Why the ozone  layer has been damaged?

  

      Versión en español                                                                    English version  

 

When the chemist Mexican-American Mario Molina and your colleague Sherwood Rowland, both award-winning in 1995 with the  Nobel Prize of Chemistry,  published  in the magazine Nature on 1974, the hypothesis that the photochemical decomposition of the chlorofluorocarbons in the stratosphere was responsible for the deterioration observed  then in the ozone layer had begun still the way toward  the explanation of the enigma and the more difficult path for the adoption of programs to reduce first and then eliminate the commercial production of the chlorofluorocarbons.  

 

For 1987, Molina had demonstrated, simulating the stratospheric conditions in the laboratory, the chain mechanism able to degrade the ozone with the participation of the chlorine free atoms, as primary radicals very reactive.     

 

The question that then gets up it is if among the numerous natural chlorated products those that rise until the stratosphere exist and cause a similar depletion mechanism.  It's well-know that the immense majority of natural chlorate compounds is soluble in water and therefore they cannot reach the stratosphere. Big quantities of chlorine (in form of sodium chloride) are evaporated of the oceans, but they are soluble in water for what they are caught by the clouds and they lower in water drops again, or snow. Another source of "chlorine" is that of the pools, but this chlorine is also soluble in water. The hydrogen chloride, product of the volcanic eruptions is a clear example of a natural pollutant, but it is transformed into hydrochloric acid, soluble in water and unable to reach the stratosphere.

 

On the other hand, the halocarbons manufactured by the man, as the CFCs, carbon tetrachloride (CCl4) and the methyl chloroform (CH3·CCl3) among other, are not soluble in the water, and they present a high inertia chemical reasons for which traffic freely pushed by the atmospheric currents toward the higher layers.

 

The following graph shows the relationship of the essential sources of chlorine in the stratosphere, about ends of ninety in last XX century.

 

 

The mechanism of destruction of the stratospheric ozone is quite complex. The chemist of Mexican origin Mario Molina received the Prize Nobel of Chemistry in 1995 for his excellent contributions to the description of this mechanism under conditions that  simulated in the laboratory what happens in the  stratosphere. 

 

The images that below schematize the process simplify their course notably. The key in the understanding of the dangerous destructive power of an free chlorine atom is the nature in chain of this process that regenerates active radical species, following the destructive cycle.  As average an atom of chlorine is able to destroy up to 100 000 molecules of ozone. This process stops finally when being formed a stable species.  

 

The radiation UV pulls up the chlorine of a molecule of chlorofluorocarbon (CFC). This atom of chlorine, when combining with a molecule of ozone destroys it, but it originates another active species (monoxide of chlorine) that restarts new stages of decomposition  

 

        

 

 

 

 

In a more rigorous form the following roads are presented:

 

The action of the ultraviolet radiation on the molecules of the freons produces those very reagents chlorine free atoms: 

CCl2F2 + hn ® CClF2 + Cl (1)

CCl3F + hn ® CCl2F + Cl (2)  

The atomic chlorine liberated in these processes is a direct agent of the destruction of ozone through diverse mechanisms, some of which are indicated next: 

Direct action: 

Cl + O3 ® ClO + O2

O + ClO ® Cl + O2

Net: O + O3 ® 2 O2 (3)  

Action through very abundant radical OH in our atmosphere:

O3 + OH ® HO2 + O2

Cl + O3 ® ClO + O2

HO2 + ClO ® HOCl + O2

HOCl + hn ® OH + Cl

Net: 2O3 ® 3 O2 (4)  

Action through the intervention of third bodies: 

Cl + O3 ® ClO + O2

2ClO + M ® (ClO)2 + M

(ClO)2 + hn ® Cl + ClOO

ClOO + M ® Cl + O2 + M

Net: 2O3 ® 3 O2 (5)  

According to these reactions, free chlorine atoms,  chlorine monoxide and hypoclorous acid are active species.

 

As competitive processes that originate inactive species regarding the ozone destruction are recognized the following ones: 

H2O + ClONO2 ® HNO3 + HOCl

HOCl + ClNO2 ® HNO3 + Cl + Cl  

However it has been suggested that it has more than enough solid surfaces as the glasses of ice of the Antarctic stratosphere they can take place quickly: 

Cl + CH4 ® HCl + CH3

ClO + NO2 ® ClONO2  

 

On the other hand the nitrogen monoxide formed in the stratosphere  enters in the chain process:

O3 + NO ® O2 + NO2

NO2 + O ® NO + O2

References:

 

Yañez M. (1993): Contaminación: Causas y posibles paliativos. La disminución de la capa de ozono. Aspectos Relevantes de la Química Actual. Ediciones de la Universidad Autónoma de Madrid.

 

UNEP (2002): Scientific Assessment of Ozone Depletion. Twenty questions and answer about the ozone lawyer. United Nations Environmental Program.

http://www.al.noaa.gov/assessments/2002/qandas.html

INDEX