Last April I reported data that suggested the air quality in many Missouri counties had markedly improved since the 1930s. In my previous post, I reported that the background level of two important pollutants (sulfur dioxide and nitric acid) had significantly improved over the eastern part of the United States, including Missouri.

The data covered in all of these posts used summary data covering a whole year. Summary data like that smooths out day-to-day variation. If, for instance, air pollution on the worst days was getting worse, but on the best days it was getting better, summary data would show no change.

This post looks at just this question: what are the background pollution trends for selected pollutants on the worst days, and on the best days? As discussed previously, to study the background level of air pollution, you can use sensing stations located in rural areas, far from big cities and far from major sources of pollution. The CASTNET system does just that. There are no CASTNET sensors in Missouri, but the Cherokee Nation Site is in Adair County, Oklahoma, not too far from the Missouri border. (If you like to play with Google Earth, you can find it at 35.7507N -94.6700W.)

I downloaded the ozone data from the Cherokee Nation Site. Ozone peaks during the day, so the data showed the daily maximum ozone concentration. The data covered the period of time from 1/1/2003 to 12/31/2012. For each year, I first calculated an average of all the readings. Then I selected the 10 highest readings and averaged them. Then I selected the 10 lowest readings and averaged them. The first chart at right shows the results. The blue line is the average of the 10 highest readings for each year, the red line is the average of all readings for each year, and the green line is the average of the 10 lowest readings for each year. The dotted lines show the trend for each.

(Click on chart for larger view.)

It is clear that the background level of ozone is not changing much, at least not at the Cherokee Nation Site. The trend for the highest days seems to be declining slightly. The trend for the overall average is also declining, though even less. And the trend for the lowest days seems to show slightly increasing ozone pollution.

I didn’t perform a statistical analysis on this data. I’m not sure the changes would be statistically significant, they are very small. Further, an article supplied to me by Dr. Jack Fishman at St. Louis University makes it clear that many factors influence ozone levels. However, there is a very slight trend: the worst days were less polluted, but the best days were more polluted.

The next 2 posts will look at more background pollution data, and one of them will draw very different conclusions.

Sources:

“Daily Ozone 8-Hour Maximum Reading, Cherokee Nation Site, Oklahoma.” EPA Home Page » AirData » Download Data » Download Daily Data. http://www.epa.gov/airdata/ad_data_daily.html.

Fishman, Jack, John Creilson, Peter Parker, Elizabeth Ainsworth, Geoffrey Vining, John Szarka, Fitzgerald Booker, Xiaojing Xu. (2010). And Investigation of Widespread Ozone Damage to the Soybean Crop in the Upper Midwest Determined from Ground-Based and Satellite Measurements. Atmospheric Environment, 44, 2248-2256.