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METO 621

METO 621. Lesson 24. The Troposphere. In the Stratosphere we had high energy photons so that oxygen atoms and ozone dominated the chemistry. In the troposphere we have lower energy photons, and the chemistry is dominated by the OH and NO 3 radicals.

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METO 621

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  1. METO 621 Lesson 24

  2. The Troposphere • In the Stratosphere we had high energy photons so that oxygen atoms and ozone dominated the chemistry. • In the troposphere we have lower energy photons, and the chemistry is dominated by the OH and NO3 radicals. • OH is generated photochemically (i.e. only during the day), NO3 is rapidly photolyzed during the day, so it can only survive at night. • NO3 is generally less reactive then OH, its peak concentration is higher. • OH provides an efficient scavenging mechanism for both natural and anthropogenic trace constituents

  3. Dry and Wet Deposition • Dry deposition – removal of gases and particles by a direct transfer from the atmosphere to the surface. • Wet deposition – removal of gases and particles carried to the surface in water – rain, snow, fog etc. • Dry deposition is known for SO2, O3, CO2, and SO3. • Wet deposition of gaseous species requires that they be water soluble. Terms used are rainout, or washout. • Acid rain is an example of the rainout of sulfurous and nitric acids, produced in polluted atmospheres.

  4. Dry and Wet Deposition

  5. Oxidation and Transformation • Let us assume that no methane has been oxidized. • Then OH is produced by the following reactions O3 + hn → O*(1D) + O2(1Dg) O*(1D) + H2O → OH + OH • It should be noted that the O*(1D) does not stay around for long, and is quenched to the ground state. The ground state then quickly combines with molecular oxygen to reform ozone. • The OH formed reacts mainly with CO and CH4 OH + CO → H + CO2 OH + CH4 → CH3 + H2O

  6. Oxidation and Transformation • These compounds then react with molecular oxygen H + O2 + M → HO2 + M CH3 + O2 + M → CH3O2 + M • If the concentration of NO is very low then further reactions convert the peroxy radicals to water vapor and carbon dioxide. • However if the nitrogen oxides are present then we get HO2 + NO → OH + NO2 CH3O2 + NO → CH3O + NO2 • This then followed by NO2 + hn → NO + O O + O2 + M → O3 + M

  7. Oxidation and Transformation

  8. Oxidation and Transformation • Analogous reactions can be written for the higher hydrocarbons, e.g. C8H18 – octane. • If we assign the formula RH to these hydrocarbons then we get RH + OH → R + H2O R + O2 + M → RO2 + M RO2 + NO → RO + NO2 • This is the basis of photochemical smog. • The photolysis of the resultant NO2 is the only known way of producing ozone in the troposphere. • The RO is further reduced to aldehydes and other organic compounds by OH, all of which can eventually produce ozone.

  9. Oxidation and Transformation

  10. Oxidation and Transformation

  11. The nitrate radical • The nitrate radical NO3 plays a significant role in the troposphere. • It is formed by the reaction NO2 + O3→ NO3 + O2 • During the day it is rapidly photolyzed NO3 + hn → NO2 + O or NO + O2 • However at night the NO3 is stable and can react with hydrocarbons NO3 + RH → HNO3 + R • R can now react with molecular oxygen and begin the oxidation process

  12. The nitrate radical

  13. The nitrate radical

  14. Chemical lifetimes wrt OH and O3

  15. Schematic of biogenic emissions

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