Many key reactions in the atmosphere are photochemical reactions which are initiated by absorption of a photon of light. Such reactions can be written as if they were normal chemical reactions by substituting photon (hv) as one of the reactants:
NO2 + hv —— NO + O
The rate constant of such a reaction is given as:
– {d[NO2]/dt} = k” [hv][NO2]
This expression is not very useful because the second-order rate constant k” would probably vary dramatically with the energy of photon involved in the reaction. However, by taking a content flux of photon with respect to wavelength and incorporating it into a pseudo first-order rate constant, the rate expression becomes:
d[NO2]/dt = J[NO2]
J is special first-order constant that embraces the absorption coefficient of the reactant, quantum efficiency of the reaction in question and the solar spectrum and intensity at the altitude and latitude under consideration. Estimates of J for many atmospheric trace gases can be made with a little information on the photochemistry. For example, a typical mid-latitude mid-day value of JNO2 , for the photodissociation of nitrogen dioxide is 5 x 10-3 s-1 which suggests a residence time of 200 s.
Many photochemical are important in the atmosphere as they yield atoms or free radicals and these species are greatly more reactive than the molecular species found in the air. For example, photodissociation of NO2 yields atomic oxygen which can subsequently lead to the formation of ozone:
O + O2 + M ——-> O3 + M
where M is a third body i.e. a molecule such as molecular nitrogen which carries off the excess energy that might disrupt the ozone molecule. The ozone thus produced might further be photodissociated:
O3 + hv ——–> O(3P) or O(1D) + O2
If wavelength of photon is less than 315 nm, the oxygen atom is produced in excited 1D state, otherwise in the 3P ground state. The ground state oxygen will probable recombine with a molecule of oxygen to for ozone again i.e. no net reaction would occur. The excited oxygen atom may be collisionally de-excited to ground state, or more importantly, may react with water molecule providing a source of hydroxyl radical (OH):
O(1D) + H2O ——–> 2OH
Hydroxyl and hydroperoxy radicals in atmosphere
Hydroxyl radical (OH) produced by reaction of excited oxygen atom (formed by photodissociation of atmospheric ozone) with water as described above is probably the most important radical in the chemistry of troposphere. A number of reactions in the troposphere involving hydroxyl radical can produce hydrogen atom or hydroperoxy radical:
OH + CO —–> CO2 + H
OH + O ——> O2 + H
OH + O + M ——> M + HO2
OH + O3 ——> O2 + HO2
H + O2 + M ——> M + HO2
Very quickly a range of radicals and atoms can be generated. These highly reactive species are basic to the gas-phase chemistry of atmosphere. Due to their high reactivity, these species are naturally found in very low concentrations in the atmosphere. Their typical background concentrations are:
Hydroxyl radical – 700000per cubic cm; Hydroperoxy radical – 20000000 per cubic cm
High reactivity of these radicals is indicated by their short residence times. The residence time of OH radical is less than 1 s while that of HO2 radical is perhaps 1 minute.
Reactions of hydroxyl and hydroperoxy radicals with atmospheric trace gases
Reactions of many trace gases found in the atmosphere with the hydroxyl radical exert a profound effect on the composition of atmosphere. Reactions with some of the atmospheric trace gases are discussed below.
1. REACTION WITH SULPHIDES
Biologically produced sulfur gases are emitted into the atmosphere mainly as sulphides such as dimethyl sulphide, hydrogen sulphide and carbon disulphide. All these react with hydroxyl radical in the atmosphere.
(a) Dimethyl sulphide: This is the major sulphide emission into the atmosphere which reacts with OH radical as follows:
CH3SCH3 + OH —–> CH3SOH + CH3
O3 + CH3SOH —–> CH3SO3H
The product of these reactions is methyl sulphonic acid (CH3SO3H) and most of it persists in the ambient atmosphere though a relatively small amount may be oxidized through sulfur dioxide.
(b) Hydrogen sulphide: This gas in atmosphere is also attacked by OH radical as follows:
H2S + OH ——-> HS + H2O
The resulting bisulphide radical (HS) is oxidized through SO2 in a number of subsequent reactions. The SO2 can also be oxidized by OH and HO2 radicals:
SO2 + OH + M —–> HSO3 + M
SO2 + HO2 —–> SO3 + OH
Bisulfite radical (HSO3) and SO3 react with OH and water respectively to yield sulfuric acid which is the ultimate product of oxidation of atmospheric sulfur.
HSO3 + OH ———> H2SO4
SO3 + H2O ———-> H2SO4
(c) Carbon disulphide (CS2): This has been experimentally shown to be oxidized by OH radical yielding equal proportions of carbonyl sulphide and sulfur dioxide as final products. However, in atmosphere CS2 may not react with OH radical principally and its reactions with oxygen atoms may be more important.
2. REACTIONS WITH AMMONIA
Most of the atmospheric ammonia is removed through dissolution in liquid water in the atmosphere. However, ammonia is also attacked by OH radical though this reaction accounts for only a few percent of the ammonia removed from Earth’s atmosphere:
NH3 + OH ——> NH2 + H2O
Various subsequent reactions are possible:
NH2 + O ——> HNO + H
HNO + O2 —–> NO + HO2
The NO can be oxidized to NO2 which subsequently may react with OH radical to yield HNO3 and this is effectively removed from atmosphere through dissolution in rainwater.
NO2 + OH ——> HNO3
3. REACTION WITH CARBON MONOXIDE
Hydroxyl radical on reaction with carbon monoxide yields carbon dioxide and hydrogen radical.
CO + OH ———-> CO2 + H
4. REACTION WITH FORMALDEHYDE
Formaldehyde found in trace quantities and formed in various atmospheric reactions is oxidized by OH radical in the following manner:
HCHO + OH ——–> HCO + H2O
5. REACTION WITH METHANE
Methane is naturally emitted from earth’s surface. In the atmosphere, methane is oxidized by OH radical yielding methyl radical and water:
CH4 + OH ——–> CH3 + H2O
CH3 undergoes following reactions in the methane cycle in the atmosphere yielding CH3O2.
CH3 + O2 + M ——-> CH3O2 + M
CH3O2 reacts with hydroperoxy radical in the following manner:
CH3O2 + HO2 ——–> CH3COOH + O2
6. REACTION AMONG HYDROXYL HYDROPEROXY RADICALS
In the presence of some suitable molecular species (M), hydroxyl radicals may react with each other to for hydrogen peroxide:
OH + OH + M —— H2O2 + M
Hydroperoxy radical may be a more efficient route for the formation of hydrogen peroxide:
HO2 + HO2 + M —— H2O2 + M
H2O2 is highly water-soluble and a strong oxidizing agent so it probably plays an important role in oxidation processes within water droplets in the atmosphere.
Environment of Earth 