Ozone, satellites, and climate change

A while ago, someone got really angry at me for calling BS on a claim that satellites cause ozone depletion. Crazy, right? But it led me down a rabbit hole, which I’ll summarize here. This should probably be a YouTube video, but I don’t do those, so I hope you’ll bear with this text instead.

First, the basics: ozone (O₃) is really important because it blocks ultraviolet radiation in the upper atmosphere. It does that by catching a short-wavelength photon, breaking apart to plain old molecular oxygen (O₂) and a free oxygen atom (O), after which the free atom will recombine with another O₂ to form a new ozone (O₃) molecule. Meanwhile, the short-wavelenght photon (UV-B radiation) was converted to heat, and didn’t reach the lower atmosphere or ground-level life, where it’d cause problems like skin burns or cancer, or similar effects to vegetation.

While ozone is continuously destroyed and regenerated in this interaction with sunlight, there are a few different chemical processes which involve permanent destruction of ozone, and in particular, one really important man-made one: that involving chlorine, such as chlorofluorocarbons (CFCl₃), which act as catalyst to continously break more and more O₃ to regular O₂ molecules:

1. Cl + O₃ → ClO + O₂
2. ClO + O₃ → Cl + 2 O₂
3. Cl is free to do this again, and again, and again

While other mechanisms of the Chapman cycle replenish stratospheric ozone, they’re much slower than the above process, so chlorine is able to reduce total ozone rapidly, causing more UV-B radiation to reach us.

Most stratospheric oxygen is O₂, though. Even at its highest concentrations, ozone is about 15 ppm of the atmosphere, while O₂ is 20.95 percent (or 209,500 ppm). And this matters for the next bit.

Ok, that’s the ozone. What about the satellites, then? Well, the gist of the claim was that as the number of satellites in Earths orbit grows (as it does — very fast), so do the number which fall down and burn up (also true).

NASA tracked orbital objects. For this article, we don’t really care about the high orbits, but there’s a lot of stuff up there in our nearby space!

Those low-Earth-orbit satellites are made up of electronics, antennas, batteries, solar panels and the structural frame holding the pieces together. That is, titanium, steel, aluminum, glass, silicon, kevlar, plastics, silver, gold and tiny quantities of other metals. When they fall down to meet the stratosphere, a couple of things happen:

1. The atmosphere breaks their speed, accelerating the fall from orbit
2. And also their structure, causing the satellite to both break up to smaller pieces
3. Everything heats up, and subsequently most of it burns up

Burning of course is a chemical process where various elements combine with oxygen. Ozone is a relatively reactive form of oxygen, so, yes, also with ozone. But since ozone is only 0.007% of stratospheric oxygen, mostly the burn is with plain old O₂.

But, goes the argument, anything we put up in the upper atmosphere is bad, we should leave it alone, and anyway, even if we’ve not studied satellite pollution, we have studied rocket pollution, and solid rocket boosters (which also burn aluminum!) are bad for ozone!

Well, yes, but no. Solid rocket boosters do indeed mostly burn a powdered mixture of aluminum and ammonium perchlorate to produce immense thrust out of the lower atmosphere on certain rockets like the former Space Shuttle, Atlas V, Ariane 5, Delta 4, SLS, and so forth. The burning powder does form particulates small enough to stay aerosolized, and have been detected in the atmosphere in some traceable quantities. But it’s not the aluminum oxide Al₂O₃ that is the concern for ozone. Rockets carry their own oxidizer, which in the case of these SRBs is ammonium perchlorate. The clue is in the name. After releasing its oxygen, we’re left with hydrochloric acid, or HCl. Yes, that Cl, the one which causes ozone depletion.

Solid rocket boosters are bad for the upper atmosphere, even if most of their emissions happen at lower altitudes.

There’s no use for chlorine on the LEO satellites, and none is left in the atmosphere by the de-orbiting of those satellites. It’d be better if we didn’t put the satellites up there using chlorine-containing rocket boosters, though.

But, and here we come to the rabbit hole, the satellites do burn, and some of that burn residue is left in the upper atmosphere as various metallic aerosols. Again in quantities which are traceable and can be attributed to satellites, rather than, for example, to meteors. So, are we polluting the upper atmosphere with stuff we don’t know the impact of?

Trace elements illustration from linked study, Metals from spacecraft reentry in stratospheric aerosol particles, by Murphy, Abou-Ghanem, Cziczo and Shen

By this stage we’re getting to the bleeding edges of atmospheric study and there’s a lot we do not know. However, most of the aerosols way up there are from volcanic eruptions and are pure sulfuric acid, with detectable quantities of both meteoric and satellite-origin metals mixed in. Lower down in the troposphere, human-origin emissions, like ocean freight and aeroplanes dominate.

And as we found starting from 9–11, then with the pandemic, and through other means of cleaning emissions, with less flights and cleaner ocean freight, we have less sulfuric acid (H₂SO₄) emissions reaching the lower stratosphere, leading to less sunlight getting caught there. Clearer days, hotter summers. Which actually might not be exactly what we want right now.

Yes, cleaning the stuff which causes climate change by releasing CO₂ can actually heat us up in the short term. Oops, shit, not what we needed. There’s a reason why some atmospheric scientists have suggested that releasing more sulfuric acid into the upper stratosphere might be a short-term solution. The trouble with that is that if it comes down to the troposphere, and it will, it’ll meet the clouds and rain down. As acid rain. Another thing we don’t want, because it does terrible things to forests. If we did put something up there, it’d be nice if it wasn’t sulfuric acid.

Rabbit holes are complicated, but satellites do not cause ozone depletion. Their rocket boosters might, but it depends on which rockets we’re talking about. High-altitude aerosols aren’t all bad, but it’d be better if the only sulfur up there was just the volcanic stuff. And it’s time to come up for fresh air because this rabbit hole might go much deeper. Thanks for reading.