Climate Dictionary

PSCs

Stratospheric Ozone

Tropospheric Ozone

Images

TROPOSPHERIC OZONE (Ground ozone, "bad" ozone, lower atmosphere ozone) What it is, what it does and why it is harmful

What is tropospheric ozone?

Ozone anywhere is a oxygen molecule comprising three oxygen atoms.

Why is tropospheric ozone called "bad" ozone? 

Whereas ozone in the stratosphere -- good ozone -- is beneficial because it shields us from ultraviolet radiation, ozone in the lower atmosphere -- bad ozone -- is harmful to living things. It harms lung tissue, and stunts the growth of plants and crops. Reactions involving ozone can damage electronic devices and materials such as rubber, plastics, outdoor paints, photographic papers, and fabrics.

Is tropospheric ozone a greenhouse gas?

Yes, tropospheric ozone is a greenhouse gas, which means it contributes to global warming. It is also involved in chemical reactions that produce the hydroxyl radical (OH). This plays a critical role in the atmosphere’s ability to cleanse itself of many pollutants. The hydroxyl radical is necessary for ozone formation in the lower atmosphere.

How is ozone formed and destroyed in the troposphere?

Ozone does form naturally in the troposphere, but it has become a pollutant as a result of human activity. Chemicals emitted from industrial processes, vehicle exhausts, and other products of fossil fuel combustion form ozone through a series of complicated chemical reactions involving sunlight.

   At ground level, ozone is formed when volatile organic compounds (VOCs) and nitrogen oxide gases react in sunlight.

   VOCs are used as ingredients in household products including; paints, varnishes, wax, fuels, cleaning, disinfecting, cosmetic, degreasing, and hobby products. In addition to all of the man made sources of VOCs, natural sources of VOCs exist. Trees naturally release small amounts of VOCs. 

   NOx, (nitrogen oxide gases) is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. The main sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels.

   High levels of VOCs and NOx react in the presence of sunlight and hot weather, producing ground ozone.

Where is ground ozone formed?

Many urban areas have high levels of ground level ozone. But even rural areas are subject to increased ozone levels because winds carry ozone, and the pollutants that form it, hundreds of miles away from original sources.

   If large amounts of NOx are transported to a rural area, filled with trees that are naturally releasing VOCs, a large amount of ground level ozone can form there.

What is the single most important source of ground ozone?

Motor vehicle emissions.

(Information courtesy of EPA, NASA, and the EU)

Table: ozone concentrations in parts per billion of air molecules

Glossary

Troposphere: The lowest layer of the atmosphere located between the earth's surface to approximately 11 miles (17 kilometers) into the atmosphere. 

Stratosphere: The layer of the atmosphere located between the troposphere and the mesosphere, characterized by a slight temperature increase with height and absence of clouds (except polar stratosphric clouds). It extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. It is the location of the earth's ozone layer.

Greenhouse gases: Natural greenhouse gases include water vapor, carbon dioxide, and ozone. The Kyoto Protocol covers a basket of six greenhouse gases (GHGs) produced by human activities: carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride.

VOCs: Organic solvents, certain paint additives, aerosol spray can propellants, fuels (such as gasoline, and kerosene), petroleum distillates, dry cleaning products and many other industrial and consumer products ranging from office supplies to building materials. VOC's are also naturally emitted by a number of plants and trees.

Nitrogen oxides (NO, NO₂, and NO_3-; or NO_x for short) are exhaust products from factories and automobiles. NOx is produced during the high temperature combustion of all fossil fuels. In addition to contributing to ozone formation, some nitrogen oxides, such as NO₂ (nitrogen dioxide), are corrosive by themselves and can cause respiratory problems.

Ozone formation: For those with some chemistry background, tropospheric ozone is formed through the following sequence of reactions: various VOC (volatile organic compounds) or CO react with the OH (hydroxyl) radical. This is followed by the conversion of NO to NO₂ (through reaction with HO2 or RO2 radicals, where RO2 represents any of a number of chains of organics with an O2 attached), which also regenerates OH. NO₂ is photolyzed (reacts with sunlight) to generate atomic oxygen, which combines with O2 to create O_3.