Ozone

Last week I was working in the lab when an an inquisitive student poked his nose round my door. He wanted to know a bit about my experiment because he thought my lab set up looked interesting (it is a pretty fun looking set-up as you can see below). I like interruptions like this, because its always fun to talk to someone about the fundamentals of what I’m doing and why it matters (especially as a break from some frustrating plumbing rearrangements, as I was doing at the time).

 

My current experiment: I'm generating small aerosol particles to test how well my instruments detect them.
My current experiment: I’m generating small aerosol particles to test how well my instruments detect them. Photo credit: Vertucci Visual (www.vertuccivisual.com)

I use ozone in part of my experiment and he asked if it was the same as oxygen and if it’s safe to breathe. It’s a good question, and I thought I’d put a more fleshed-out explanation here than the one I was able to give in the lab before he had to catch up with his class.

Ozone is trioxygen, with the chemical formula O3, meaning it’s a molecule made up of 3 oxygen atoms bound together. The common form of the oxygen atom in our atmosphere (because it’s the most stable), and the kind it’s good for us to breath, is diatomic oxygen, O2.

Dioxygen: the most common form of molecular oxygen in the lower atmosphere, made up of 2 oxygen atoms
Dioxygen: the most common form of molecular oxygen in the lower atmosphere, made up of 2 oxygen atoms
Ozone: a molecule made up of three oxygen atoms
Ozone: a molecule made up of three oxygen atoms

So is ozone safe to breath? Well very in dilute concentrations, it’s not going to kill you. Ozone exists in unpolluted air at ground level at concentrations around 10-15 parts-per-billion (ppb, meaning for every billion liters of air that we breathe, 10-15 of them are ozone). When areas get polluted the concentration of ozone in the air increases because emissions for vehicle exhausts and industry contain hydrocarbons and nitogen oxides. In the presence of sunlight these react to form ozone. Too much ozone in the air that we breathe is bad and can trigger problems in the chest and throat, reduce lung function and worsen conditions like bronchitis, emphysema, and asthma. It also damages vegetation. Since October 2015 the US National Ambient Air Quality Standards (NAAQS) for ground-level ozone are 70 ppb).

The brown haze in this photo I took on an afternoon walk is smog over Denver. Part of the pollution here is ozone.
The brown haze in this photo I took on an afternoon walk is smog over Denver. Part of the pollution here is ozone.

At this point, I know many people will be thinking “if ozone is bad for us, why are we worried about the ozone hole?”. Well it turns out that while ozone down in the lower atmosphere is bad because that’s what we’re breathing, ozone higher up, in the stratosphere is good, because it protects us from UV radiation from the sun.

Recently I was lucky enough to catch a seminar by Susan Solomon, who played a huge role in explaining the cause of the Antarctic ozone hole while working here in the Chemical Sciences Division at NOAA, where I work (she’s now at MIT). She and her colleagues postulated that chlorofluorocarbons (CFCs, the ones we used to put in the atmosphere from refrigerants and aerosol spray cans) react with ozone when they’re on the surface of ice particles in polar stratospheric clouds (also known as nacreous clouds).

In 1986/87 she led expeditions to Antarctica, measuring 100 times more chlorine dioxide in the stratosphere when the ozone hole opened up in spring than the background levels. This provided this link showing the CFCs were causing the ozone depletion and was a substantial part of the research behind the Montreal Protocol, regulating and phasing out the use of CFCs in order to protect stratospheric ozone.

Polar Stratospheric Clouds. Chlorine reacts with ozone on the surface of these cloud droplets, depleting stratospheric ozone. Photo credit: Zhibin Yu/CIRES
Polar Stratospheric Clouds. Chlorine reacts with ozone on the surface of these cloud droplets, depleting stratospheric ozone. Photo credit: Zhibin Yu/CIRES

So ozone is a complex beast. It is made up of oxygen atoms, but in a different molecular form from the type of oxygen we need to breathe makes. This makes it bad for humans if we breathe too much of it. Pollution increases ozone levels above what is naturally there. However, while we don’t want it in the air we’re breathing low in the atmosphere, we really need it up in the stratosphere to protect us from UV radiation. Ozone occurs naturally in the stratosphere, but is removed when we pollute it with CFCs.

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