Arizona Air Quality Information

Why less traffic doesn't necessarily mean less ozone


When you think ozone, we often immediately link it to air pollution generated by cars. With that link in mind, you would think with fewer vehicles on the road lately due to COVID-19, ozone levels at the surface would immediately drop, right? Not so fast!

Ozone is a very complex pollutant, with a lot of chemistry involved. Don't worry, we will keep it simple here, but we do have to understand the two components that help in the formation of ozone:

(1) Volatile organic compounds (VOCs): plants can be a big contributor to VOCs, with other sources including, but not limited to: paint, glue, that new car smell from the plastic and fabric, etc. Car emissions are also a source of VOCs; however, they play a bigger role in the next ingredient.

(2) Nitrogen oxides (NOx): the most significant source of NOx is from the combustion of fuel in an engine; this is where cars and trucks come into the picture.

Now that we have the essential ingredients for ozone formation, how they come together is the next key to understand. Studies have found that there is a sweet spot for the ratio of VOCs to NOx to most effectively form ozone.

To make this more clear, think about making pancakes from a boxed mix, you need the right ratio of water to dry mix. If you put in too much water, you get a runny mess (can barely even make one pancake); if you put in too much mix, you can still make pancakes, but they might be a little dense. Trust me; this is going to relate to ozone.

With that example in mind, let's consider the mix to be VOCs and the water to be NOx. Since the number of cars on the road has decreased in the Phoenix valley, NOx would be expected to decline, as well. If you reduce the amount of NOx in the air, you get an environment that is called NOx limited. Consider that a reduction in the amount of water when talking about pancakes; this would mean you can still make pancakes because the ingredients are still there, but you have more mix than water. The same is true for ozone - it is still generated because both VOCs and NOx (the ingredients) are still there.

As you can see, even though we have reduced cars, ozone will not necessarily drop right away as you still have the ingredients. There are still plenty of trucks on the roads, cars idling in drive-thrus, people driving just to get out of their homes, etc. However, there is a tipping point.

The benefit of a NOx limited environment is there becomes a point where if you remove enough NOx, ozone will quickly drop even if you still have a lot of VOCs. Liken that back to the pancake example - if you don't have any water or just a few drops, the mix won't be able to create a pancake. Hence, the key in the current situation is continuing to reduce the amount of NOx significantly, so there isn't enough of it to create ozone.

That's why less traffic due to current events many not result in a big drop in ozone levels - yet. We need to get to that tipping point. It remains essential that we continue every effort to maintain and improve air quality across the State. Ozone in the air we breathe can be harmful to our health and worsen bronchitis, emphysema, asthma and other respiratory issues ꟷ including those symptoms related to COVID -19. See how you can help HERE .

The Phoenix valley is also just starting to enter "ozone season," so we will continue to track ozone across the Valley and how the reduced amount of traffic will play into the equation.