In several previous posts, I have discussed the air quality data maintained in the EPA’s Air Quality System Data Mart. The EPA calculates an Air Quality Index (AQI) that combines the various pollutants into an index intended to represent overall air quality. But the EPA also identifies how many days each pollutant was the main one.

I identified 19 Missouri counties for which AQI data was available in more than 6 of the 12 years for which I downloaded data (1983, 1993, 2003-2012). The main air pollutant varies among them. For instance, in 2012, the main pollutant in Taney County was ozone on all 214 days monitored. However, in Iron County, sulfur dioxide was the main pollutant on all 351 days monitored.

Despite the fact that it might not apply to all locales, I thought it might be useful to explore how the most important pollutant has changed over time. The method I used to do this is described below. The graph at right shows percentage of days each pollutant is the main pollutant (for all 19 counties combined). In the graph, CO = carbon monoxide, NOX= nitrogen oxides, O3 = ozone, SO2 = sulfur dioxide, PM2.5 = inhalable particulates smaller than 2.5 micrometres, and PM10 = inhalable particulates between 2.5 and 10 micrometres.

PM2.5 was the most important zero percent of the time in 1983 and 1993, but increased in importance markedly thereafter. I believe this occurred because of changes in national standards and monitoring practices–the current standard for PM2.5 was established in 1997. EPA is in the process of changing this standard.

Sulfur dioxide has declined significantly in its importance. Where once it was the most important pollutant on almost half of all monitored days, now it is most important on less than 10%. Carbon monoxide has also declined in importance. In 1983, it was most important on almost 20% of days. Since 2003, however, it has not been the most important pollutant on any monitored day in any county in Missouri.

On the other hand, PM2.5 and ozone have each increased in importance. Although it would not be true in every location, across the state ozone is now the most important pollutant on approximately 50% of all days.

Sources:

Air Data, EPA, http://www.epa.gov/airquality/airdata.

For PM2.5 standard: Overview of EPA’s Proposal to Revise the Air Quality Standards for Particle Pollution (Particulate Matter), EPA, http://www.epa.gov/pm/pdfs/PMNAAQSProposalOVERVIEW61512UPDATED.pdf.

Method used to construct the graph:

For each county, the EPA reports the number of days each year a pollutant is the main one. For each pollutant, I summed the values across the 19 counties, yielding the total number of county-days for which each pollutant was most important (call this number “A”). I summed these, yielding the total number of county-days for which importance estimates were made (call this number “B”). Then, I divided A by B, and multiplied by 100, yielding the percentage amounts used in the graph.