Image: Patrick Ferguson

I may not live in NYC, but I do know concrete’s not supposed to glow. The issues with the Big Apple’s subways have continued to mount as the rail systems decay, but this one is definitely new.

Sadly, for those who saw the mysterious green goop in NYC recently, swimming in it will not give you superpowers. It’s not a mutagen, nor Chemical X. In fact, it’s hard to tell what exactly is in those bins. But, as I currently work at an environmental lab [Disclaimer: my views and statements here are not meant to be official statements by my employer], and have worked in automotive paint before, I thought I might be able to puzzle out what the witches’ brew could be.

The many forms of phosphorus: (left to right) white, red, red, and violet.
Image: Wikipedia

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While it would take a thorough chemical analysis to make any definitive statement, some of my co-workers thought that phosphorus could be the cause. Famous for its greenish glow, the element lent its name to the term phosphorescence. In urban settings, phosphorus is usually found in the form of phosphates, which are a key part of the fertilizer and agricultural industries. My lab regularly checks phosphate levels from various sources—which include construction site runoff, due to permit regulations—because they can be harmful to people in large concentrations.

No wonder white phosphorus caputred alchemists’ imagination.
Photo: Endimion17 (Flickr)

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However, I don’t think phosphorus is responsible for the ooze’s hue. While pure white phosphorus can glow like Nickelodeon slime, phosphates shine a different shade. Also, ironically, phosphorus doesn’t phosphoresce. Fluorescence and phosphorescence occur when chemicals absorb certain wavelengths of light, then emit other ones. Phosphorus’ light, on the other hand, comes from a chemical reaction with oxygen (chemiluminescence). In the photo, note that the only bins glowing are the ones lit up by the construction workers’ lights. If phosphorus had been found in large concentrations, they would’ve all been glowing.

Speaking of concentrations, there would’ve been serious repercussions if that much phosphorus had gotten into a water supply. It seems unlikely for a working crew to dispose of that much all at once.

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Finally, phosphorus smells like garlic. No one smelled garlic at the site. They smelled sulfur.

From this angle, in this light...sulfur kinda looks Corn Pops.
Photo: Wikipedia

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Sulfur subtly shines forest green (sorry, out of S-words) under 254 nm UV light.
Photo: James Horste (Database of Luminescent Minerals)

Sulfur seems like an odd candidate for the eldritch light. Its pure form is a yellow rock, after all. However, in the presence of UV light, sulfur can fluoresce green.

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Unlike the sulfur above, this sample is lit under long-wave UV light, not short-wave. But it still appears green, because it’s interfering with the fluorescent mineral beneath it.
Photo: Mark Cole (Nature’s Rainbows)

In addition, there are quite a few ways for sulfur to get into a construction site. My lab also tests for the presence and concentration of sulfides in environmental samples, again, due to permit regulations. Considering the subway lines were being worked on, based on OSHA information, it isn’t infeasible for sulfur to be the color culprit. Unfortunately, I don’t think sulfur is actually what’s responsible for that shade of green.

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For sulfur to fluoresce that brightly would require the construction workers’ lights to produce UV rays of the correct wavelength. While some kinds of lighting can produce UV radiation, manufacturers take steps to prevent those waves from leaking out. Even full-spectrum LEDs don’t emit UV light unless something’s physically wrong with the lights themselves. But, that doesn’t mean the lights can’t be the guilty party in this.

A diagram of how light interacts with a yellow school bus
Image: HunterLab

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The standard angles used to measure color intensity and metallic sparkle on a car body. Each angle reveals something different.
Image: Jose M. Medina & Jose A. Diaz (Photonics Media)

Here’s where it ties back to cars. After working in automotive coatings alongside experts in the field of paint and color development, one of my key takeaways was that color is contextual. Change the light source, the surface it reflects off of, and/or where you’re standing, and your car’s color changes. My first car, an 02 Malibu, demonstrated this very well. During the day, it was a metallic green. But when I drove into gas stations at night, it turned completely blue under the fluorescent lights.

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Compare this photo of a Caterham, taken using one white balance setting...
Photo: Scouting For Zen
...with this photo, taken immediately afterwards with a different setting. Notice the changes in colors?
Photo: Scouting For Zen

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This is why it’s sometimes difficult to accurately photograph certain colors. I struggle sometimes to find the right white balance settings, or the correct angle, to snap some shades of red. Sometimes they look more orange than they are IRL. Ever notice how playing with the white balance settings often noticeably changes a photo’s colors? That’s because those settings are what help the camera’s sensors interpret the data they’re receiving.

No wonder my parents had so many arguments about their kitchen redecoration: so many light temperatures to pick from!
Image: Shakir Williams (HomElectrical Electrical Supply)

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Helpful guide to all us photographer-types.
Image: Olympus

Incoming light is characterized by color temperature. Know how metal glows when it heats up? Color temperature basically uses an ideal glowing body as a reference point. At 3000 K (Kelvin) light’s colored like this, at 6000 K like that, and so on. White balance settings are based on the temperatures of reference lights, such as fluorescent bulbs and sunlight. But sunlight’s temperature can change. That’s why some cameras include settings for cloudy days, shade, and sunset. As sunlight passes through the atmosphere, it travels different distances at different times of the day, which shifts the light to a different part of the visible spectrum. Sunrise and sunset wouldn’t color the Sun red if that weren’t the case.

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Why is all this important? Because that’s what commenter Bdog thought might be the reason behind the funky bins. In a comment to the Jalopnik post, Bdog noted that the workers could have been using recently switched-on metal halide lamps. Having no experience with the bulb technology, I did some more digging. Turns out, it’s an absolutely solid hypothesis.

These lamps work by passing an electrical arc through vaporized mercury and metal halides. When it’s cold, and when there are several of these bulbs lit close together, the perceived color temperature shifts (as shown in the above video) as the vaporization process hasn’t really started, and the bulbs’ individual beams interfere with one another. Add in some plausible screwiness with automatic white balance setting, plus the effects of observation angle andend time of day, and you have slime seemingly straight from the set of Ghostbusters.

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As much as I laugh/cry at the scenes from New York’s subways today, I’m glad we can take solace in knowing the MTA isn’t trying to cover up a spill of cartoon nuclear waste. Instead, we have—quite literally—a trick of the light.