (eventually reaching altitudes up to 35 km) over a period of nine hours, and resulting in the collapse of the mountain.

The immediate consequences of the eruption luckily were limited by the preventive measures that had been taken, though many lives were still lost (an estimated 300-350 deaths were directly caused by the eruption, due to roofs collapsing under the weight of the ashfall) and the material damage was substantial. Heavy rains that created destructive flows of volcanic debris (called Lahars) continued to plague the surrounding population in the ensuing years, adding up to a total death toll estimated between 750 and 850.

Impact of the Mt. Pinatubo eruption on the chemical composition of the stratosphere

The scope of the consequences went much further than that, though. The Mt. Pinatubo eruption of June 15, 1991, was the largest eruption in the last 100 years. The amount of sulphur injected - and dispersed in the entire stratosphere over a few months - was so large that it disrupted the propagation of light in the atmosphere for more than 5 years. In addition to this direct impact, which resulted in a cooling of the atmosphere by about 0.2 to 0.7°C (for the global troposphere), the volcanic cloud had multiple secondary effects. There were changes in the atmospheric dynamics and a huge impact on the chemistry and the chemical composition of the atmosphere (among which a decrease in the ozone layer of about 25%) and eventually, on the whole Earth climate system.

The plumes were mostly composed of ash and sulphur in the form of sulphur dioxide (SO2), but also a whole range of other chemicals, including water vapour (H2O), were transported high up into the normally dry stratosphere. The extra water vapour allowed the SO2 to oxidise into sulphuric acid (H2SO4), and to form condensation nuclei for sulphate aerosols (fine droplets in suspension in the air) within weeks.  In fact, volcanic eruptions, because of their violent power, are a very effective way of providing the necessary ingredients for the formation of aerosols in the stratosphere, a very stable and difficult layer to penetrate.

Volcanic eruptions influence global temperatures

Pinatubo from space
Pinatubo eruption seen from space. Credit: NASA/JSC Digital Image Collection

The stratospheric aerosol layer that Mt. Pinatubo created was able to block substantial amounts of solar radiation from reaching Earth’s surface, resulting in a heating of the stratosphere by 3.5°C, and a cooling of the troposphere of the northern hemisphere by 0.2 to 0.7°C. These may seem like small numbers, but in the atmospheric system, small changes have big consequences. It led to an increase in the temperature difference between the equator and the poles, which resulted in a strengthening of the atmospheric dynamic at the global scale, of the polar vortex at mid-latitudes (a closed wind stream loop that isolates the polar atmosphere for several months in winter), and of a suite of other atmospheric oscillation phenomena.

At first, it was suggested that the Pinatubo eruption of 1991 also caused the unusually warm winter of the same year (in the northern hemisphere). However, recent studies performed with climate modelling fail to reproduce this effect, showing that the two events were likely unrelated.

Volcanoes have always been important drivers of climate changes in Earth’s history, since long before human activity became a factor. This is why we need to monitor the nature and volume of the matter that volcanoes eject, but also the frequency of eruptions.

Monitoring stratospheric aerosols

We have discussed the effects that SO2 and aerosols of volcanic origin have on the Earth’s climate and on the ozone layer, but these are not the only dangers. Volcanic eruptions like that of Mt. Pinatubo also form a direct threat to aviation. Both the ash and gases that are released can cause substantial damage.

All these reasons illustrate the necessity for high quality data on both stratospheric and tropospheric sulphate aerosols. However, compared to atmospheric gases, aerosols are a particularly challenging object of study. The way they scatter or absorb light is dependent on many variables like their size, composition, history and origin (there are other sources like desert dust, anthropogenic pollution etc., though volcanism remains the main source in the stratosphere). Atmospheric conditions or local features of atmospheric dynamics also influence their microscopic characteristics.

This variability and the varying characteristics of the aerosol particles make it difficult for scientists to decipher what the satellite observations actually represent. Researchers need to piece together what is happening in the atmosphere based on incoming streams of numbers, which represent the light signatures that remote-sensing satellite instruments have captured from their orbits.

At BIRA-IASB, we work on translating satellite data into meaningful information that can be used for other applications. We provide long-time series, global datasets, or more detailed local data for various atmospheric species, including stratospheric aerosols, to the European Copernicus programme. This information is centralised in international databases after precise and detailed characterisation, to be integrated in climate models, among other things.

Grpah
Figure 3 from Bingen et al., JGR, 109, 2004. This graph shows a time series produced from measurements of the satellite instrument "SAGE II", and representing the evolution in time and latitude of the aerosol particle density in number per cm-3, between 1985 and 2000, at an altitude of 22.5 km. The letters "R" (Nevado del Ruiz), "K" (Kelut) and "P" (Pinatubo) indicate the main volcanic eruptions during this period. The logarithmic colour scale shows that the density can vary over 3 orders of magnitude. The white areas correspond to a lack of measurements, due to “orbit gaps” (the satellite's orbit does not cover the area concerned), with the exception of the extended area over the tropics between June 1991 and mid-1992: the volcanic cloud, whose reddish-orange tones show the propagation towards the poles, was so dense during these measurements in the tropics that the light reflected back to the satellite was too weak to be correctly measured. The resulting lack of measurements means that the eruption is still studied using models and that there are still uncertainties as to its exact size and course.

References

 

News image 1
News image legend 1
View from Clark Air Base of the major eruption of Mt. Pinatubo on June 15, 1991. The June 15 eruption lasted nine hours, sent debris, ash and gases into the stratosphere, up to 35 km, generated voluminous pyroclastic flows (extremely fast “avalanches” of very hot gas and ash), and left a caldera (the crater subsequent to the collapse of the volcano) 2.5 km in diameter in the former summit region. Credit: R. Lapointe, U.S. Air Force
News image 2
News image legend 2
A 9 cm-thick layer of ash covering Clark Air Base, 25 km east of Mt. Pinatubo. Credit: R.P. Hoblitt
News image 3
News image legend 3
Space Shuttle (Mission STS-43) photograph of the Earth over South-America taken on August 8, 1991, showing double layer of Pinatubo aerosol cloud (dark streaks) above high cumulonimbus tops. Credit: NASA