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. OH is generated photochemically (i.e. only during the day), NO 3 is rapidly photolyzed during the day, so it can only survive at night. NO 3 is generally less reactive then OH, its peak concentration is higher. OH provides an efficient scavenging mechanism for both natural and anthropogenic trace constituents

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 SO 2, O 3, CO 2, and SO 3. 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.

Dry and Wet Deposition

Oxidation and Transformation Let us assume that no methane has been oxidized. Then OH is produced by the following reactions O 3 + h  → O*( 1 D) + O 2 ( 1  g ) O*( 1 D) + H 2 O → OH + OH It should be noted that the O*( 1 D) 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 + CO 2 OH + CH 4 → CH 3 + H 2 O

Oxidation and Transformation These compounds then react with molecular oxygen H + O 2 + M → HO 2 + M CH 3 + O 2 + M → CH 3 O 2 + 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 HO 2 + NO → OH + NO 2 CH 3 O 2 + NO → CH 3 O + NO 2 This then followed by NO 2 + h  → NO + O O + O 2 + M  → O 3 + M

Oxidation and Transformation

Analogous reactions can be written for the higher hydrocarbons, e.g. C 8 H 18 – octane. If we assign the formula RH to these hydrocarbons then we get RH + OH → R + H 2 O R + O 2 + M → RO 2 + M RO 2 + NO → RO + NO 2 This is the basis of photochemical smog. The photolysis of the resultant NO 2 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.

Oxidation and Transformation

The nitrate radical The nitrate radical NO3 plays a significant role in the troposphere. It is formed by the reaction NO 2 + O 3 → NO 3 + O 2 During the day it is rapidly photolyzed NO 3 + h  → NO 2 + O or NO + O 2 However at night the NO 3 is stable and can react with hydrocarbons NO 3 + RH → HNO 3 + R R can now react with molecular oxygen and begin the oxidation process

The nitrate radical

Chemical lifetimes wrt OH and O 3

Schematic of biogenic emissions