Tropospheric Ozone

Ozone (O3) is a molecule which can be both beneficial and detrimental to life. In the stratosphere it protects us from harmful ultraviolet radiation, and is essential to life. In the troposphere, however, ozone is a dangerous pollutant, it is a key component in the photochemical smog which effects such cities as Los Angeles and Atlanta. Ironically, there are some human activities which create the dangerous ozone that is increasing in the troposphere, while others destory the ozone of the stratosphere which protects us. A current problem is the reduction of stratospheric ozone due to pollutants such as CFC's (chloroflourocarbons), causing the emergence of an ozone "hole" around Antarctica. Ozone is considered to be a secondary pollutant, as it is not directly emitted into the atmosphere, but is a result of the reactions of pollutants that are emitted.

The reaction to form ozone is:

O2 + O --> O3

Two of the substances which are most directly involved in forming ozone are NOx (Nitrogen Oxides) which consist of NO and NO2, and VOC (Volatile Organic Compounds). NOx are derived mainly from the combustion of fuel in cars, VOC come both from fossil fuel combustion and from natural sources such as some trees (isoprenes and pinenes) and decomposition of organic material. Although NOx and VOC are the two main sources of ozone, other important precursors include methane (CH4), Carbon Monoxide, and the ever-important hydroxyl radical (OH).

Quick Quiz: what are two of the most important precursors for tropospheric ozone?
Sulfur Oxides and VOC
VOC and NOx
Sulfur Oxides and NOx
O2 and Sulfur Oxides

The following chart identifies some of the main sources of pollutants, many of which are precursors to ozone:

pie chart of emissions

Some regions in the world are more susceptible to ozone than others, usually due to a number of factors. Obviously the main factor which effects ozone formation is the prevalance of pollutants. Large cities with many commuters tend to be the places with the largest ozone problems. Additional complicating factors include geography and weather. One can look at Los Angeles as a case study of the characteristics of a city with high ozone levels:
  • Many sources of primary pollutants, especially automobiles which emit high levels of NOx
  • Temperature Inversions which inhibit the turbulent mixing of air, causing the pollutants to remain concentrated in one area
  • Few clouds, allowing for high UV intensity, a key ingredient in ozone formation.
  • Light winds which do not disperse pollutants well.
  • Coastal Mountain terrain which traps pollutants in the area.

The following link shows the ozone levels throughout the world on a monthly basis, note the amount of ozone over the Los Angeles area, especially in months like June and July. Compare that to the amount of ozone over the ocean, as well as to the amount of ozone during cooler months such as January and February.

Map of Ozone concentrations
Confused? Have a question? If so, check out the Frequently Asked Questions (FAQ) page or send mail to the OS411 tutor (os411tutor@shodor.org) with your question!
Report technical/content problems here