https://www.wired.com/story/tragic-physics-deadly-explosion-beirut/
The Tragic Physics of the Deadly Explosion in Beirut
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 a 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 6 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 nonnuclear 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.
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 a 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 Os. It’s chemically unstable, meaning the bonds between the Ns and the Os 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.”
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 they 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 gray 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 speed 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 6 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.
As an aside, nuclear weaponry is set to detonate several hundred feet above ground level, and therefore doesn’t exert enough force directly on the soil to create a crater. The detonation of the first atomic weaponry above Hiroshima occurred almost exactly 75 years ago to the day, and despite its historically unprecedented trauma to the city and populace, it left behind no crater.
Germany, too, knows the destructive power of improperly stored ammonium nitrate, and an accident in that country reinforces the calculation of the charge size. In 1921, a fertilizer explosion in Oppau, Germany, carved a remarkably similar crater. At 120 meters in diameter, following the explosion of 4.1 million kilos of ammonium nitrate, the size of the Oppau crater supports the idea that the Beirut pier, which early reports said held only 2,750 metric tons—2.75 million kilograms—may have held some number of millions more kilograms of charge. However even using only those 2,750 metric tons, Special Agent Wojtylak says his preliminary calculations indicate that those safe from all risk of carnage would have needed to be at least 15 kilometers from the Beirut pier.
Within that radius, the injuries from such a massive blast in a downtown location can be as varied as the victims who experience them, but a number of them are likely from glass and other flying projectiles. Flat, delicate, frangible, and installed in large sheets, glass is the perfect target for a blast wave of even minuscule magnitude; it shatters and flies easier than any other substance.
In Halifax, the fire that eventually triggered the main explosion meant that citizens were already positioned at their windows to watch the excitement when the bomb detonated. As a result, of those wounded on that morning in 1917, injuries of shattered glass penetrating the eyes of onlookers were described as “extraordinarily prevalent.” The accident of the fireworks storage area and its alarming plume of smoke similarly guaranteed plenty of onlookers for Beirut. Without smoke drawing these witnesses, we would not have such ample cell phone footage, but there also might have been fewer injuries.
The Halifax explosion resulted in the same series of events that, according to Duke University Hospital emergency room physician Dan Buckland, still occurs in emergency rooms today after mass casualty incidents. Buckland treated casualties following a natural gas explosion in Durham, North Carolina, in 2019 which, while substantially smaller in size, occurred in a densely populated downtown area. The first wave of patients are those who were physically near emergency services, he says, those unlucky enough to get hurt but lucky enough to do it in close proximity to an ambulance. After that, the hospitals get flooded by “walking wounded,” those who are wounded enough to seek medical care but intact enough to transport themselves. The key to increasing the number of survivors, Buckland says, is getting the third wave of patients through the door, through the crowds of this second wave. The third wave is made up of those at the core of the site, too injured to walk, whom the emergency responders combing the wreckage medically prioritize next.
After a blast in a civilian area, emergency responders aren’t the only heroes. Just after 9 am on December 6, 1917, in Halifax, mustachioed 45-year-old train dispatcher Vincent Coleman knew the lazy plume of smoke coming from the explosive-laden vessel in Halifax harbor was a terrifying portent of worse to come. But as the other dispatchers ran for their lives, Coleman realized the number of train passengers set to arrive at Halifax any moment, and so he stayed long enough to send one final telegram: “Hold up the train. Munitions ship on fire and making for Pier 6 ... Goodbye boys.” Coleman died in the blast at 9:05, but his final message saved thousands, not just the passengers on the trains that were able to stop before entering the zone of destruction, but also the citizens already in Halifax: The telegram signal reached every operator in the surrounding region, and because of Coleman’s quick thinking, every doctor who felt the earth rumble, up to 160 km away, had almost immediate access to news about what had happened. They rushed in to help.
Medical personnel dealt with as many of the first, second, and even third waves of injured patients as fast as humanly possible, some of them even creating makeshift treatment centers on trains. From Beirut too, will gradually emerge stories of heroism and savvy, along with the final accounting of lives lost. But in the meantime, between those who filmed the blast and the application of physics, we can prevent any escalation of conspiracy theories or misunderstandings. The explosion wasn’t a military-grade bomb; it most certainly wasn’t a nuclear one. It was, sadly, tragically, history repeating itself yet again: Explosives can be devastatingly lethal, and we should never underestimate their destructive fury.
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