Home » Air » Background Sulfur Dioxide and Nitric Acid Pollution Improve

Background Sulfur Dioxide and Nitric Acid Pollution Improve


The program to reduce the air pollution that causes acid rain has been one of the most successful in our nation’s history.

As described in the previous post, CASTNET is a network of 81 sensing stations located in rural areas, far from cities and major sources of air pollution. They are meant to capture the background level of pollution, unaffected by any local source.

Two of the principal causes of acid rain are sulfur dioxide and nitrogen dioxide. When these gases are emitted by power plants and vehicles into the atmosphere, they mix with water vapor already present in the air to form sulfuric acid and nitric acid. Even in this diluted form, these powerful acids fall with the rain, killing plants and dissolving metals, stonework, and concrete. Forests are affected, of course, but in addition, billions of dollars of damage has been done to buildings, bridges, and roads.

The first set of maps on the right shows the background concentration of sulfur dioxide over 3 periods: 1989-1991, 1999-2001, and 2009-2011. The second set of maps shows the background nitric acid concentration over the same time periods. The scale runs from green, which represents less of the pollutant, to dark red, which represents the most.

(Click on maps for larger view.)

First, notice that the white space on the maps disappears over time, This represents the development of the CASTNET system to cover the whole country.

Second, notice that in 1989-1991, the area of high pollution concentration extended from roughly Missouri to the east and northeast portions of the country. The prevailing wind blows west-to-east, blowing pollution from the Midwest towards the East.

Third, notice that over time the amount of red, orange, and yellow has decreased. In fact, by 2009-2011 areas of red and orange had disappeared entirely from the maps.

The maps are sufficiently complete in all 3 time periods to infer change from the Missouri-Kansas border eastward. The background level of sulfur dioxide and nitric acid appear to have improved significantly in Missouri. However, west of the Missouri-Kansas border, the maps are not sufficiently complete in all time periods to infer change. I included a photo of haze at Bryce Canyon National Park in the previous post. These maps don’t provide much information about the air quality there over the whole time period.

A couple of cautions need to be mentioned here. First, these data only cover two pollutants, sulfur dioxide and nitric acid. The two pollutants of most concern in Missouri are currently ozone and small particulate matter (PM2.5).

Second, these maps show average concentrations over three years. What if pollution was decreasing at some times of years, but increasing at others? These maps would not show it, a more detailed analysis would be required. The next post will look into that issue.


Ambient Air Concentrations, Clean Air Status and Trends Network (CASTNET), EPA, http://epa.gov/castnet/javaweb/airconc.html.

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