How did COVID-19 travel restrictions impact air quality in Chicago and St. Louis?

How did COVID-19 travel restrictions impact air quality in Chicago and St. Louis?

Post by Charlie Blake and Josh Gifford

During the pandemic lockdown period in 2020 when travel was restricted, the natural world experienced big changes. We saw beaches around the world become healthier because people visited them less. Animals began to frequent areas they had not been visiting in quite some time. Water around the world became clearer than normal because of less boat travel. There was an eerie quietness with less vehicle, train, and airplane travel. But how did this period of travel restriction impact the quality of our air?

A great approach to answer this question is to check out purpleair.com. These low cost air quality sensors track particulate matter, a key factor in calculating air quality. For more information about air quality check out our previous blog, Air Pollution! Dr. Charlie Blake, post-doctoral fellow at the SIUE STEM Center, analyzed data from PurpleAir sensors in the Chicago and St. Louis areas to see what impact the travel restrictions had. They compared pre-pandemic (2019) to lockdown (2020) PM2.5 levels to see how much air quality had changed during these times.

When comparing 2019 to 2020 PM2.5 levels, Dr. Blake created the graph in Figure 1. The lockdown in Chicago began on March 18th, what differences between the two years can you see?

Figure 1. 2019 and 2020 PM2.5 levels from Chicago’s McKinley Park

We can see a strong peak in PM2.5 levels around April 7th, 2019 that is not seen anywhere in 2020. For the next six months the 2020 PM2.5 level remains lower than the previous year’s levels. Another way to look at these levels is by monthly average. This allows us to dig deeper and see if there is a significant difference between 2019 and 2020 PM2.5 levels.

Figure 2. 2019 and 2020 monthly mean PM2.5 levels in Chicago’s McKinley Park

This bar chart shows that every month in 2019 had higher PM2.5 levels than 2020 did during the pandemic. Figure 3 shows the results from a single factor ANOVA test which compares means between different categories. The results from this test provide a statistically significant difference between Chicago’s PM2.5 levels from 2019 and 2020.

Figure 3. ANOVA results for Chicago’s McKinley Park

You might think this air quality change would be seen everywhere across the country; however, St. Louis shows a slightly different story. Dr. Blake looked at St. Louis’s University City in the same manner they did for Chicago. Dr. Blake created Figure 4 to visually see the PM2.5 levels of 2019 and 2020.

Figure 4. 2019 and 2020 PM2.5 levels from St. Louis’s University City

This graph shows lower levels of PM2.5 for most of this 6 month period besides a sharp peak on the 4th of July. This peak in 2020 was caused by the celebration of the holiday with fireworks and the resulting smoke from the explosions in the air.

Figure 5. 2019 and 2020 monthly mean PM2.5 levels in St. Louis’s University City

By looking at St. Louis’s monthly means, we can see that not every month’s PM2.5 levels were higher in 2019. The beginning of the lockdown period showed lower levels while the late summer months were actually higher than the previous years. Figure 6 shows another single factor ANOVA. The results from this test do not show a statistically significant difference between St. Louis’s PM2.5 levels in 2019 and 2020.

Figure 6. ANOVA results for St. Louis’s University City

The results from this investigation into air quality during the lockdown period might be surprising to some. However, the geographic and regional differences between Chicago and St. Louis might have impacted the results. Many factors could have contributed to PM2.5 levels during the lockdown period such as weather patterns and cultural norms that centered on compliance with local lockdown ordinances. Why do you think Chicago’s air quality was different from 2019 to 2020 while St. Louis’s was not?

Feature Image by Max Bender, downloaded at Unsplash