As Phoenix residents, we are all aware of high ozone alert days. We have all seen the less-than-clever air quality messages on the highway message boards urging us to carpool the following day. Ozone is the primary pollutant that contributes to poor air quality. However, what is ozone, and why does the American Lung Association give Maricopa County an F on its State of the Air Report?

High Ozone on Roadway Phoenix

What is Ozone?

Ozone is a molecule that consists of three oxygen atoms bonded together, represented by the chemical formula O3. It is important to note that ozone is not the same as the oxygen we breathe, composed of two oxygen atoms bonded together (O2). O2 is the most common stable form of the highly reactive atom oxygen on Earth. Our bodies cannot use ozone the same way they use O2 because of their differing properties. 

Energy in the form of light, electricity, or heat can break the bonds between the atoms. We call the breaking of bonds between atoms a “chemical reaction.” Ozone is a highly reactive oxidant, readily undergoing chemical reactions and breaking its bonds. Ozone is becoming more prevalent with the release of other reactive chemicals emitted from cars, power plants, industrial boilers, refineries, and chemical plants. Ozone pollution can come from paints, cleaners, solvents, and motorized lawn equipment. The increase in human activities producing ozone is elevating the concentration of it close to the Earth’s surface.

Helpful versus Harmful Ozone

Ozone is a naturally occurring compound on Earth that forms naturally through the decay of living things, forest fires, and other biological processes. In the upper atmosphere, it forms naturally through a chemical reaction between O2 and the energy from the sun. In the upper atmosphere, O2 (the oxygen we breathe) reacts with the energy released from the sun to break apart into two unattached oxygen atoms. These free oxygen atoms then react again to join with O2, forming O3 or ozone. Here, about 6 miles above our heads, ozone plays a vital role in life on Earth by absorbing 97 to 99 percent of the sun’s harmful medium-range UV rays. Without this shield, life could not survive on Earth.    

Atmospheric Ozone Formation

In addition to this protective layer high above us, there is “ground-level” ozone. Low-level, ground-level, or tropospheric ozone is a secondary pollutant that results from chemical reactions between primary pollutants emitted directly from car exhausts, industries, and UV rays. Fossil fuels, volatile organic chemicals (VOCs), and nitrogen oxides (NOX), released by cars and industries, use the energy in the UV rays to combine and form ozone. The by-product of this reaction is ozone (or smog), which is often visible on the horizon in cities like Phoenix. Ozone also absorbs heat reflected from Earth and traps it, making it a greenhouse gas. Car emissions are the primary source of low-level ozone. Therefore, road announcement boards encourage people to drive less in Phoenix when conditions are ripe for high ozone.

Ground level Ozone Formation

Phoenix Air Quality and Our Health

Exposure to ground-level ozone can lead to many harmful effects on our respiratory system. Some immediate effects are the onset of wheezing, coughing, and shortness of breath- symptoms that can be particularly stressful for people with pre-existing conditions such as asthma. Additionally, ozone inhalation can cause inflammation of lung tissue, leading to further complications. A 2019 study found that people exposed for years to higher-than-average concentrations of ground-level ozone developed lung changes similar to those seen in smokers. This noticeable effect is because our lungs are susceptible to ozone. 

Ozone on horizon in Phoenix

Humans can detect 0.1 ppm of ozone due to its oxidative nature. One part per million, or one milligram per liter (mg/l), would equal putting ONE drop of food coloring into 10 gallons of water.  This video can help you visualize just how small that amount is. Seemingly as a built-in defense against ozone, humans can smell just a tiny amount of ozone. The “smell of rain” is ozone generated by high-energy electrical charges within the clouds of a storm.

When ozone is present at ground level, it becomes a dangerous pollutant for all living beings, including plants. Plants are crucial for life on Earth as they provide oxygen and glucose.  Ozone reduces photosynthesis rates in plants, leading to lower crop yields. The ozone can enter a plant through the stroma or tiny pores on leaves, burning the plant and ultimately leading to decreased photosynthesis, productivity, disease, and even death.  

Phoenix Air Quality and Our Geography

Phoenix experiences high ozone levels due to the creation of ozone from sunlight and pollutants. Both are abundant in the Valley of the Sun. Phoenix is the 5th largest US city and one of the largest growing metropolitan areas in the US. In the American Lung Association’s 2023 State of the Air Report, Phoenix was ranked 5th in ozone and 7th in year-round particulate pollution nationwide. But one more factor makes Phoenix an ozone trap: geography.  

The Valley of the Sun is just that, a valley. Phoenix is located in the Salt River Valley, surrounded by mountains to the north and east and low mountain ranges on all other sides. These mountains act as walls, trapping pollution within the Valley. Additionally, the monsoon season brings cloud cover that acts as a lid, further trapping the ozone in Phoenix. During the intense heat of the summer and during times of dense cloud cover, high ozone days will increase.  

Google Earth Phoenix

Fortunately, when the monsoon rains finally arrive, they help to “clean” the ozone from the air by dissolving it and bringing it down to Earth. The strong winds in Phoenix also contribute to clearing the ozone from the city. Ultimately, though, the long-term solution to ozone is fewer cars on the road and more environmentally responsible industries. As the climate generally warms, high ozone days will only increase, and so will the health issues it creates.

Cities Heating Up

But there is still one more factor: the urban heat island effect. All large cities experience the heat island effect; however, it is more pronounced in Phoenix because Phoenix is the 6th hottest city in the world. Concrete, asphalt, and metal all pull in and hold onto heat. The urban heat island, or UHI, makes Phoenix’s nighttime lows 10 to 15 degrees Fahrenheit warmer than surrounding rural areas. Remember that energy-hungry ozone traps heat. Now add a higher-than-average amount of ozone, trapping even more heat coupled with living in the hot desert, and you have a perfect recipe for adverse ozone effects. Ozone levels rise as much as 20% when it is hot. The combination of heat and pollutants becomes even more alarming when you consider the world is only getting hotter and Phoenix is getting dryer. As Phoenix dries, the rains that clean the air will become less prevalent.  2023 was the hottest year on record, with no cooling in sight; ozone pollution will continue to worsen.  

The Air Quality Index (AQI)

The Environmental Protection Agency (EPA) has created air quality guidelines for ozone and other air pollutants. This information is available to the public through the Air Quality Index (AQI). When a message is posted on the board to “carpool tomorrow,” conditions are ideal for high particulate and ozone pollution. Consider carpooling or taking the bus or the light rail to avoid exposure on such days. Limit your time outside as well. I don’t let my small children out to play on high-ozone days. We find indoor entertainment instead. However, what concerns me is that, like many Phoenicians, I did not grow up here. Hence, I haven’t encountered high levels of ozone for the entirety of my life, but my children will. Hopefully, with everyone’s help, my children’s lives will see ever-decreasing ozone levels in the city of their birth, and they will not be exposed to it their whole lives. So, the next time you see an alert for a high ozone day, consider asking a friend to carpool.

Ways to lower ozone