On August 4, 2020, a massive explosion blasted deadly waves through
downtown Beirut.
Then, video of the fireball rippled around the world almost as quickly.
Now, details of the blast that started in a fireworks storage area by a
small storage building at the end of Beirut pier trickle in as the
world waits to hear what the final death, injury, and destruction
tallies will be. However, in a way, the world already has some idea what
to expect, because similar blasts have occurred before.
As a biomedical engineer with a doctorate in the patterns of injury
and trauma that follow an explosion, scraping together information from
accidental blasts is part of my daily work. The more mundane explosions
are rarely this size, but the same principles of physics and chemistry
apply. Science, along with a few case studies from history, let me do
some preliminary calculations to puzzle out this explosion, too.
The Frisky Chemistry of Ammonium Nitrate
Every fire is a rearrangement of molecules, and an explosion is
basically a fire turbocharged into a hyper-energy-fueled frenzy.
Unstable structures barter and swap atoms with one another until all of
them, happy with their trades, blissfully settle into more relaxed,
lower energy states, like rocks reaching the bottom of a hill. But their
excess energy has to go somewhere. In a campfire, where the chemical
reactions are facilitated in a leisurely way by the oxygen in the air
alone, energy is released slowly as enjoyable levels of heat and light.
In an explosion, however, the devilish little instigator that is oxygen
shoves the process into overdrive.
Early reports of the blast revealed that the building that sparked
the eruption may have been storing large quantities of ammonium nitrate,
a flammable chemical that has relatively harmless manifestations as
fertilizer but has also been experimented with as a rocket fuel. Oxygen
is the key to ammonium nitrate’s
deadly habit of exploding, and given that
47 known,
major, accidental ammonium nitrate explosions have occured in the last
century, it is undeniably a habit. “Ammonium” is a nitrogen atom with
four hydrogens, written NH
4+, whereas the “nitrate” part of the mix is a nitrogen with three oxygens, NO
3-. Under boring everyday conditions, the
+ of the ammonium and the
–
of the nitrate pull the two molecules into a harmless hug, but when you
add spark—or a firework—the molecules realize that their very atoms can
get a little friskier and convert into something completely new.
When ammonium nitrate is manufactured as fertilizer, it is mixed with other chemicals that usually
stop this reaction from happening, though as the 2013 explosion at the West Fertilizer Company proved, those chemicals are
not always successful. The first reports out of Beirut suggested fertilizer may have once again been the culprit. However,
photos shared on social media
showed bags marked “Nitroprill HD” supposedly being stored at the
Beirut pier, and some have speculated that if those photos are accurate,
Nitroprill may be a knockoff of the name-brand blasting agent Nitropril. Nitropril is
designed for use in coal mines,
so this particular breed of ammonium nitrate would not have been mixed
with quieting chemicals like a fertilizer would be; rather, it would
have been mixed to blow.
And nitrate, when mixed to blow, wants to ditch those little O’s.
It’s chemically unstable, meaning the bonds between the N and the O’s
vibrate with an unhappy level of physical tension. Overloaded with three
oxygens, NO
3- is eager to shove some onto any neighbor, and with a little bit of heat to get things moving, it will do so willingly. NH
4+ is all too happy to accept.
The chemical rearrangement of ammonium nitrate answers a lot of the
public questions about the videos, including the source of the startling
red color of the plume. One of the byproducts of NO
3- as it sheds all that oxygen is nitrogen dioxide, which has a logically obvious chemical structure of NO
2, and looks
deep, blood red.
Many explosive materials give off tints and hues during a blast that
suggest their chemical composition—chemical additives to color both
smoke and explosions have been around since
before the 1920s and are how we get different-colored fireworks and
signaling flares—and
it’s nitrogen dioxide that gives an ammonium nitrate explosion its
signature, ominous blood-like tone. A small blast can look subtle and
orange-ish, but on a large scale like at Beirut, the sunlight helps
deepen its hue.
According to Brad Wojtylak, a special agent bomb technician and
certified explosive specialist with the Bureau of Alcohol, Tobacco,
Firearms, and Explosives, when smoke plumes are large enough they begin
to catch the sunlight, and refraction will darken the normal colors
produced by any explosion. Wojtylak is not directly involved in the
Beirut investigation but has 16 years of experience investigating blast
accidents. He says as sunlight bounces around within the cloud of
contaminants, other, less determined wavelengths get refracted off in
different directions. When a smoke plume happens on such a large scale
only the longest wavelengths, the red shades, persevere all the way
through to the viewer on the other side. So, the natural reddish color
becomes even deeper, richer, darker than it would be for a small blast.
An explosive with a pure burn, like any explosive used in
military-grade weaponry, will produce smoke that looks equally pure:
snowy, billowy white, or sometimes a pale grey. But accidental
explosions are far less tidy, and their sloppy combustion also produces
ash, particulates, and gross black charred contaminated matter. This
black gunk billows into the sky along with the other byproducts,
coloring the smoke plume, like the charcoal residue left behind after
the more efficient parts of the campfire wood have burned away. To a
blast expert, the videos, with their roiling cloud of black and red
curling over the pier of Beirut, scream “ammonium nitrate.”
Not a Shock Wave
The videos also show an unnervingly uniform hemisphere of white
propagating outward from the blast site, a dome of vicious vapor that
eventually hurtles toward every person filming and announces its arrival
in the audio with a crash. This hemisphere is the pressure wave
produced by the explosion.
