The air we breathe is this extremely precious thing. Especially, because there is so little of it - the atmosphere is really thin, it's like if you wrapped a single layer of plastic wrap around a basketball. That thin sheet is what makes this tiny little planet in the vast solar system, and as far as we know - universe - able to sustain life. We're going to look at some ways in which human beings have negatively affected the thin layer of earth’s atmosphere by filling it with pollutants. And spoiler alert - some of the main culprits are these guys: vehicles and power plants - particularly those that burn fossil fuels. There’s a theme in this episode - burning fossil fuels puts a lot of pollutants in the air we breathe. But before we get into that, let’s first take a look at what ‘air’ or more accurately the atmosphere is. This is the atmosphere. It’s mostly nitrogen, some oxygen, a dash of argon, and then everything else, like Neon, Helium, and Water, is a tiny sliver. And notably, carbon dioxide, a molecule we’re going to be talking about a lot during this series, because it is so important for understanding climate change, makes up only a very small percentage of the atmosphere. All those gases together, plus any pollutants and dust that end up floating around make up the atmosphere. We can then divide up the atmosphere by distance from the surface. These layers are the troposphere, stratosphere, mesosphere, thermosphere and then the exosphere on top. We're only going to focus on two layers of the atmosphere. The troposphere and the stratosphere. The troposphere is where most of environmental science happens: All life forms, all biogeochemical cycles, weather patterns big and small, everything. The stratosphere is important, because that’s where the ozone layer is. Now, a small point of clarification. Ozone is three oxygen molecules bonded together. Down here, in the troposphere, you don’t want it, it’s real bad. It’s a pollutant that people make, a component of photochemical smog which is just as bad as it sounds, and is related to a whole host of respiratory issues. However, in the stratosphere, stratospheric ozone is amazing. It’s our planet’s sunscreen. The ozone layer blocks out the vast majority of the sun's harmful ultraviolet (UV) radiation and without it, life would not exist as we know it. All organisms on this planet have evolved with this small amount of UV radiation that makes it onto the surface of our planet. And that includes us - that’s why when people go into space, radiation is a huge worry, because we aren’t built to experience the full power of the sun. Fortunately for our soft UV-sensitive bodies, most of the ozone in the atmosphere is up in the stratosphere, in the ozone layer. Up there, the physical properties of the ozone molecule allow it to absorb all the UVC and most UVB radiation. UVA, and some UVB, still makes its way to the earth’s surface. The difference between UVC, UVB, UVA - is the energy levels, UVC has a longer wavelength, but they’re all still within the ultraviolet section of the light spectrum. Hmm. This is getting dangerously close to physics, so back to the ozone layer. Most ultraviolet radiation gets stopped at the door, but some of it gets through, and it has some benefits - it helps people create Vitamin D - but it also has risks. Exposure can cause cataracts in your eyes or skin cancer. Wear sunscreen and sunglasses, people. However, if we didn’t have the ozone layer, there is no amount of sunscreen that could protect us. And that almost happened. People started using these miracle chemicals called chlorofluorocarbons, or CFCs. Long story short - that chloro - which stands for chlorine pops off the carbon chain when exposed to UV radiation. Then that rogue chlorine tears an oxygen out of an ozone molecule. The fancy name for this is ozone depletion, and that led to what you might have heard referred to as the ozone hole. Fortunately, humans collectively got their act together and banned CFCs under The Montreal Protocol in 1987. You can learn all about that in this video. Here’s the important take away - the story of stratospheric ozone depletion is a great example of science understanding a problem, identifying the culprit, and legislative and international bodies coming together to ban that very substance. And eventually, seeing a recovery in our earth’s system. Now, let’s bring it down a layer, back home into the troposphere to talk about air pollution. Air pollution is a pretty broad category that encompases anything in the air that harms us or any other organism. And it’s a pretty big deal: an estimated five to six million people die every year due to air pollution - primarily from related respiratory problems. Breathing in foreign particles and chemicals is not good for the precious system in here In the United States, the body that is tasked with monitoring and protecting us from air pollutants is the Environmental Protection Agency or EPA. The EPA has six air pollutants that they continuously monitor all over the country and they're called criteria pollutants. [I used to be really good at this] NOx, SOx, Particulate Matter, Lead, Carbon Monoxide, and ozone. There's dozens and dozens, if not hundreds, of different types of air pollutants out there, but these six are consistently tested and measured all over the country because we can tie each one of them to specific human activities. Which makes monitoring and enforcing air quality standards easier. You might have noticed a pattern in these names. A lot have ‘Ox’ or ‘oxide’ - nitrogen dioxide, sulfur dioxide, carbon monoxide. That oxide is a result of combustion. When things are burned that have sulfur in it, that makes SOx, sulfur oxides. When things are burned that have nitrogen in them, they make NOX, nitrogen oxides. If they have carbon – carbon monoxide. So if you hear oxide, or Ox - that means something has probably been burned. For reasons I will make painfully obvious, we’re going to talk a lot about power plant emissions this episode, so I think it is worth taking a second to know what we’re looking at. This is a power plant and you can see two types of smoke stacks or flues. The large one, with curved sides, that’s a cooling tower. All that’s coming out of there is water vapor. The smaller, thinner smoke stacks - that’s where you’ll find the SOX, NOX, leads, and other dangerous pollutants. Okay, let’s start with NOx and SOx - why should we be worried about these oxides? Both are highly reactive and are emitted when fuels are burned at high temperature. For NOx, the big emitters are cars, construction equipment, boats, and power plants. SOx comes almost entirely from coal-fired power plants and some other industrial processes. And lucky for us, nitrogen oxides and sulfur oxides are poisonous. They’re bad for our respiratory system, can impact plant growth, and react with the air to make acid rain. The formal term for this is acid deposit. In the air NOX reacts to create nitric acid and SOx creates sulfuric acid. Which then fall to the ground - its an acid so it can burn through structures we’ve built, damaging buildings, statues, you name it. And of course, it is dangerous to humans and other organisms. Here’s a chart showing the critical pH levels that organisms cannot survive in. Take this cute crayfish - below 5.5, it dies. Acid deposition lowers the pH of water and soil, making them more acidic, and that can have serious ramifications throughout an ecosystem. The US has actually done a pretty okay job of reducing SOx emissions from power plants, partly through regulation, improvements in smokestack scrubbers and other cleaning technology, but also the economic viability of coal isn’t so hot these days. So while acid deposition still has serious consequences, it isn’t as prevalent as it once was. There’s another serious consideration when it comes to oxides, especially NOx, and that’s photochemical smog - which we’ll come back to later. So next on the list - this is particulate matter - the solid stuff in the air. So not a gas like NOx and SOx, but solid particles that are small enough to be airborne - this could be smoke, dust, or soot particles, like ash and lead, or pollen. Near coal-power plants, having tiny pieces of mercury in the air is not uncommon - and most of these whether from natural or anthropogenic (the fancy word for human) sources can be dangerous to our health. The US EPA categorizes particulate matter into two sizes, PM10, anything smaller than 10 micrometers across, and PM2.5, anything smaller than 2.5 micrometers across. In general, the smaller the particulate matter, the more dangerous it is to us. Particulate matter comes in many shapes, sizes and origins. Some of which are even beneficial, important elements of the earth system. Dust from windstorms in the Saharan desert in Africa add phosphorus to the soils in the Amazon - an ocean away - to make up for nutrient loss from flooding and surface runoff. Although dust particles are small, no larger than around a tenth of the width of a human hair, they form massive plumes that can be seen from space. This footage is from NASA's Calypso satellite. This data shows that on average, 182 million tons of dust leaves Africa each year. 27 million tons of which ends up in the Amazon basin. However anthropogenic sources of particle pollution are often not so beneficial. Power plants, especially coal-fired power plants are responsible for large concentrations of particulate matter. This map of the United States shows the concentrations and sources for particulate matter, notice it is especially dense in the eastern US, which is also where the majority of the US population lives. So those pollution generating plants are providing electricity but they are also poisoning the air we breathe, where we breathe it. Relevant to right now, the spring of 2020, the novel coronavirus has had a dramatic impact on the amount of particulate matter in the air. This example happens to come from India. You can see in 2016, 17, 18, 19, 20 there are heavy layers of particulate matter in the atmosphere. But now in March of 2020, we see a big drop. With much of the world sheltering-in-place