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Background Air Quality


Bryce Canyon Visability PhotosThe two photos at right show Bryce Canyon National Park on a clear day and a hazy day. 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. It is also dry, so the haze in the photo is not from humidity. Previous posts in this blog suggested that air quality was improving. What’s going on?

The air pollution 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 large lead smelters. The sites monitor pollution where it is most likely to be directly harmful. But they don’t tell us much about the background level of pollution that has dispersed into the atmosphere.

There are two ways to measure the background level of pollution. One method involves a network of rural monitoring sites far from cities and significant sources of pollution. Only there can you measure the degree to which pollutants have dispersed into the ambient air.

Spurred by the problem of acid rain, the federal government established just such a network in 1990, the Clean Air Status and Trends Network (CASTNET). Smaller at first, and focused on the eastern half of the country, it has grown into a national network of 81 monitoring sites. Each site is located in a rural area, far away from cities and significant sources of air pollution. CASTNET focuses on only a few pollutants most relevant for acid rain: sulfur dioxide and sulfates, nitric acid and nitrates, and ozone.

There are no CASTNET monitoring sites in Missouri; the nearest sites are in Alhambra, IL, Riley County, KS, Adair County, OK, Clark County, AR, and Trigg County, KY. I don’t know why Missouri is absent from the network.

The second way to measure the background level of pollution is to measure it from space. Several pollutant gases can be measured from space,and they include some important ones: ozone, carbon monoxide, carbon dioxide, methane, sulfur dioxide, nitric oxide, and formaldehyde.

My next few posts will focus on the ambient background levels of certain pollutants, as measured by the CASTNET program and from space. We’ve made progress in reducing the air pollution in our cities. Has the background level of pollution decreased as well? Or as the photos above might suggest, has it increased?


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

Remote Sensing of Tropospheric Pollution from Space. (2008). Fishman, J., K.W. Bowman, J.P. Burrows, A. Richter, K.V. Chance, D.P. Edwards, R.V. Martin, G.A. Morris, R. B. Pierce, J.R. Ziemke, J.A. Al-Saadi, J. K. Creilson, T.K. Schaack, and A.M. Thompson. Bulletin of the American Meteorological Society, 89,(6), 805-821.

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