In my previous 3 posts, I have noted that in most Missouri counties air quality improved from 2013 to 2014: the percentage of unhealthy air quality days decreased, and the percentage of good air quality days increased. However, 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 2014.

Data source: Environmental Protection Agency Air Quality Index Report

The chart at right shows the percentage of days for which each of the criterion pollutants was the most important one. The chart combines all 20 counties together. In 2014, as in 2013, ozone was the most important pollutant, followed by small particles (PM2.5). One or the other of these two pollutants was the most important on 87% of all days statewide.

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. It is now the most important pollutant on about half the time (49% in 2014, 51% in 2013). 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 (think 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 (38% of the time in 2014, 35% in 2013). 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 their 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 a couple of years ago, and the Missouri Department of Natural Resources is in the midst of deploying a program to come into compliance with the new standards. 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 has been made on reducing SO2 emissions (48% of the time in 1983, 8% in 2014). For the role of SO2 in background air pollution, see this post.

In Jackson County, the county with more unhealthy air days than any other, the most important pollutant in 2014 was PM2.5. It was most important on 200 days, while sulphur dioxide was most important on 97. In St. Louis City, the county with the second most unhealthy air days, the most important pollutant was also PM2.5. It was most important on 251 days, while ozone was most important on 54.

Don’t forget that the chart at right does not show the levels of the six pollutants, it only shows the number 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 become more important.

I’ll offer some summary conclusions about this air quality data and climb on my soapbox briefly in the next post.

Sources:

Environmental Protection Agency. Air Quality Index Report. This is a data portal operated by the EPA. Data for 2014, Missouri, and grouped by County downloaded on 11/6/2015 from http://www.epa.gov/airdata/ad_rep_aqi.html.