Global Tropospheric Ozone Production

Most of us have heard about ozone in the context of the stratosphere's ozone layer (the "good" ozone, 10-15 miles above our heads, that shields us from harmful ultraviolet radiation). Most of us have also heard about ozone in the context of pollution problems in smoggy cities and even in some rural areas (the "bad" ozone at the Earth's surface that harms crops, forests, and human health). This is the story of yet another of ozone's roles in our atmosphere, one that you might not be as familiar with. Here we will consider the bulk of ozone that exists in the entire lower atmosphere ("troposphere"), not just the "smog" ozone at surface locations.

The Earth's atmosphere is a reactive chemical system. Ozone, as one of the most reactive gases in the atmosphere, is one of the key players in chemical processes that "cleanse" it of various substances. For example, ozone either directly or indirectly contributes to the breakdown of a wide range of substances containing carbon and hydrogen, such as methane (CH4) and the hydrochlorofluorocarbons (HCFCs) that are being used as substitutes for ozone-destroying chlorofluorocarbons (CFCs). The cleansing action causes these and other gases to be removed from the lower atmosphere, or troposphere, and limits (or, in some cases, eliminates) their ability to reach the stratosphere.

Many of these gases are infrared-active-- "greenhouse gases." They absorb infrared (heat) radiation that would otherwise escape to space, lending a warming term to the heat budget of our planet. And ozone itself is a major greenhouse gas.

Changes in tropospheric ozone and its chemical precursors are thus coupled to the Earth's climate system, both directly (because ozone itself absorbs infrared radiation) and indirectly (because ozone, its precursors, and its close relatives influence the amount of other greenhouse gases in the atmosphere).

What processes govern the amount of ozone in the global troposphere? How are the activities of humans affecting that ozone and, more generally, the oxidizing capabilities of the lower atmosphere? What are the implications of changes in global tropospheric ozone for climate? At CSD, scientists are engaged in field, theoretical, and laboratory studies that address these questions (see below).

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