ozone layer depletion

Ozone Layer Depletion

Introduction

Ozone is a bluish gas located in the stratosphere which protects the earth by absorbing UV-B and prevents this harmful radiation from reaching the earth. Research has shown that the ozone is slowly being depleted. We will discuss: The causes of ozone depletion The impacts ozone depletion has on the environment The current status of the ozone Solutions to the problem

What is ozone?Ozone is a stable molecule composed of three oxygen atoms. While stable, it is highly reactive. The Greek word ozein means to smell and O3 has a strong pungent odor. Electric discharges in air often produce significant quantities of O3 and you may have smelled O3 near these sources.

Ozone in the atmosphere

The ozone layer

Ultraviolet protection by ozone

Ozone absorbs UV light in the solar irradiation that is harmful to life

Ultraviolet protection by ozone

The overlap of ground level radiation with the sunburn sensitivity curve would be much greater without the filtering effects of the ozone layer.

Ozone formation and destruction in the stratosphere

Chapman Theory O2+ hv ( 2O b) O+O2+M -> O3+M c) O3 + hv (25 km, the net effect is to destruct O3. (NOx accounts for >50% of total ozone destruction in the middle and upper troposphere.) In the lower stratosphere, the net effect is to protect O3 from destruction.

The catalytic destruction reactions described so far, together with the Chapman cycle, account for the observed average levels of stratospheric ozone, they are unable to account for the ozone hole over Antarctica. The ozone depletion in the Antarctica is limited both regionally and seasonally. The depletion is too great and too sudden. These observations can not be explained by catalytic O3 destruction by ClOx alone.

Causes of Depletion

According to the Environmental Protection Agency, the discovery of an ozone hole over Antarctica in 1985 focused attention on the idea that humans can have a significant impact on the global environment. There are also a number of natural causes of ozone depletion. When the following substances reach the stratosphere, they break down under intense ultraviolet light, and release chlorine or bromine atoms, which degrade the ozone.

Chlorofluorocarbons (CFCs)

CFCs is the abbreviated form of ChloroFluoroCarbons, a collective name given to a series of compounds containing chlorine, fluorine and carbon atoms. Examples: CFCl3, CF2Cl2, and CF2ClCFCl2. Related names HCFCs: Hydrochloroflorocarbons, halocarbons containing hydrogen atoms in addition to chlorine, fluorine and carbon atoms. HFCs: hydroflorocarbons, halocarbons containing atoms of hydrogen in addition to fluorine and carbon atoms. Perhalocarbons: halocarbons in which every available carbon bond contains a haloatoms. Halons: bromine-containing halocarbons,

Chlorine atomSources: Photolysis of Cl-containing compounds in the stratosphere. CFCl3 + hv (185-210nm) CFCl2. + Cl. CF2Cl2 + hv (185-210nm) CF2Cl. + Cl. Subsequent reactions of CFCl2 and CF2Cl more Cl atoms The principal Cl-containing species are: CF2Cl2, for Cl-containing compounds (need Sources CFCl3, CFCl2, CF2Cl, CCl4, CH3CCl3,to be CF2HCl, CH3Cl long-lived in the troposphere) Man-made: e.g. CFCs Natural: e.g. methyl chloride from biomass burning.

Chlorine atom (Continued)Termination reactions for Cl Cl. + CH4 CH3. + HClStable in the stratosphere Removed from air by precipitation when it migrates to the troposphere

ClO. + NO2 + M ClONO2 + MReservoir species Relatively unreactive but can regenerate reactive species upon suitable conditions ClONO2 + hv ClO + NO2

Nitric oxide

NO is produced abundantly in the troposphere, but all of it is converted into NO2 HNO3 (removed through precipitation) NO in the stratosphere produced from nitrous oxide (N2O), which is much less reactive than NO. N2O + hv N2 + O (90%) N2O + O 2 NO (~10%)

Removal processes: Inhibit theNO2 + .OH HNO3 ClO. + NO2 ClONO2 HOx and ClOx cycles

Hydroxy radical

Accounts for nearly one-half of the total ozone destruction in the lower stratosphere (16-20 km). Sources O3 + hv (