Atmosphere ozone layer

UV-irradiated cells. Using cell-free cytosolic keratinocyte extracts, Simon and colleagues26 confirmed the role of membrane oxidation in NF-kB activation. Particularly important aspects of the experimental design employed by Simon and colleagues was the use of keratinocytes versus cells derived from a cervical cancer patient, and the use of biologically relevant UVB (290 to 320 nm) radiation versus UVC (200 to 290 nm) radiation, which is filtered out by the atmospheric ozone layer and does not reach the earth s surface. Overall, these data indicate that the activation of cytokine transcription, a step essential for the induction of immune suppression, can occur independently of UV-induced DNA damage and suggest that membrane lipid oxidation can serve as a UV photoreceptor. 

Halon - As employed in the fire protection industry, a gaseous fire suppression agent. Halon is an acronym for halogenated hydrocarbons, commonly bromotrifluoromethane (Halon 1301) and bromochlorodifluoro-methane (Halon 1211). Considered obsolete for fire protection purposes due to a possible environmental impact to the Earth s atmospheric ozone layer and beginning to be phased out or eliminated. 

Thus far we ve focused on the rate law, an equation that tells how a reaction rate depends on reactant concentrations. We re also interested, however, in how reactant and product concentrations vary with time. For example, it s important to know the rate at which the atmospheric ozone layer is being destroyed by air pollutants, but we also want to know what the ozone concentration will be 50 years from now and how long it will take for the concentration to be reduced by a given amount, say, 10%. 

Another important mutagen is ultraviolet light. Recent concern about the depletion of the atmospheric ozone layer by chlorofluorocarbon compounds (CFCs) is due to the role of the ozone in absorbing UV radiation before it can cause mutations in the organisms at the earth s surface. All the DNA bases efficiently absorb UV and become chemically reactive as a result. The formation of pyrimidine dimers from adjacent thymidine residues in DNA interferes with replication and transcription of DNA. See Figure 8-14. 

UV radiation from the Sun is a ubiquitous and important source of DNA damage that contributes to the high incidence of skin cancer in exposed individuals. The UV radiation spectrum has been arbitrarily divided into the UV-A component (wavelengths 320 00 nm), the UV-B component (wavelengths 295-320 nm), and the UV-C component (wavelengths 100-295 nm). Penetration of the atmospheric ozone layer is weak for wavelengths below 300 nm. Thus, solar UV radiation primarily comprises the UV-A and UV-B components. However, experimental studies typically involve the use of UV-C radiation from germicidal lamps with a sharp emission maximum at 254 nm. Since... 

CFCs (chlorofluorocarbons) - As applied to polyurethane foams, blowing agents having chlorine and fluorine in their chemical structure. CFCs are gradually being replaced by other blowing agents because of concern about their adverse effect on the atmospheric ozone layer. 

It should be mentioned that in the atmospheric reservoir gases other than those listed in Table 1 can be found. For the sake of simplicity the atmospheric cycle of these gases will not be presented. However, the effect of different halogen compounds, including anthropogenic chlorofluorocarbons, on the stability of atmospheric ozone layer will be discussed, in relation to the ozone budget. 

The emission of chlorine and bromine compounds also is a matter of interest, because stratospheric ozone is easily destroyed by these halogen species (Stolarski and Cicerone, 1974 Molina and Rowland, 1974 Wofsy et al, 1975). Chlorine and bromine atoms are present in a variety of industrial products dispersed in the atmosphere, but many of these are rapidly destroyed at low altitudes and therefore do not reach the middle atmosphere where the bulk of the atmospheric ozone layer resides. Some industrial compounds do, however, have sufficiently long... 

The possible effects on the atmospheric ozone layer of volatile organic pollutants, inclusive of acetylene, were assessed by measuring the reaction rate with OH radical at room temperature, in the presence of oxygen pollutants were classified as reactive or non-reactive according to US-EPA ". ... 

Many years ago, the USA agreed with other comitries to stop using CECs (chlorofluorocarbons) as propellants in aerosols because they contribute to the destruction of the atmospheric ozone layer. Until recently, formulary exemptions were made for inhalers that contained CECs. The pMDI remains the most popular type of inhaler used by people with asthma, with a different propellant called hydrofluo-roalkane (HEA), also referred to as CFC-free propellant. It may be less powerful and have a different taste which could impair compliance and efficacy in children accustomed to using their old inhaler. The DPI is the... 

Another example of problems resulting from the release of chemicals into the environment is displayed by the chlorofiuorocarbons (CFCs). These very stable compounds, which have been widely used as refrigerants and aerosol propellants, are believed to be damaging the atmospheric ozone layer. They are now being replaced by more ozone-friendly products. 

Fluorochlorinated hydrocarbons have many of the properties of the ideal blowing agent. They are nonflammable, are nontoxic, and have high heat stability. Monofluorotrichloromethane (Freon 11) and 1,2,2-trifluorotrichloro-ethane (Freon 113) are liquid at ambient temperature and they are used in the production of rigid and flexible foams of polyurethanes, PVC, and so on. But they have a negative effect on the atmospheric ozone layer that protects the surface of the earth from short-wavelength radiations from the sun, and for this reason, their use is limited by legislation in many countries. 

Ozone depletion potential (ODP) (cleaning) A rating for the potential of a vapor to deplete the atmospheric ozone layer. See also Global warming potential (GWP). 

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