The COVID-19 outbreak has had major economic implications both locally and across the nation, but some notable environmental trends have also appeared. Specifically, some cities across the nation and the globe have seen decreases in air pollution. The reduction in air pollution is driven by factors such as government-ordered business shutdowns and fewer motor vehicles on the road. The Tucson Metropolitan Statistical Area (MSA) experienced an improvement in air quality through the end of May which is, in part, likely due to less traffic on the streets, according to Pima County’s Department of Environmental Quality (PDEQ).1 The current fire in the Santa Catalina Mountain range has negated the recent air quality improvements.
Ozone and PM 2.5 are two major pollutants of concern in Southern Arizona. Ozone is formed through chemical reactions between oxides of nitrogen and hydrocarbons, while PM 2.5 is particle matter smaller than 2.5 micrometers. Both of these pollutants can be the result of operating motor vehicles.2 Figure 1 shows the air quality trend for Ozone and PM 2.5 in Tucson from 2017 through May 2020. The COVID-19 outbreak and resulting stay-at-home orders picked up around the end of February through April so this is the timeframe of interest. The average ozone AQI (Air Quality Index) values for the month of March were 49 in 2017, 47 in 2018, 50 in 2019, and 43 in 2020. The average PM 2.5 AQI values showed a larger range in March, dropping to an average of 6 in 2020, more than four times lower than in previous years. AQI values for April were lower in 2020 than previous years, however there was no stark drop in ozone or PM 2.5, as there was for March. Air quality in Tucson can also have a seasonal component during the springtime.
Figure 1: Average Monthly Tucson Ozone and PM 2.5
Figure 2 displays a drop-down menu with charts for percent change of overall daily AQI values from 2019 to 2020 for peer western MSAs from January through May. Each chart also has a weekly average percent change comparing 2019 to 2020. When comparing peer MSAs, daily year-over-year percent changes were volatile. Weekly averages were volatile too, but for half of the metros, an interesting trend appeared between February and April. For Denver, El Paso, and Las Vegas, the average weekly percent change from 2019 to 2020 noticeably declined from the end of February to about mid-April, when compared to 2019. San Diego, Phoenix, and Tucson showed a similar downward trend from March through April before trending up just before May and then spiking down at the end of May. These trends are not definitive as air quality is impacted by several factors such as seasonality, but the year-over-year change is an interesting measure to explore. As more data is collected in the coming months, a clearer picture might appear.
Figure 2: Percent Change Air Quality Index (AQI) MSAs 2020
Some Arizona counties had similar trends to the metros explored, while others were very volatile across the board. However, for almost all counties, there was a large drop in average weekly percent change around mid-to-late February 2020, meaning that air quality was significantly better during this time in 2020 than in 2019. Counties such as Cochise, La Paz, Navajo, Maricopa, Pinal, Pima, and Yuma had better air quality from the end of February through April in 2020 than in 2019. Figure 3 shows year-over-year percent changes for overall daily AQI values for Arizona counties and weekly average change. Note: April and May data were not available for Apache and Mohave.
Figure 3: Percent Change Air Quality Index (AQI) Arizona Counties 2020
There is still a lot to learn about these air quality figures in the coming months, but it appears that air quality improved during the height of the COVID-19 restrictions, at least compared to 2019. Less motor vehicle travel due to COVID-19 is likely part of the reason for this air quality improvement.
How is it Measured?
Air quality data come from the U.S. Environmental Protection Agency’s Air Quality Index Daily Values Report, which provides AQI values daily for major air pollutants, though many areas do not have monitoring stations for all pollutants. The AQI has a range of 0-500 broken down into six levels: good (0-50), moderate (51-100), unhealthy for sensitive groups (101-150), unhealthy (151-200), very unhealthy (201-300), and hazardous (301-500). Overall AQI values for Metropolitan Statistical Areas and Arizona Counties are the maximum of the individual pollutant AQI values on a given date.
With fewer cars on the streets, Tucson's air quality is improving. (April, 2020), Arizona Public Media, https://news.azpm.org/p/news-topical-nature/2020/4/28/171171-with-fewer-...
- Guidlines for Developing an Air Quality Program, U.S. Environmental Protection Agency (EPA), https://www3.epa.gov/airnow/aq_forecasting_guidance-1016.pdf