In the atmosphere, the molecules which are present are constantly colliding and moving with each other, as illustrated by “Kinetic-Molecular Theory”. During the daytime, also the atmosphere is constantly lit up. As a consequence, light energy absorption held by the molecules of atmosphere can generate “photochemical reactions.” Such reactions don’t appear at regular atmospheric temperatures without any sign of light.
These corresponding reactions play a significant role to determine atmospheric composition itself and the luck of several chemical species which contributes to pollution of air.
NO2, Nitrogen Dioxide, is one of the most extremely active photochemical species present in the atmosphere. The molecule of NO2 is the example based on free radical as it consists of the unpaired electron.
When the molecule of NO2 absorbs a light photon consisting of ‘h’ energy, the molecule is all-together elevated to higher energy level; It results in becoming the “electronically excited molecule”, which is signified by the asterisk (*).
. NO2 (g) ——> . NO2*
The stimulated or excited molecule might result in re-emitting a ‘photon of light’, or energy required to make a nitrogen monoxide (NO) molecule, using NO bond and the (O) oxygen atom.
. NO2 *(g) ——> . NO(g) + . O . (g)
O and NO both are free radicals as they have one or more than one unpaired electrons which are individually represented by the dot. Another mechanism called, “Photodissociation” is a process of radical formation, in which the molecule absorbs the ultraviolet photon and it produces 2 free radicals in the form of products.
Molecular oxygen can be photo dissociated so as to form 2 – O (Oxygen) atoms. A few of the free radicals, like oxygen atoms, react almost with other molecules or atoms instantly.
O2(g) —–> . O .(g) + . O . (g)
Some molecules such as NO2 are not that reactive and they are very much stable to have their existence for quite a long time. Most of the radicals are having highly reactive nature and they are for short-term.
Particles have the probability to enter the exhibited environments, for instance, as a consequence of industrial operation or dust storm. In the atmosphere, they may even be formed through chemical reactions. Also, they may be chemically and physically changed as a consequence of these types of reactions.
The formation of Photochemical smog particles takes place when Nitrogen Dioxide (NO2) is photolyzed and as a result, atomic oxygen has a reaction with specific organic vapors having their presence in such ‘smog’ to give rise to an array of complex reactions.
There is ample proof that such reactions are also occurring in circulating environments, including essential oils which are emitted through vegetation. Both Sulfur dioxide and Hydrogen sulfide are oxidized in smog and the ambient environments by different processes. The final products contain droplets of sulfuric acid and different sulfates.
Drops of sulfuric acid produced by oxidation of SO2 and H2S which reacts with atmospheric NH3 i.e. ammonia to make ammonium sulphate.