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 + 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
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
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 + 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
The nitrate radical
The nitrate radical
Chemical lifetimes wrt OH and O3
Schematic of biogenic emissions