On January 15th, 2022, the Hunga Tonga Hunga Ha'apai volcano in the nation of Tonga violently
exploded with an energy equivalent to the detonation of between 9 million and 37 million
tons of TNT.
As a large swath of the 4 kilometer or 2.5 mile long island was instantly destroyed,
the tallest eruption plume ever recorded shot to a height of 58 kilometers or 190,000 feet.
This plume expanded outwards in all directions and within an hour covered an area approximately
the size of France.
While this eruption had many long term effects such as deepening the volcano's caldera by
750 meters or 2,460 feet, long reaching tsunami damage, and the destruction of all life in
a 15 kilometer radius around the volcano, there is a lesser noted effect which was equally
spectacular albeit nondestructive.
Due to the large amount of fine particulates, sulfur dioxide, and other aerosols sent high
into the atmosphere, the eruption has caused some beautiful and in some cases eerie sunsets.
These sunsets are unusually bright and vivid, take up larger sections of the sky than typical
sunsets, and contain unusual colors typically not seen such as purple.
While these sunsets are being reported around the globe, they are most prominent in the
southern hemisphere, specifically around Tonga and New Zealand.
However, since much of the initial VEI 6 eruption plume drifted to the west, places such as
Johannesburg in South Africa are also seeing these highly unusual sunsets, although not
to the same extent as observed in Tonga.
Generally speaking, the two dominant gasses released by large volcanic eruptions are carbon
dioxide and sulfur dioxide, the latter of which is more important for causing unusual
sunsets.
Although sulfur dioxide is emitted by human activities such as ore smelting, its effects
are stronger when placed at a higher level in the atmosphere via a highly explosive volcanic
eruption.
There, much of the sulfur dioxide slowly combines with oxygen and water to produce sulfuric
acid.
The more particles light travels through in the atmosphere, the more scattered it becomes.
As an aerosol high in the atmosphere, sulfuric acid and other secondary sulfate compounds
scatter a larger percentage of incoming light back to earth as light now has to pass through
more molecules than usual.
Thus, whereas at noon the sky would normally be blue, and yellow orange and red at sunset,
if a layer of volcanic aerosols were present the sky would be a more prominent red and
orange as other wavelengths had scattered by then.
The cause for the high altitude purple coloration at sunsets after volcanic eruptions relates
to already scattered blue light scattering downwards once again at the aerosol layer,
and combining with red to form the color purple.
This effect scales to a certain point, but generally is only noticed in the aftermath
of a volcanic eruption rated as a 4 or higher on the 0 to 8 volcanic explosivity index.
Since 2008, at least 5 other volcanoes have also caused unusually colored sunsets.
In each case, the phenomena lasted for only a few weeks.
However, when an eruption 1 or 2 orders of magnitude larger occurs, the effect can last
for months to years, even up to a decade.
This was the case in the aftermath of the 1815 eruption of Tambora, or the 1883 eruption
of Krakatau.
We know that these colorful sunsets occurred as not only were they widely documented in
literature, but famous painters made pieces of artwork which featured them.
For example, Edvard Munich painted "The Scream" in 1893 which featured an unusual sunset caused
by Krakatau's explosive eruption a decade earlier.
The Hunga Tonga Hunga Ha'apai eruption was initially not expected to create unusual sunsets
as it did not eject a similarly large amount of sulfur dioxide when compared to other recent
voluminous eruptions which produced the phenomena.
However, the Hunga Tonga eruption was the only eruption of this group to largely occur
below sea level, and its plume was unusually white suggesting a composition unusually rich
in water and steam.
Thus, much of the sulfur dioxide that normally would have risen high into the atmosphere
instead was almost immediately converted into sulfuric acid and other secondary compounds.
In my opinion, we should continue to see these unusual sunsets for 6 to 18 months following
the eruption, as it represented the largest volcanic aerosol event since Pinatubo erupted
in 1991.
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