Reactions in Atmosphere

Reactions in Atmosphere

The various chemical and photochemical reactions taking place in atmosphere, mostly depends upon, temperature, composition, humidity and intensity of sunlight. Photochemical reaction take place by the absorption of solar radiations in the UV region. Absorption of photons by chemical species gives rise to electrochemically exited molecules, which can bring about cetain reactions. The electrochemically excited molecules may undergo any of the following changes-
• Reaction with other molecules on collision.
• Polymerization.
• Internal rearrangement.
• Dissociation
• De-excitation or de-activation of fluorescence.

Photochemical reaction included the following three steps-
1. absorption of radiation
2. Primary reaction
3. Secondary reaction.

Three steps of compound mainly gives photochemical reaction-
1. reaction involve in oxide of N2 (NO, NO2, NO3 etc)
2. Reaction involve in oxide of sulphuric (SO2, SO3 etc).
3. Oxidation of organic compound.


Reaction involve in oxide of Nitrogen

The oxide of nitrogen in the atmosphere are NO, NO2 and N2O. In the stratosphere N2O undergo photochemical decomposition to NO, which in turn depletes the protective ozone layer.

N2O + hv N2 + O.

N2O + O. N O. + N O.


N O. + O3 NO2 + O2

The most important photochemical reaction is dissociation of NO2

N2O + hv NO + O.

This nitric acid (NO) and pungent red-brown nitrogen dioxide (NO2) are important constituent of polluted air. The NO so formed may be O3 or more slowly by O2. thus leading to a cyclic chain reaction. The chain may be broken when the nitrogen dioxide is completely converted into HNO3 by hydration and cyclic oxidation in presence of fog or photochemical smog.

The reaction may take place are-

2NO + O2 2NO2

NO + O 3 NO2 + O2

2NO2 + O3 N2O5 + O2

4NO2 + O2 + 2H2O 4 HNO3

N2O5 + H2O 2HNO2 + O2

The HNO2 and HNO3 may also undergo photochemical dissociation as follows-


HNO3 + hv NO2 + HO.

HNO2 + hv NO + HO.

HNO2 + hv NO2 + H.

In strarosphere, NO2 may reacts with HO. And form nitric acid.

NO2 + HO. HNO3

The HNO3 so formed as acid rain or is converted into particulate nitrates due to neutralization by NH3 or particulate lime.


Reaction involves oxides of sulphur

SO2 present in the atmosphere, absorb solar radiation in the range of 300-400 nm and produces electrochemically exited ststes of SO2. This undergo oxidation to SO3 and in presence of water vapour
This converted to H2SO4.

SO2 + hv SO2.

SO2. + O2 SO4.

SO4. + O2 SO3 + O3

SO3 + H2O H2SO4

The overall reaction in the presence of sunlight and relative humidity 30-90 % may be represented as-

SO2 + O2 + H2O H2SO4

This photochemical oxidation of SO2 to H2SO4, aerosol is greatly accelerated in the presence of olefinic hydrocarbons and oxides of nitrogen which are usually present photochemically smog.

The oxidation of SO2 also take place by interaction with the free radical HO. Present in photochemical smog.


SO2 + HO. HOSO2.


HOSO2. + O2 HOSO2O2.

HOSO2O2. HOSO2O. + NO2.

In relative humid atmosphere, SO2 may also oxidize by water droplets of aerosols, which are accelerated in presence of ammonia catalysts such as oxides of Mn, Fe, Cu, Ni etc.

NH3 + SO2 + H2O NH4+ + HSO3-

NH3 + HSO3- NH4+ + SO32 –

SO2 is a pollutant responsible for smog formation, acid rain and corrosion of metals and alloys.


Reaction involves in oxidation of organic compound

Organic compounds, e.g., hydrocarbons, aldehyde, ketone, absorb solar radiations and undergo various photochemical and chemical reaction involving free radicals, some of these reactions are catalysed by sort and metal oxides-

CH4 + O2 H3C + HO.

CH4 + HO. H3C + H2O


H3C + O2 + M (third body) H3COO. + M

CH3CH CH2 + HO. CH3C.H CH2OH

RCHO + hv R. + HC.O

RCRO + hv R. + R.CO

RCOOO. + NO RCOOONO2 (peroxyacetyl nitrile PAN)

Where, R is an alkyl or aryl radical or even a H2 atom. The alkyl radical, which may reacts with oxygen to form a peroxy radical, which in turn may reacts with another oxygen atom to give O3. The peroxy radical may reacts with NO2 to give peroxy acyl nitrile(PAN), formaldehyde and various polymeric compounds which reduce visibility.