The Air Quality Index is a measure that combines the level of pollution from six criterion pollutants: ozone (O3), sulphur dioxide (SO2), nitrous oxide (NO2), carbon monoxide (CO), particulate matter smaller than 2.5 micrometers (PM2.5), and particulate matter between 2.5 and 10 micrometers (PM10). For a brief discussion of these pollutants, see Air Quality Update 2017.

Figure 1. Data source: Environmental Protection Agency.

Figure 1 shows the percentage of days for which each of the criterion pollutants was the most important one. The chart combines all 24 counties together. Since 2009 ozone has been the most important pollutant on more days than any of the other pollutants, and it extended its “lead” in 2017. PM2.5 was the most important pollutant on the second highest number of days. Since 2007, however, the percentage of days on which it was the most important pollutant has been trending lower. One or the other of these two pollutants was the most important on 77% of all days statewide.

(Click on figure for larger view.)

Thirty years ago, ozone was a much less important pollutant than it is now. In 1983, it was the most important pollutant on fewer than 30% of the days statewide, but in 2017 it was the most important pollutant on 54% of the days. While we need ozone in the upper atmosphere to shield us from ultraviolet radiation, at ground level it is a strongly corrosive gas that is harmful to plants and animals (including us humans). We don’t emit it directly into the air. Rather, it is created when nitrogen oxides and volatile organic compounds (vapor from gasoline and other similar liquids) react in the presence of sunlight. These pollutants are emitted into the atmosphere by industrial facilities, electric power plants, and motor vehicles.

The second most important pollutant was PM2.5 (23% of days in 2017, sharply reduced from 2016). These tiny particles were not recognized as dangerous until relatively recently, though now they are thought to be the most deadly form of air pollution. I can’t find anything that says so specifically, but I believe the zero readings in 1983 and 1993 means that PM2.5 wasn’t being measured in Missouri, not that it wasn’t a significant pollutant back then. The EPA significantly tightened its regulations for PM2.5 in 2012. In 2015, no Missouri county was determined to be noncompliant with the new standards, however data gaps from sensors just across the Mississippi River prevented determination of whether pollution from Missouri was causing a violation of standards in the Illinois side of the metro area. Thus, the counties of Franklin, Jefferson, St. Charles, St. Louis, and St. Louis City were all called “unclassified.” Road vehicles, industrial emissions, power plants, and fires are important sources of PM2.5.

Sulphur dioxide used to be by far the most important pollutant. While it has not been eliminated and was still the most important pollutant on some days, good progress was made on reducing SO2 emissions: 6% of days in 2011. Since then, however, its relative importance has been on the increase, and in 2017 it was the most important pollutant on 16% of days. For a discussion of the role of SO2 in background air pollution, see this post. For my most recent update on the concentration of sulfur dioxide in background pollution, see here.

Don’t forget that Figure 1 does not show the levels of the six pollutants, it shows the percentage of days on which each was the most important. As previous posts have clearly shown, air quality is better. As we have reduced some types of air pollution, apparently, other types have increased in relative importance.

Missouri has come a long way in improving its air quality. To a large extent, it did so in two ways: by kicking some of its coal habit (replacing coal with natural gas and oil as sources of energy), and by requiring large coal-burning power plants to install pollution control equipment. We have more work to do, especially with regard to O3 and PM2.5, but it has been a significant environmental success story.

In the next post, I will discuss the health effects of air pollution. Spoiler alert: air pollution isn’t good for you!

Sources:

Environmental Protection Agency. Air Quality Index Report. This is a data portal operated by the EPA. Data downloaded on 7/31/2018 from http://www.epa.gov/airdata/ad_rep_aqi.html.

Missouri Department of Natural Resources. Missouri State Implementation Plan: Infrastructure Elements for the 2012 Annual PM2.5 Standard. Viewed online 3/30/2017 at https://dnr.mo.gov/env/apcp/docs/adopted-isip-2012-pm2.5-naaqs.pdf.