Photochemical
smog is a mixture of pollutants including particulates , nitrogen oxides , ozone
, aldehydes , peroxyacetyl nitrate (PAN) , unreacted hydrocarbons , etc . A
brownish haze and painful eyes are often indicators of photochemical smog .
Nitrogen dioxide is responsible for the brownish colour of the haze .
Conditions
for the formation :
The reactions leading to the formation of photochemical smog are
initiated by sunlight and involve hydrocarbons and nitrogen oxides emitted from
automobiles . The combination of sunlight , the catalysis by particulates and
the abundant pollutants present in modern cities provide favourable conditions
for smog formation .
Process
of formation :
Nitrogen dioxide from automobile exhaust first absorbs sunlight and
breaks down into nitrogen monoxide and reactive oxygen atom:
The oxygen atom then reacts with other components of automobile
exhaust(e.g. unburnt hydrocarbons) and those of the atmosphere (e.g. oxygen) in
a complex series reactions to produce a variety of lachymatory and toxic
chemicals (e.g. peroxyacetyl nitrate (PAN)).
Very much simplified, some of the principal reactions in the formation of
photochemical smog are:
Effect
of photochemical smog:
It can cause headaches, eye, nose and throat irritations impaired lung
function, coughing and wheezing.
Ozone attacks the C=C linkage in rubber, in a process known as
ozonolysis which we have met in organic chemistry:
It
can therefore cause rubbers and fabrics to deteriorate and car tires to crack.
It can damage plants, leading to the loss of crops.
