When writing my previous post, I needed to research some things about atmospheric dynamics that wound up including a lot of information on the ozone layer. This is a topic that I think most people have heard about but don’t understand, but also one that doesn’t get talked about as much as it used to. So, let me explain what the ozone layer is, the risks to it, and how many of those risks have been dealt with.
Ultraviolet Light
I’m sure many of you have heard about the light spectrum, also called the electromagnetic spectrum, but I’ll give a recap just in case. Light travels as waves* and how broad or narrow these waves are determines what properties this light will have. For example, what we call visible light (all the light we can see), is a band of wavelengths that are able to pass through air and water with minimal blockage, hence why so many organisms evolved to see these wavelengths. But the visible spectrum is a comparatively tiny portion of the electromagnetic spectrum; if you represented the entire light spectrum as a piano with 350 white keys, the visible part of the spectrum would be only seven keys (one octave out of over fifty).** Lower on the light spectrum (light with wavelengths broader than what we can see) we have infrared, microwaves, and radiowaves. These are broad categories for ranges of frequencies far larger than the visible spectrum, each with different properties. We tend to use radiowaves for communication because they pass through most materials, from air to most buildings, and because they’re easier to generate. On the other side of the spectrum (light with wavelengths narrower than what we can see) we have ionizing light such as ultraviolet, x-rays, and gamma rays. Smaller wavelengths mean that the energy in light is packed into a smaller area, so they pack a far greater punch. At these frequencies, light has enough energy to rip atoms apart. This is why radiation such as x-rays and gamma rays are so dangerous, because they can tear apart the atoms making up DNA and cause mutations. Fortunately, these frequencies can’t pass through Earth’s atmosphere, but some frequencies of ultraviolet light can.
Ultraviolet light is a broad category of light wavelengths that are narrower and more energetic than visible light. At the lower end of the spectrum, just past what humans can see, we have what you may have heard being called UV-A, ultraviolet light that can’t rip apart atoms but can trigger certain chemical reactions. Slightly more energetic is UV-B, which is powerful enough to knock electrons off of atoms and turn them into extremely reactive ions. UV-B rays can directly damage DNA as well as collagen proteins, causing sunburn in the short-term and cataracts and skin cancer in the long-term. UV-A is less dangerous but it can trigger reactions that form DNA-damaging molecules and can cause skin damage with long-term exposure. And that’s just us; plants are extremely vulnerable to excessive UV radiation due to having their metabolic machinery stored in their skin. Roughly 10% of the Sun’s light is ultraviolet and if all of that UV radiation made it to Earth’s surface, complex life would not be possible there.
The Atmosphere
So, why are we not all dead? Well as I said before, different spectrums of light interact with different materials in different ways. Our atmosphere is largely transparent to visible light, but it looks very different when viewed under different types of light. Really dangerous wavelengths of ultraviolet light (sometimes called UV-C) can’t pass through the atmosphere at all. But UV-A or UV-B can pass through the vast majority of our atmosphere without too much absorption. Just our atmosphere alone isn’t enough to protect us. But as it turns out, the chemistry of ultraviolet light works in our favor.
Remember, high-energy ultraviolet light ionizes atoms it impacts. When UV-B rays hit oxygen in the atmosphere, the molecule breaks apart. In nature, oxygen is almost universally found as two oxygen atoms bonded together (O2), so breaking it creates two highly reactive oxygen atoms called free radicals. Each atom is prepared to attach itself to the first molecule it finds, and if that molecule happens to be another molecule of O2, it forms a molecule of ozone (O3). Over time, ozone molecules build up in the upper atmosphere to form a layer of ozone. Ozone is very opaque to ultraviolet light, with 90% of UV-B being blocked by the ozone layer and 50% of UV-A being blocked. This band of ozone sits in the upper stratosphere about 15 to 40 kilometers (10-25 miles) above us and due to atmospheric dynamics is very isolated from the lower atmosphere*** (which is good, ozone is toxic to humans). The ozone layer formed about 600 million years ago and it’s believed that this formation was a key factor in the Cambrian Explosion, a massive increase in global biodiversity around 550 million years ago.
Holes
Since the ozone layer is so important to life on Earth, any news of its depletion is a cause for concern. You see, while UV light breaks apart O2 so it can reform as O3, UV can also break apart O3 and let it reform as O2. This is normally not a problem, ozone is generated at the same rate as it is destroyed. Problems arise when other substances are added to the mix that also react with ozone and oxygen free radicals. You may have heard of CFCs, also known as chlorofluorocarbons. This is an umbrella term for multiple chemicals made up of chlorine, fluorine, and carbon atoms. They are structurally similar to hydrocarbons like methane, ethane, and propane, but with the addition of chlorine and fluorine atoms. Most CFCs are gaseous at room temperature (though a few are liquids), they are excellent solvents (other substances easily dissolve in them), and they are all very stable and unreactive, which makes them nonflammable and often non-toxic. These qualities make them useful for any application requiring a gas or liquid that people might be exposed to, including coolants for air conditioners and refrigerators, fire extinguishers, foaming agents, aerosol spray propellants, and as solvents for purposes such as dry cleaning and degreasing.
But being a gas means CFCs readily leak into the atmosphere. Atmospheric currents tend to isolate the ozone layer from the troposphere, but the stability of CFCs means that they don’t dissolve or break down for a long time, giving them time to eventually reach the ozone layer after a lot of circulation. Very few things break down CFCs, but one thing that does is ultraviolet light. Being outside the ozone’s protection, CFC molecules easily break down into chlorine and fluorine free radicals. Just like the oxygen free radicals, these unstable atoms bond to whatever is nearby, including O2 and ozone molecules. The oxygen free radicals that create ozone now have competition for reaction partners, so fewer molecules are being produced. But ozone molecules break apart at the same rate. With ozone being destroyed faster than it is generated, the ozone layer slowly thins.
There are several factors behind why the large holes in the ozone formed over Antarctica and the Arctic. The prevailing winds in the stratosphere and upper troposphere blow from lower latitudes toward the poles, so CFCs that make it to the stratosphere accumulate above the poles. Once there, the polar vortex traps CFCs inside, concentrating them further. Once trapped, the ice molecules in polar stratospheric clouds act as catalysts for the chemical reaction between CFCs and ozone (the stratosphere is normally too dry for clouds to form, but polar winters are cold enough to allow it). Add that with six months of night, ozone formation slows down at the same time these clouds are forming. All these factors together result in significant ozone depletion over the Arctic and especially over Antarctica. How much depletion occurs can vary throughout the year and between years as regional temperature impacts the rate of reaction. But the lowest recorded amount of ozone above Antarctica (in September 1994) was 30% of its healthy concentration, with the lowest above the Arctic being 70% of normal. This is enough to almost double the amount of UV rays making it to the ground, posing a health hazard to animals and plants in the region.
Fortunately, the problem of ozone depletion has been largely addressed by the international community. In the 1970s, the first studies were published that found CFCs were leaking into the atmosphere in significant proportions and proposed that they could react destructively with ozone. By 1985, these suspicions were confirmed by studies showing a significant decrease in ozone over the Antarctic. This same year, the first negotiations for the Montreal Protocol began, an international treaty which would phase out the production and use of CFCs and similar ozone depleters. Chemicals which were worse for the ozone were phased out faster, with exceptions being made for critical uses with no known substitutes, such as asthma inhalers. A fund was created to help developing nations make the transition. While there was pushback from CFC manufacturers, this pushback wasn’t universal and industry experts were involved in the process. An international panel of scientists was created to advise future policy development with signatories required to base their decisions on this panel’s findings. The treaty was put into effect in 1989, with nine revisions being made since then as our understanding of the problem was refined. It was the first UN treaty to be ratified by every UN member nation as well as all non-member observer states.
The Montreal Protocol has been one of the most successful international treaties in history, not to mention its success as international climate policy. Since 2010, CFC manufacture and use has been banned globally. Today, the ozone layer above Antarctica averages around 60% of healthy ozone concentration at its worst time of year. Ozone concentrations are trending upward, with full recovery expected by the 2060s. Now, it should be noted that illicit manufacture and use of CFCs do still occur and that many of the chemicals that replaced CFCs are extremely potent greenhouse gasses (as were CFCs), so the Montreal Protocol has been amended to phase these chemicals out as well over the coming decades. And it is fair to say that it is easier to phase out CFCs than it would be to phase out fossil fuels, as CFCs are just one family of chemicals we use and not the backbone of our energy sector. However, the success of this treaty is considered cause for optimism for climate policy. It is proof that we can deal with climate issues with international policy that is science-based, practical, fair, and enforceable. The fact that our ozone is recovering is evidence that climate change isn’t an unfixable problem and shows how we could do so.
For More Details
This entry is dedicated to Rumba Rhodes, who sadly passed away recently from complications of feline kidney disease at age 17. She will be missed.
*This is an oversimplification, but explaining why would require discussing quantum mechanics. For the purpose of this entry, light behaves like a wave.
**I use a piano for this metaphor because sound and light work in similar ways. Both are waves and are thus measured in terms of wavelength and frequency. Each musical note is a different wavelength of sound, just as each color is a different wavelength of light.
*** Quick primer on atmospheric layers; the bottom-most layer of Earth’s atmosphere is called the troposphere (0-13 km up). It gets most of its heat from the ground, so it is defined as the region of air that gets colder the higher up it goes. All weather occurs in the troposphere. The next layer up is the stratosphere (13-50 km up) which contains the ozone layer. Most of its heat comes from the chemical reaction creating and destroying ozone, so it is defined as the region of air that gets warmer the higher up it goes. As such, the coldest part of the troposphere and the stratosphere is where they border, at the top of the troposphere and the bottom of the stratosphere. Vertical air mixing occurs when there is warmer air below cooler air (hot air rises and cold air sinks, so they mix as they move past each other), so the stratosphere has very little air mixing with the troposphere.
Comments