The world is complicated. The air is thin.  Nature is subtle.  Our capacity to cause harm is great.  We must be much more careful and much less forgiving about polluting our fragile atmosphere.  Carl Sagan, from  Billions and Billions

from: http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/ATM_CHEM/earths_atmosphere.html

Remote Sensing of Ozone

Ozone is a very important trace gas in the atmosphere.  As I have mentioned in previous lectures on the temperature profile of the atmosphere, and when we discussed absorption by atmospheric gases, ozone plays a role in both the stratosphere and the troposphere, and has absorption bands in both the ultraviolet and the infrared portion of the electromagnetic spectrum.  Ozone in the stratosphere acts as a natural sun screen absorbing the high energy ultraviolet wavelengths, this results in an increase in the thermal energy (increased temperature) in the stratosphere, and a decrease in the amount of UV radiation penetrating through to the surface.     In addition to stratospheric ozone, there is also ozone in the troposphere, where it acts as a very important oxidant in a large range of chemical reactions.  A signficant portion of this ozone comes from anthropogenic precursors, nitrogen oxides and reactive hydrocarbon compounds.  These compounds undergo photochemical reactions that result in the formation of ozone near the surface (which is where most of the pollutants are located).   There are two very important current problems which are being investigated by atmospheric chemists, one is related to the decrease in stratospheric ozone, catalyzed by anthropogenic pollutants know as chlorofluorocarbons; the other is related to the increase in tropospheric ozone, triggered by increasing anthropogenic emissions of nitrogen oxides and reactive hydrocarbons in urban areas.  

My own research here at the University is directed at understanding the relative contributions of natural and anthropogenic (pollutant) ozone in the remote marine atmosphere over the North Atlantic.  Natural ozone comes primarily as air exchanged from the stratosphere to the troposphere driven by dynamics in the upper portion of the troposphere.  Additonal sources of natural ozone come from natural sources of nitrogen oxides and reactive hydrocarbons, such as lightning, biomass burning, soil emissions, vegetation emissions, etc.

There are several way that we try to measure O3 in the troposphere, we will discuss these methods and learn how we can use transport models, weather models, and remote sensing products to identify the origin and motion of ozone through the troposphere.  One approach is to use remotely sensed water vapor and its utility as a tracer of ozone which is exchanged from the stratosphere into the troposphere.

In preparation for these lectures on current research topics, we will review material assembled from several tutorials on ozone chemistry.  We will begin with a discussion of stratospheric ozone and ozone depletion.

The ozone layer was discovered by the French in 1933.  In 1936 a scheme was proposed by Chapman that showed  some oxygen (O₂) molecules absorbed energy from the Sun's ultraviolet (UV) rays (at wavelenghts < 220 nm, or .22um) and they split to form two oxygen atoms. These atoms combine with remaining oxygen (O₂) to form new ozone (O₃) molecules.  Ozone molecules themselves are very effective at absorbing UV rays, this occurs at slightly longer wavelengths (200-300nm, or .2-.3um) than the photodissociation of the oxygen molecules. The thin layer of ozone that surrounds Earth acts as a shield, protecting the planet from irradiation by UV light.  Together, these two processes, the photodissociation of O2 and the formation of uv-absorbing O3, remove almost all of the radiation at wavelenghts shorter than 300nm.  

The amount of ozone required to shield Earth from biologically lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is believed to have been in existence 600 million years ago. At this time, the oxygen level was approximately 10% of its present atmospheric concentration. Prior to this period, life was restricted to the ocean. The presence of ozone enabled organisms to develop and live on the land. Ozone played a significant role in the evolution of life on Earth, and allows life as we presently know it to exist.

Stratospheric Ozone Depletion:  Work resulting in the 1995 Nobel Prize in Chemistry (the first environmental science Nobel Prize)

Atmospheric Chemistry Educational Resources