Following September 19's "Beirut Port Explosion Probe Gains Momentum With Extradition Request".
From Wired, August 6, 2020:
A blast injury specialist explores the chemistry—and history—of explosions like the one captured in videos that swept across the world.
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.
In 1917, an accidental detonation of six million pounds of hodgepodge high explosives in the harbor of Halifax, Nova Scotia, left a swath of wreckage that, at least until Tuesday, was the largest non-nuclear explosion ever created by humanity. As we learn more about Beirut, which could possibly challenge that record, the story of Halifax tells us what we might expect to learn about the ensuing trauma, and the modern cell phone videos, along with the blast physics gleaned by scientists in the intervening century, tell us why those patterns of trauma occurred in quite the way they did.
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 NH4+, whereas the “nitrate” part of the mix is a nitrogen with three oxygens, NO3-. 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, NO3- is eager to shove some onto any neighbor, and with a little bit of heat to get things moving, it will do so willingly. NH4+ 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 NO3- as it sheds all that oxygen is nitrogen dioxide, which has a logically obvious chemical structure of NO2, 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.
No, it’s not a shock wave. It’s a pressure wave, and that key difference affects the number of casualties expected. A shock wave goes from zero pressure to its absolute maximum pressure in literally zero seconds. The impact of a pressure wave is like hitting the ground after rolling down a steep cliff; the force of a shock wave is like hitting the ground after falling through the air and reaching terminal velocity. High explosives produce shock waves; low explosives, like ammonium nitrate, produce pressure waves, which have a bit of slope to their shape, a period of time over which the pressure increases more gradually.
Shocks, because of their fascinating and complex physics, travel faster than the speed of sound, and cause far more damage than pressure waves. Thankfully, we know this blast did not produce a shock because the speed of the water vapor-filled white dome can be measured.
The speed of sound in air is 343 meters per second. Based on the viewing angle and distinctive red chairs pictured in some of the later frames, I traced one of the Beirut videos posted by The Guardian to its filming location on the rooftop terrace of La Mezcaleria Rooftop Bar, and measured it to be 885 meters from the center of the blast. From that vantage, the pressure wave can be seen neatly traveling from the center of the blast first to the point halfway between the end of the pier and the edge of the long, massive grey grain silo building, a distance of 151 meters, then to the end of the pier, 262 meters, then eventually to La Mezcaleria.
By measuring the times at which the pressure wave reaches these landmarks on the video, we know that, as it blazed down the pier, its rampage occurred at a speed of only 312 meters per second. That’s slow for a bomb. Then by the time the audible crash and mayhem reached the formerly peaceful and picturesque outdoor bar, it had slowed to at most 289 meters per second. The pressure wave, slower than the 343 meters per second of sound, caused destruction, horror, confusion, shattered glass, torn apart flat surfaces, and disorientation for onlookers as their ears were subjected to the rapid pressure fluctuations. But a shock wave could have caused them to drop dead from lung trauma as they watched.
In the six million pound Halifax explosion of 1917, the propagation of the shock wave through downtown left a swath of fatalities reaching 1.5 miles from the center of the blast, killing an estimated 1,950 and leaving another 8,000 with devastating injuries. (The ships that exploded in the harbor were known to be carrying high explosives, which by their nature always make shock waves.) In Beirut, thankfully, while building damage has been reported up to 5.6 miles away, because the low-explosive ammonium nitrate made a pressure wave rather than a shock wave, the fatality estimates so far are still in the hundreds, even though the charge size was likely larger than the bomb in Halifax.
Thanks to modern technology that charge size can be calculated scientifically too, even while waiting for more complete information to trickle out, using the size of the telltale crater. Analysis of the aerial photographs of the pier shows a crater in the range of 120 to 140 meters in diameter; blast physics mixed with history tell us that to carve a chunk that size from the side of the planet requires a charge equivalent to 1.7 to 5.4 million kilograms of TNT (that’s 3.8 to 11.8 million pounds for any Americans dragging their feet on converting to metric). For reference, the bombing of the Murrah Building in Oklahoma City in 1995 used the equivalent of 1.8 thousand kilograms of TNT. So, Beirut was at minimum a thousand times more boom than Oklahoma City....
....MUCH MORE
