How bushfire smoke traveled around the world
A study uncovers how some Australian fires produced a spreading
stratospheric haze rivaling that of a volcanic eruption

Date:
March 18, 2021
Source:
Weizmann Institute of Science
Summary:
Scientists managed to track puzzling January and February 2020
spikes in a measure of particle-laden haze to bushfires, and then
uncovered the 'perfect storm' of circumstances that swept the
particles emitted from those fires into the upper atmosphere and
spread them over the entire Southern Hemisphere.



FULL STORY ==========================================================================
It's not just how hot the fires burn -- it's also where they burn that
matters.

During the recent extreme fire season in Australia, which began in
2019 and burned into 2020, millions of tons of smoke particles were
released into the atmosphere. Most of those particles followed a
typical pattern, settling to the ground after a day or week; yet the
ones created in fires burning in one corner of the country managed to
blanket the entire Southern hemisphere for months. A pair of Israeli
scientists managed to track puzzling January and February 2020 spikes in
a measure of particle-laden haze to those fires, and then, in a paper
recently published in Science, they uncovered the "perfect storm" of circumstances that swept the particles emitted from those fires into
the upper atmosphere and spread them over the entire Southern Hemisphere.


========================================================================== Particles reaching the stratosphere -- the upper layer of the atmosphere
- - most often get there through volcanic eruptions. The ash emitted in
the more extreme eruptions dims the sun and cools the planet, as well as producing spectacular sunsets. Prof. Ilan Koren of the Weizmann Institute
of Science's Earth and Planetary Science Department, who conducted
the study together with his former student, Dr. Eitan Hirsch, now the
Head of the Environmental Sciences Division at the Israel Institute for Biological Research in Ness Tziona, had noticed an extreme increase in
a satellite-based measure of particle loading in the atmosphere called
AOD -- or aerosol optical depth. In January 2020, those measurements,
plotted in standard deviations, showed a deviation three times the normal
-- some of the highest readings ever obtained, higher even than those from
Mt. Pinatubo in 1991. But the timing did not coincide with any volcanic activity. They wondered if fires might be to blame, even though it is rare
for the smoke from fires to escape the lower layer of atmosphere known as
the troposphere in significant amounts. The troposphere extends from the
ground to a height of several kilometers, and if smoke particles manage
to rise that high, they hit an inversion layer called the tropopause that
acts as a sort of ceiling between the troposphere and the stratosphere.

Working backwards and using data from several satellites, including,
in addition to AOD, LIDAR readings that revealed how the particles were distributed vertically in "slices" of atmosphere, the two were able to
prove that the source of the spikes was bushfires -- specifically those
burning in Southeastern Australia. Further analysis of satellite data
revealed the broad band of haze in the stratosphere spreading to cover the Southern hemisphere, peaking from January to March and persisting through
July; reaching all the way around and back to Australia's west coast.

How did these smoke particles penetrate through the tropopause ceiling
and why did they come from these fires and not the others? One clue,
says Hirsch, lay in another, distant forest fire that had occurred
several years ago in Canada.

Then, too, high AOD levels had been recorded. Both of these fires occurred
in high latitudes, away from the equator.

The height of the troposphere shrinks at these latitudes: Over the tropics
its upper ceiling can reach up to 18 km above the surface, while somewhere above the 45th parallel -- North and South, it takes a sudden step down
to around 8- 10 km in height. So the first element enabling the particles' trans-layer flight was simply having less atmosphere to cross.

Pyrocumulus clouds -- clouds fueled by the fires' energy -- were
considered as a means of transporting smoke to the stratosphere. However,
when inspecting the satellite data, Hirsch and Koren noticed that
pyrocumulus clouds formed only over a small fraction of the fires'
duration, and they were mostly seen over fires burning on the central part
of the coast. In other words, these clouds could not explain the large
amounts found to be transported to the stratosphere, and an additional mechanism for lifting smoke downwind from the sources was missing.

This brings up the second element: the weather patterns in the strip
known as the mid-latitude cyclone belt that runs through the southern
end of Australia, one of the stormiest regions on the planet. The smoke
was first advected (moved horizontally) by the prevailing winds in the
lower atmosphere to the Pacific Ocean, and then some of it converged
into the deep convective clouds there and was lifted in the clouds'
core into the stratosphere. An interesting feedback mechanism known as
"cloud invigoration by aerosols" can further deepen the clouds. In a
previous study, the authors had shown that in conditions such as the
pristine environment over the Southern Ocean, the convective clouds are "aerosol limited." The elevated smoke levels could thus act as cloud condensation nuclei, allowing the clouds to develop deeper and thus
increasing the number of clouds that able to penetrate the tropopause
and inject the smoke in the stratosphere.

Up in the stratosphere, the particles found themselves in a different
world than the one they had just left. If below they were at the mercy
of mixing and churning air currents, up on top the air moves in a steady, linear fashion.

That is, there was one strong current, and it was moving them eastwards
over the ocean to South America and back over the Indian Ocean toward Australia, and slowly settling around the entire hemisphere. "People
in Chile were breathing particles from the Australian fires," says
Hirsch. By sailing on an endless air current, these particles remained
airborne for much longer than lower atmosphere smoke particles.

"For people on the ground, the air may have just seemed a bit hazier
or the sunsets a bit redder. But such a high AOD -- much, much higher
than normal - - means sunlight was getting blocked, just as it does
after volcanic eruptions," says Koren. "So the ultimate effect of that
smoke on the atmosphere was cooling, though we still do not know how much influence that cooling and dimming may have had on the marine environment
or weather patterns.

"There are always fires burning in California, in Australia and in the tropics," he adds. "We might not be able to stop all of the burning, but
we do need an understanding that the precise locations of those fires
may grant them very different effects on our atmosphere." Prof. Ilan
Koren's research is supported by the de Botton Center for Marine Science;
the Sussman Family Center for the Study of Environmental Sciences;
the Dr. Scholl Foundation Center for Water and Climate Research; the
Ben May Center for Chemical Theory and Computation; Scott Eric Jordan;
the Yotam Project; the estate of Emile Mimran; and the European Research Council. Prof. Koren is the incumbent of the Beck / Lebovic Chair for
Research in Climate Change.

Video: https://www.youtube.com/watch?v=x6vjPauMi_0 ========================================================================== Story Source: Materials provided by Weizmann_Institute_of_Science. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Eitan Hirsch, Ilan Koren. Record-breaking aerosol levels explained
by
smoke injection into the stratosphere. Science, 2021 DOI: 10.1126/
science.abe1415 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/03/210318142440.htm

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