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Importance of stratospheric ozone

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The Importance of Ozone The chemistry of the stratosphere is dominated by the chemistry of ozone. Ozone is of importance because it absorbs damaging solar ultraviolet radiation in the 220 - 290 nm range (although the absorption tails off to wavelengths longer than 300 nm). Nothing else present in the atmosphere absorbs in this region, and ozone, as we shall see has a somewhat tenuous existence. We can see how well it protects the Earth's surface by considering absorption at 250 nm, the position of the ozone maximum. Using the Beer-Lambert Law, ln(I0/I) = ?cl, with ? ? 10-17 cm2 molecule-1, c ? 2.5 ? 1019 molecule cm-3, and l ? 0.3 cm,1 log(I?/Isurface) = 10-17?0.3?2.5?1019/2.303 = 32.6 i.e., less than one part in 1032 of the radiation at ? = 250 nm reaches the Earth's surface. Life on the surface of the planet could not survive without the protection of the ozone layer. (See Wayne, pp. 665 ? 673 for a description of the evolution of this aspect of the Earth's atmosphere.) Oxygen-only Chemistry Stratospheric ozone is generated from the photolysis of O2 in an oxygen-only scheme known as the Chapman Scheme O2 + h? ? O + O (3.1) O2 + O + M ? O3 + M (3.2) O + O3 ? O2 + O2 (3.3) O3 + h? ...read more.


Models can predict the position of the ozone maximum quite well, but predicts ozone concentrations that are a factor of two too high, when nothing but oxygen-only chemistry is used. Something is destroying ozone, but it was not immediately clear to scientists what the problem was. The problem is that there are many things that can destroy ozone, but they are not present in very high concentrations. For example, Cl atoms will react with ozone, but [O3]/[Cl] is typically 10 000. The problem is solved when catalytic cycles are considered. Cl atoms can destroy O3 and then be regenerated in cycles such as: Cycle 1 Cl + O3 ? ClO + O2 (3.6) ClO + O ? Cl + O2 (3.7) Net O + O3 ? O2 + O2 (3.3) The net result of the cycle is reaction (3.3), the only reaction that destroys odd oxygen in the oxygen-only scheme. The impact of Cl atoms is to increase the rate of this process by reducing the activation barrier. Typically, one Cl atom can destroy ca. 10 000 O3 molecules. In this cycle, Cl could be replaced by NO, H, OH, Br or I. Various other cycles can be written. At high altitudes Cycle 2 O + HO2 ? OH + O2 (3.8) O + OH ? H + O2 (3.9) Net O + O + M ? ...read more.


HNO3 + M (3.18) HNO3 + h? ? OH + NO2 (3.19) Typically 50 % of the stratospheric load of NOx exists as HNO3.2 Cycle 11 Cl + CH4 ? HCl + CH3 (3.20) OH + HCl ? H2O + Cl (3.21) Typically 70 % of the stratospheric load of ClOx exists as HCl. Other reservoir compounds are, HOCl, HO2NO2 and ClONO2. Sources of Stratospheric Trace Constituents NOx Natural: N2O formed from incomplete microbiological nitrification (NH4+ ? NO3-) or dentrification (NO3- ? N2). N2O + O(1D) ? NO + NO Polluted Direct injection of NO by SST (Concorde). HOx Natural: CH4 from enteric fermentation in ruminants (cattle) CH4 + O(1D) ? OH + CH3 Polluted: Direct injection of H2O by SST. H2O + O(1D) ? OH + OH ClOx Natural: Emission of CH3Cl from oceans, and burning vegetation. CH3Cl + h? ? CH3 + Cl ? ? ClO CH3Cl + OH ? H2O + CH2Cl ? ? ClO Polluted: Emission of CFCs in the troposphere, followed by photolysis (Similarly for BrOx and IOx.) 1 although the concentration of O3 varies through the stratosphere, the total ozone column abundance = 300 Dobson Units (which means at STP, the ozone layer would be 3 mm thick), and the molecular concentration at one atmosphere is about 2.5 ? 1019 molecule cm-3. 2 Note that NOx refers to NO and NO2. NOy refers to NOx plus, N2O5, HNO3, HO2NO2, PAN (but not N2O). ?? ?? ?? ?? ...read more.

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