Introduction: Photochemical Smog

Good air quality is important to public health. Several studies have linked high levels of air pollution to increased hospital admissions for respiratory and heart disease. Autopsies reveal more lung and airway damage in people exposed to high levels of air pollution. This tutorial will discuss the harmful components of polluted air, describe the important sources of air pollution in the United States, and examine several strategies for reducing air pollution. Redox reactions, where electrons are transferred from one compound to another, are the foundation for understanding each of these subjects; redox reactions are involved in creating the molecules that comprise pollution, they are central to understanding why polluted air poses a health threat, and they are at the heart of strategies to reduce air pollution. Thus, to understand air pollution and its effects, we must understand electrochemistry, the study of the connection between chemical and electrical energy, and redox reactions.

In the modern industrial world, photochemical smog is the most common air-quality problem. Smog is a mixture of pollutants including nitrogen oxides, ozone, aldehydes, peroxyacetyl nitrate, particulates and volatile organic hydrocarbons (see Table 1).

Molecule Name and Formula	Two-Dimensional Representation	Three-Dimensional Representation
Nitric Oxide, NO
Nitrogen Dioxide, NO₂
Ozone, O₃
Acetaldehyde, C₂H₄O (an example of an aldehyde)
Peroxyacetyl Nitrate, C₂H₃O₅N
1,3-Butadiene, C₄H₆ (an example of a volatile organic hydrocarbon)

Table 1

The 2D representations shown in this table were drawn using CS ChemDraw Pro. In the 3D representations, carbon atoms are gray, hydrogen atoms are light blue, oxygen atoms are red, and nitrogen atoms are dark blue. The coordinates for the 3D representations were obtained from molecular-modeling calculations using ChemDraw 3D, and the images were rendered using SwissPDB Viewer and POV-Ray (see References).

These small molecules confer a variety of characteristics on the atmosphere. Nitrogen dioxide ( NO₂) is a brownish color, and is responsible for the visible "brown cloud" associated with smog. Many people notice eye irritation when smog is severe because peroxyacetyl nitrate is a lachrymator (irritates the eyes), and some people experience difficulty breathing. In addition, visibility is impaired on smoggy days. Figure 1, below, shows the difference in visibility on high-pollution vs. low-pollution days in St. Louis, Missouri.

Figure 1a. Low Pollution Day

Figure 1b. High Pollution Day

Figure 1

Figure 1a is a picture of downtown St. Louis on a clear (low pollution) day. Figure 1b is a similar picture on a smoggy day. Notice that it is difficult to see the arch in Figure 1b. These pictures were taken by the Missouri Department of Natural Resource's Visual Air Pollution Camera. Click on the preceding link to see these pictures as well as a photo of today's visibility conditions.

In the United States, the Environmental Protection Agency (EPA) is charged with monitoring air quality and ensuring that air pollution is not allowed to become so severe that it endangers public health. How does the EPA evaluate air quality?