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Background Air Quality 2017


Several times this blog has reported on air pollution, especially focusing on the Air Quality Index published by the EPA. In general, air quality has improved significantly. (The most recent series starts here.) The Air Quality Index monitoring program in Missouri focuses on large metropolitan areas or potentially large sources of pollution. Monitoring sites are often located next to pollution sources such as busy highways, industrial areas, or smelters. The sites monitor pollution where it is most likely to be most intense, but they don’t tell us much about the background level of pollution that has dispersed into the atmosphere.

Air Pollution Affects Visibility at Bryce Canyon National Park. Source: National Park Service, 2017.

The photos at right show Bryce Canyon National Park on three days ranging from clear to hazy. Bryce Canyon is dry, so the haze is not caused by humidity, it is air pollution. But Bryce Canyon is one of the remotest locations in the continental United States. It is close to no cities and no major sources of air pollution. The haze is caused by pollution that has dispersed widely into the atmosphere.

Spurred by the problem of acid rain, in 1990 the Environmental Protection Agency, National Park Service, and Bureau of Land Management established a network of rural monitoring sites far from cities and significant sources of pollution, called the Clean Air Status and Trends Network (CASTNET). These cites monitor the degree to which pollutants have dispersed into the ambient air. CASTNET has grown into a national network of 95 monitoring sites. CASTNET focuses on only a few pollutants most relevant for acid rain: sulfur dioxide and sulfates, nitric acid and nitrates, and ozone. (Clean Air Status and Trends Network 2017a)

No CASTNET monitoring sites are located in Missouri. Sites are located in Clark County Arkansas, Champaign, DuPage, Jo Daviess, and Madison Counties in Illinois, Brown and Riley Counties in Kansas, and Adair County in Oklahoma. (Clean Air Status and Trends Network 2017b)

The program to reduce the air pollution that causes acid rain has been one of the most successful environmental programs in our nation’s history. Two of the principal causes of acid rain are sulfur dioxide and nitrogen dioxide. When these gases are emitted by power plants and vehicles, 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 metal, stonework, and concrete. Forests are affected, of course, but in addition, billions of dollars of damage has been done to buildings, bridges and roads.

Figures 1-4 map the average background concentration of sulfur dioxide over 4 periods: 1989-1991, 1999-2001, 2009-2011, and 2011-2014. Figures 5-8 map the average background concentration of nitric acid over the same 4 periods. (Be sure to notice that there is a decade between the first three maps in each series, but fewer years between the final two.)

Suflfur Dioxide Maps

Figure 1. Source: CASTNET 2017a.

Figure 2. Source: CASTNET 2017a.









Figure 3. Source: CASTNET 2017a.

Figure 4. Source: CASTNET 2017a.











Nitric Acid Maps

Figure 5. Source: CASTNET 2017a.

Figure 6. Source: CASTNET 2017a.









Figure 7. Source: CASTNET 2017a.

Figure 8. Source: CASTNET 2017a.











First, notice that the white space on the maps disappears over time. The CASTNET system did not cover the whole country at first, and this represents the development of the system.

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

Third, notice that over time the areas of red and orange have disappeared, and the area of yellow has been much reduced. The background atmospheric concentration of these two pollutants is much less than it was in 1989.

The background level of sulfur dioxide has improved significantly in Missouri and across the entire eastern portion of the country. Across the West, it does not appear to have been very high when measurements started. On the other hand, across the West the high background concentration of nitric acid appears to have occurred primarily in Southern California. It has improved. So has the background concentration of nitric acid across Missouri and the entire eastern portion of the country.


Clean Air Status and Trends Network. 2017a. Ambient Air Concentrations. Downloaded 12/1/2017 from https://www3.epa.gov/castnet/maps/airconc.html.

Clean Air Status and Trends Network. 2017b. Clean Air Status and Trends Network (CASTNET): Program Overview. https://www3.epa.gov/castnet/docs/CASTNET-Factsheet-2015.pdf.

Clean Air Status and Trends Network. 2017c. Site Information, Clean Air Status and Trends Network, EPA, http://java.epa.gov/castnet/epa_jsp/sites.jsp.

National Park Service. 2017. Air Pollution Impacts, Bryce Canyon National Park. Downloaded 12/2/2017 from https://www.nature.nps.gov/air/Permits/aris/brca/impacts.cfm?tab=0#TabbedPanels1.

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