My Chemical Romance

Did your chemistry teacher tell you that chemistry is all around you? It very literally is! You can find chemistry in many places, and the air around you is one of those places.

The air we breathe is very important to us. To stay healthy and work (or play!) at our best, we need air that is clean and not too hot or cold. City planners, your state legislature, and the Environmental Protection Agency (EPA), among many other people, work hard to protect your air. Scientists find out what they need to know to make these decisions.

You can’t see it, but it’s there 

Chemistry in the atmosphere. Image created by author.

The air is full of gases and particles. We are very glad that there is oxygen in the air for us to breathe, but there are other things in the air that could make us sick if we breathe in too much of them. When people discuss “air quality” they are often referring to the level of two specific things in the air: (1) ozone, which makes up smog and can increase asthma attacks, and (2) very small particles, much smaller than pollen, that can get stuck in your lungs and blood. The EPA knows that these things can make people sick and regulates how much can be in the air.

It’s getting hot in here

The air can also make people sick when it gets very hot. This can especially be a problem in cities, due to the urban heat island effect. Because cities have so many buildings and pavement, instead of plants, they can get hotter than surrounding areas. One way that people try to reduce this is by designing buildings with “cool” walls and roofs. These design features bounce solar energy off of the building and away from the city, usually based on the color. This is similar to wearing breezy whites in the summer instead of all black.

Before and after photos show the installation of a “cool roof” that reduces heat. Source: US Coast Guard 

They’ve got chemistry

The atmosphere is very complicated, however. The gases and particles in the air undergo chemical reactions, that are temperature dependent. If you ever used a hot plate in your chemistry class, or even cooked an egg or baked a cake, you know that some chemical reactions happen faster at hotter temperatures. This happens in the atmosphere, too.

Let’s experiment

Some scientists used a computer simulation (the weather research and forecasting model coupled with chemistry) to see if these cool buildings might change air quality. They studied Los Angeles, CA and the surrounding area. The cool buildings did reduce the temperature (about 0.5 degrees F). This reduction in temperature leads to less ozone. This is because the chemical reactions that form ozone happen more often with higher temperatures.

Image shows smog in Los Angeles, which can be reduced with cool roofs and walls. Source: Ben Amstutz 

However, there are more particles in the cooler air. This is because there is less movement in the cooler air. This movement takes the particles out of the city where people are breathing them. There are some chemical reactions that affect the particles as well, but the effect of the air moving around outweighs the reduction from the chemistry.

What’s the verdict?

This is important information for people making decisions. Reducing urban heat islands is good for the people living in cities and for the environment. Reducing ozone is an excellent additional benefit. However, it is important to be aware that there can be an increase in particles that are harmful to breathe. When decision-makers try to balance all the different consequences of their decisions (good and bad) they are better able to make decisions that are good for people. Because one decision can have many different effects, making the right choice is complicated. Scientists can help make complicated decisions by providing this information.


Citation Zhang, J., Li, Y., Tao, W., Liu, J., Levinson, R., Mohegh, A., Ban-Weiss, G. 2019. Investigating the urban air quality effects of cool walls and cool roofs in southern California. Environmental Science and Technology 53(13) 7532-7542

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Kristen Brown

Kristen Brown

I am a postdoctoral researcher at the EPA where I specialize in evaluating environmental impacts of our energy system. I have a PhD in Environmental Engineering from CU Boulder where I also received a master’s in Mechanical Engineering, and I have a BA in Physics from Cal Berkeley. Outside of work, I’m an amateur boxer and have two spoiled dogs. You can follow me on twitter at @Kris10BrownPhD and find out about my research at

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