Chemistry in the
Sunlight

by Jeannie Allen January 27, 2002

 

The sun’s awesome power drives a multitude of chemical reactions that are critical for life on Earth. In the atmosphere, ultraviolet radiation at wavelengths smaller than 242 nanometers splits molecular oxygen (two atoms bonded together) into atomic oxygen (individual atoms). Then when some energetically excited individual oxygen atoms encounter molecular oxygen, they can bond to form three-oxygen molecules, ozone.

Photograph of
Sunrise
Solar ultraviolet (UV) radiation drives the chemical reactions that produce ozone in both the Earth’s upper atmosphere (stratosphere) and its lower atmosphere (troposphere). (Photograph copyright National Center for Atmospheric Research Digital Media Catalog)

Ozone packs a punch in our lives that’s out of proportion to its small concentrations in the atmosphere. Its influence can be for good or ill depending on where it is. Ozone far above us in the upper atmosphere (stratosphere) absorbs and protects us from deadly ultraviolet radiation. Ozone in the lower atmosphere (troposphere) where we live is toxic. It reacts easily with biological tissue, donating oxygen atoms in the process known as oxidation. Breathing too much ozone over time impairs human lung capacity and causes illness and, for a few, premature death. Other animals and some plants also suffer from ozone overexposure. Several important crop plants such as soybeans and tobacco respond to currently common concentrations of ozone with lower rates of photosynthesis and reduced productivity.

Ozone formation occurs naturally throughout the atmosphere. From a few percent to as much as 50 percent of the ozone in the troposphere intrudes from the stratosphere. (The exact amount depends on location and time of year.) The rest forms from two groups of chemical compounds that occur both naturally and as by-products of fossil fuel combustion: nitrogen oxides (NOx) and volatile organic compounds (VOCs), which are carbon-containing gases and vapors such as gasoline fumes. Carbon monoxide also plays a critical role in some ozone formation reactions. Sunlight must be present for ozone to form, hence the term, “photochemical” smog. Without sunlight, no ozone forms.

As the human population has risen sharply and we have rapidly industrialized our economies over the last century, our consumption of fossil fuels has risen dramatically. Amounts of the byproducts of fossil fuel combustion emitted into the atmosphere have risen just as dramatically. Some of these byproducts contribute to ozone formation, therefore ozone concentrations in the air we breathe have risen as well. Ozone concentrations in the mid-1880s peaked somewhere around 10-15 parts per billion (ppb) in a given volume of air (Finlayson-Pitts and Pitts, 1999). Ozone levels in the troposphere now average 35-40 ppb around the globe in even the most remote regions (Finlayson-Pitts and Pitts, 1999; Fishman 1999; Madronich 1993). Typically in some suburban and rural areas in summer, ozone levels range from 80 to 150 ppb for several days at a time, far exceeding the U.S. National Ambient Air Quality Standard of 80 ppb averaged over an eight-hour period. According to the American Lung Association’s State of the Air 2002> report, unhealthy levels of ozone reached fully half of the American public during each of the last three years. In the most polluted urban areas of the world, ozone concentrations occasionally reach 500 ppb. (Finlayson-Pitts and Pitts, 1999

Map of Air
Quality

The air in many parts of the U.S. frequently contains unhealthy concentrations of ozone. This map from the Enivronmental Protection Agency shows ozone levels on August 12, 2002. Direct summer sunlight catalyzes emissions from cars and industry to create high ozone levels around many urbanized areas. (Map courtesy U.S. Environmental Protection Agency AIRNow)

Of all our common air pollutants, ozone has proven the most elusive to control. It forms through a highly complex series of reactions that take place over several hours, and that shift according to the presence of a multitude of other chemical species. Some governments have attempted to reduce ozone levels by mandating the reduction of hydrocarbons in motor vehicle emissions. But in recent years, it has become apparent that we must control NOx as well if we are to breathe healthy levels of ozone.

Photograph of Cloudtops
Understanding where ozone is likely to form and where it will travel means understanding the physical dynamics of the atmosphere: wind directions and speeds, humidity, and pressure. (Photograph courtesy NOAA Aircraft Operations Center)

Because tropospheric ozone forms over time, controlling it entails understanding the physical dynamics of chemical transport through the atmosphere: winds, humidity, atmospheric pressure, and so on. Since ozone and the chemicals that participate in its formation (precursors) can travel several hundred kilometers or farther on the wind, controlling ozone also requires monitoring and other research all around the globe. Since 1978, NASA and the European Space Agency (ESA), with the participation of Japanese and Canadian science organizations, have monitored tropospheric ozone using increasingly sophisticated satellite instruments. The Aura satellite will go into orbit in 2004 to make daily global maps based on high precision measurements of tropospheric ozone and its precursors.

next: Ozone, Space, and Time

  Ozone forms through a highly complex series of reactions that take
place over several hours.

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