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 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

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.

Dry and Wet Deposition

Oxidation and Transformation

• Let us assume that no methane has been oxidized.• Then OH is produced by the following reactions

O3 + h→ O*(1D) + O2(1g)

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

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 + h→ NO + O

O + O2 + M → O3 + M

Oxidation and Transformation

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.

Oxidation and Transformation

Oxidation and Transformation

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 + h→ 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

The nitrate radical

The nitrate radical

Chemical lifetimes wrt OH and O3

Schematic of biogenic emissions