Cleaner Air, Higher Ozone?

Cleaner AIr

Studying the environment is difficult because we often can’t isolate the world from all of the factors that make it up. For example, it’s difficult to measure the effects of air quality on populations of the world because conditions are so varied. Many underdeveloped countries have poor air quality, but also have populations for which smoking is a societal norm.

Because it is not ethical to expose healthy people to a polluted environment, it is difficult to elucidate which poor health outcomes are due to the direct environment, cultural norms, or the combination of the two. Environmental scientists face similar issues, as they are not able to change weather patterns or request that other countries stop burning coal to assess the effects on the atmosphere. Although the pandemic is a difficult time for us all, it does provide useful metrics that are nearly impossible to obtain without the specific set of conditions we find ourselves in.

Energy usage, especially that due to transportation and manufacturing, has plummeted in the last few months. Some of the most dramatic changes have been in underdeveloped countries. The picture below shows the difference in visible air pollution before and during the pandemic in New Dehli. This information has helped researchers pin down sources of pollution based on what activities have ceased and what energy consumption at home still contributes.

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New Delhi on Nov. 1, top, and again in April. Manish Swarup/Associated Press

 

Interestingly, one of the most talked-about pollutants is on the rise: Ozone. Dr. Guttikunda, the director of Urban Emissions.info speaks about this temporary spike. “This is a theory that atmospheric chemists learn in class, but we haven’t seen it work in real time.” So, if emissions are decreasing, why is Ozone increasing?

To explain this, we encounter the “Ozone Weekend Effect.” Ozone is the result of a chemical reaction in our atmosphere. Emissions react with sunlight to produce Ozone. The amount of a specific kind of emission Volatile Organic Compound, or VOC is usually the “limiting reactant” or “limiting ingredient.” This means that the production Ozone possible in a day is directly proportional to the amount of VOC emissions. You can think about this as if you were baking and had enough of all of your ingredients but eggs. The eggs limit how much cake you can make regardless of how much flour you have and is your “limiting reactant.”

These emissions are largely tied to manufacturing, and decrease drastically over the weekends. One would expect that ozone would therefore decrease proportionally. However, the opposite is true. While VOC is the limiting reactant, it turns out that this is only true when there is more VOC compared to another type of emission: Nitrogen Oxides (NOx.) When VOC emissions decrease, the ratio of which emissions are present changes. When this changes, the ratio of NOx emissions to VOC flips (high NOx to low VOC). Surprisingly, NOx causes Ozone to be formed at a much higher rate than VOC, but only when the ratio is flipped in this manner. This causes spikes of Ozone production when manufacturing emissions decrease, which may be seen over weekends or drastically, in a global pandemic.

As the NOx emissions decrease, however, the Ozone production will decrease again as well.
What does this mean? While you may hear reports of ozone increasing, it’s important to remember that this is because manufacturing emissions have decreased. The resultant spike in Ozone production is due to the remaining transportation emissions in our air, but these should decrease as they are “used up” in the reaction. Over the next few months we should see a decrease in Ozone due to this.

This reminds us that caring for our environment is something we all have a part in! Though a large portion of manufacturing has temporarily ceased operations, there are still effects from the choices you and I make every day, like energy usage in our homes and transportation.
When we recover from the pandemic we hope to walk away with insight into ourselves and our environment that can be catalysts for positive changes.

Kate Roberts, USA.

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