Residents of Phoenix are all familiar with high ozone alert days. We have all seen the not-as-witty-as-usual air quality messages on the highway message boards asking us to carpool the next day. The primary pollutant contributing to the bad air quality is ozone. But what is ozone; why does the American Lung Association give Maricopa county a grade F on its State of the Air Report?
What is Ozone?
Ozone is O3, or three oxygen atoms bonded together. Energy, in the form of light, electricity, or heat, can break the oxygen bonds. The usual reaction to the discovery that ozone is oxygen is usually, “Oxygen is good, right?”, after all, it is what we breathe. However, the oxygen we breathe is O2, or two oxygen atoms bonded together. That one little atom makes a huge difference! Our bodies cannot use ozone the way our body uses O2 because ozone is the wrong size and has very different properties. For starters, ozone is a highly reactive oxidant. An oxidative molecule is one that wants to react or break its bonds; we call this a chemical reaction. If energy (heat, light, electricity) is available, ozone will take it and become something else, giving off a by-product as a result of the reaction.
Air Quality and Our Health
When we breathe in ozone, it causes many adverse side effects because it wants to react with our bodies, such as wheezing and coughing, shortness of breath, lung tissue inflammation, and asthma attacks. In plants, it affects photosynthesis rates, resulting in reduced crop yield. Ever notice an increase in respiratory irritation/illness when we have stretches of high pollution days? Our lungs are sensitive to disease due to the oxidative nature of the molecule and the fact that cells are sensitive to minimal amounts of it (0.1 ppm). This video helps you visualize what that means: https://ed.ted.com/lessons/how-to-visualize-one-part-per-million-kim-preshoff-the-ted-ed-community. We are so sensitive to it we can smell it in minimal concentrations. Everyone knows what rain “smells like,” coming from the midwest this smell triggers a euphoric nostalgia in me. However, very few know that what you are detecting is ozone. Humans can smell a teaspoon of ozone in the area of an Olympic sized swimming pool. As high energy electrical charges are released as a storm comes in, it forms ozone.
The Ozone Layer
The ozone layer is, at its closest, 6 miles above us. When ozone is ground level and able to be breathed in, it becomes a dangerous pollutant to all life, including plants. Plants make life possible on Earth by providing O2 and glucose, two essential components for life. Low-level ozone or tropospheric ozone is a secondary pollutant. Meaning, it is not directly emitted from a source. Instead, it is the result of chemical reactions between primary pollutants emitted directly from car exhaust and industry and UV rays. Hydrocarbons, or fossil fuels, and nitrogen oxides (NOX) given off by cars and industry, use the energy in the UV rays to rearrange themselves into ozone. A by-product of this reaction is smog, often seen just at the horizon in Phoenix. Energy hungry ozone also absorbs heat bouncing off Earth and traps it. It is this heat-trapping behavior that classifies ozone as a greenhouse gas. The primary source of low-level ozone is car emissions. Hence the call to drive less from the City of Phoenix.
Air Quality and Geography
Phoenix is the perfect place for high ozone because of three major factors: climate, geographic location, and population. The more people you have, the more car exhaust and industry you have. Therefore, just generally, there is higher ozone in urban areas than in less populated areas. However, the geography of our beautiful city compounds this. The Valley of the Sun is just that, a valley. Think of Phoenix as the bottom of the bowl, where you mix your ingredients. The mountains around us are like the sides of the container, keeping the contents trapped within it. Cloud cover in Phoenix acts as the lid to the bowl, trapping the contents of the bowl. Ozone gets trapped in Phoenix, and the cloudy skies of monsoon trap the ozone.
When the monsoon rains finally come, they help to “clean” the air of the ozone, dissolving it and pulling it down within earth. The fantastic Phoenix winds also help to clear the ozone from the city. According to U.S. Climate Data, Arizona has 3835 hours of sunshine in a year. That is an unusually high amount of available UV rays to help create ozone. Our beautiful blue skies and sunshine compound our ozone production. Add that to our population size and our geography, and you have the perfect environment for ozone to form and stay.
Cities Heating Up
But there is still one more factor, the climate coupled with the urban heat island effect seen in all cities generates an enormous amount of heat. Remember that energy-hungry ozone traps heat. Cities are already naturally warmer than the surrounding environment because of all the heat-absorbing concrete and steel. Still, Phoenix is unique because of its desert biome. Phoenix is the 6th hottest city in the world, according to the World Atlas. Now add in a higher than average amount of ozone that is trapping even more heat. Ozone levels rise as much as 20% when it is hot.
The Air Quality Index
When one considers all this, Phoenix seems like an unsafe, perhaps illogical city to live. Luckily we do have a system in place that allows us to make choices to limit our production of ozone and our exposure to ozone when it is high. The Environmental Protection Agency (EPA) has created air quality guidelines for ozone and other air pollutants and made this information available to the public through the Air Quality Index (AQI). The information contained in the Air Quality Index has been gathered through controlled, replicated scientific study. Learn more about the AQI here: https://www.airnow.gov/index.cfm?action=aqibasics.aqi.
When the message board says “carpool tomorrow,” it is based on the AQI and recommendations of the EPA, which are both based on sound, solid science. Consider carpooling, taking the bus, or the light rail. To avoid exposure, limit time outside. I don’t let my small children out to play on high ozone days. We find things to entertain us inside. What does worry me is this – I, like many Phoenicians, did not grow up here. Therefore I have not been exposed to high levels of ozone my whole life; my children will be. Hopefully, with everyone’s help, my childrens’ lives will see ever decreasing levels of ozone in the city of their birth, and they will not be exposed to it their whole lives. So next time you see an alert for a high ozone day, ask a friend to carpool, seriously, do it.
Author’s Note: This was just released August 13, 2019
For More Information:
- AQI: https://www.airnow.gov/index.cfm?action=aqibasics.aq
- EPA: https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics
- AirNow: https://airnow.gov/index.cfm?action=aqibasics.ozone
- NASA: https://ozonewatch.gsfc.nasa.gov/facts/SH.html
- The American Lung Association: https://www.lung.org/our-initiatives/healthy-air/outdoor/air-pollution/ozone.html
- NOAA: https://www.ozonelayer.noaa.gov/science/basics.htm
- National Geographic: https://www.nationalgeographic.com/environment/global-warming/ozone-depletion/