Chemistry of Ozone in the Stratosphere Levels of stratospheric ozone have been d
Gunn, Liezel, Contributing Editor/Writer has reference to this Academic Journal, PHwiki organized this Journal Chemistry of Ozone in the Stratosphere Levels of stratospheric ozone have been dropping NASA – http://toms.gsfc.nasa.gov Decreasing Levels of stratospheric ozone is harmful There has been an increase in the number of cases of skin cancer in addition to cataracts Evidence of damage to plant in addition to marine life Note: tropospheric ozone is harmful, stratospheric ozone is beneficial.
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Increase in yearly ultraviolet radiation: The % increase from 1980 to 1997 in UV radiation (causing the skin to turn red) is calculated using observed total ozone values from the TOMS satellite instruments in addition to assuming clear sky conditions. Environment in the European Union at the turn of the century, European Environment Agency, Chapter 3.2. Ozone-depleting substances Structure of Ozone, O3 Where is ozone found in the atmosphere NASA Goddard Space Flight Center Note, higher concentration in stratosphere, compared with troposphere
Chemical Kinetics in addition to Photochemical Data as long as Use in Stratospheric Modeling – JPL Publication97-4 Solar Flux Role of Ozone in the Stratosphere Solar Flux Chemical Kinetics in addition to Photochemical Data as long as Use in Stratospheric Modeling – JPL Publication97-4 Role of Ozone in the Stratosphere Absorption Spectrum of Ozone Role of Ozone in the Stratosphere
The Ozone Depletion Phenomenon, Beyond Discovery, National Academy of Sciences Role of Ozone in the Stratosphere UV A (~400 to 350 nm) not absorbed by earths atmosphere UV B (~ 350 to 270 nm) partially absorbed by earths atmosphere UV C (~270 to 150 nm) completely absorbed by earths atmosphere Role of Ozone in the Stratosphere Chapman mechanism – Sidney Champman, 1930 How is ozone as long as med in the stratosphere O2 + hn (l < 242 nm) -> O + O k1 ~ 5 x 10-11 s-1 2[O + O2 + M-> O3 + M] k2 ~ 5.6 x 10-34 cm6 mol-2 s-1 O3 + hn -> O + O2 k3 ~ 9.5 x 10-4 s-1 O + O3 -> 2 O2 k4 ~ 1 x 10-15 cm3 mol-1 s-1 Note: k1 in addition to k3 depend on intensity of light; above values are as long as mid day
Ozone: What is it in addition to why do we care about it, NASA Facts, Goddard Space Flight Center This mechanism, which describes how sunlight converts the various as long as ms of oxygen from one to another, explains why the highest contents of ozone occur in the layer between 15 in addition to 50 km – the ozone layer Kinetics of Chapman Mechanism Rate of as long as mation of O in addition to O3 d[O]/dt = 2k1[O2] -k2[O][O2][M] + k3[O3] – k4[O][O3] d[O3]/dt = k2[O][O2][M] – k3[O3]-k4[O][O3] Steady-State Approximation d[O]/dt = d[O3]/dt= 0
Kinetics of Chapman Mechanism Can re-write [O3] as: Since the rate constants in addition to concentration of species are known, can shown that: Hence, [O3] depends on rate of reaction 2 in addition to the intensity of light Kinetics of Chapman Mechanism Reaction 2 is slow (termolecular); makes ozone vulnerable to ozone-depleting reactions 2[O + O2 + M-> O3 + M] k2 O3 + hn -> O + O2 k3 Later measurements showed appreciable deviations from Chapman’s theory. Calculations of ozone concentration based on the Chapman mechanism were considerably higher than observed ones. Must be other chemical reactions contributing to the reduction of the ozone content.
Competing Reactions Marcel Nicolet: HOx cycle H, OH in addition to HO2 species as long as med by reaction of excited O atoms with H-containing atmospheric species like H2O in addition to CH4 O3 + hn (l < 310 nm)-> O + O2 O + H2O -> OH + OH O + CH4 -> CH3 + OH H2O + hn -> H + OH Reactions of HOx species with O3 OH + O3 -> HO2 + O2 HO2 + O -> OH + O2 Net Reaction O + O3 -> 2O2 Ozone Depletion Paul Crutzen: NOx Cycle Competing Reactions NOx species are produced during the reaction of O atoms with N2O (produced in the soil by bacteria) O + N2O -> 2 NO
Reactions of NOx species with O3 NO + O3 -> NO2 + O2 NO2 + O -> NO + O2 Net Reaction O + O3 -> 2O2 Ozone Depletion Paul Crutzen, ~ 1970 The first man-made threat to the ozone layer was noted by Harold Johnston (1971): supersonic aircrafts These aircraft would be capable of releasing nitrogen oxides right in the middle of the ozone layer at altitudes of 20 km. This was also the start of intensive research into the chemistry of the atmosphere. Competing Reactions Mario Molina, Sherwood Rowl in addition to (1974): ClOx cycle ClOx species are produced from chlorofluorocarbons (CFCs) in addition to methyl chloride CFCs are artificially produced; methyl chloride is a naturally occuring chemical. Examples of CFCs : Freons (CFCl3, CF2Cl2) CCl2F2 + hn -> CF2Cl + Cl CCl2F2 + O -> CF2Cl + ClO
Reactions of ClOx species with O3 Cl + O3 -> ClO + O2 ClO + O -> Cl + O2 Net Reaction O + O3 -> 2O2 Ozone Depletion 1974 – Mario Molina, Sherwood Rowl in addition to Paul Crutzen, Mario Molina, Sherwood Rowl in addition to 1995 Nobel Prize in Chemistry – as long as their work in atmospheric chemistry, particularly concerning the as long as mation in addition to decomposition of ozone” http://www.nobel.se/chemistry/laureates/1995/press.html Consequences of Competing Reactions Catalytic Reactions Cl + O3 -> ClO + O2 ClO + O -> Cl + O2 – lower activation energy Ea as long as Chapman mechanism = 17.1 kJ/mol Ea as long as ClOx reaction = 2.1 kJ/mol catalyst catalyst intermediate intermediate
Depleting reactions are NOT independent of each other; all occur simultaneously Effect of competing reaction on rate of ozone as long as mation Consequences of Competing Reactions NET LOSS OF OZONE Sources of ozone depleting molecules in the stratosphere Naturally occuring species (H2O, N2O, CH4) Artificial, man-made species CFCs (CCl3F,CCl2F2, etc.) CCl4, CHCl3 HBFC (CHFBr2,CHF2Br) CH3Br NO from supersonic aircrafts The artificial compounds have the most severe effect What is the Ozone Hole First observed in 1985 by the British Antarctic Survey – realization of ozone depleting reactions Every spring, a huge hole in atmospheric levels of ozone is observed over the Antarctic. NASA Goddard Space Flight Center July – Sept 2001
References NASA Goddard Space Flight Center (www.gsfc.nasa.gov/) EPA (www.epa.gov) Center as long as Atmospheric Science, Cambridge University (www.atm.ch.cam.ac.uk/tour/index.html) British Antarctic Survey http://www.antarctica.ac.uk/ Chemical Kinetics in addition to Dynamics,Ch 15, J. Steinfeld, J. Francisco, W. Hase
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