Photochemical smog is a type of air pollution caused by sunlight reacting with primary pollutants like nitrogen oxides and volatile organic compounds in the atmosphere. This process forms secondary pollutants such as ozone. The reactions that produce photochemical smog involve nitrogen oxides reacting with oxygen to form nitrogen dioxide and ozone, and hydrocarbons reacting with oxygen radicals to form aldehydes, ketones, and peroxide radicals, which further react to produce more ozone and compounds like peroxyacetyl nitrate (PAN).

1. Photochemical Smog
(Formation & Reactions)
ES 2209
ATMOSPHERIC POLLUTION

2. What is Photochemical
Smog
Photochemical Smog means the
condition called haze in the atmosphere
accompanied by high levels of ozone and
nitrogen oxides, caused by the action of
sunlight on pollutants.
Photochemical Smog is composed of
primary and secondary pollutants.
Primary pollutants, which include
nitrogen oxides and volatile organic
compounds.
Secondary pollutants, like ozone, result
from the reaction of primary pollutants
with ultraviolet light

3. REACTIONS DURING PHOTOCHEMICAL SMOG:
1. NO+O2 NO2 O+NO
2. O+O2 O3
3. O3+NO O2+NO2
4. Hydrocarbon + O RCO● RCO3
●
5. RCO3
● +O2 RCO2
●+O3
6. RCO3
● +NO RCO2
● +NO2
7. RCO3
● +NO2 RCO3NO2 (PAN)
+UV
O2

4. Reaction processES:
1. Nitric Oxide combine with oxygen to form Nitrogen dioxide. The u.v. rays strike the NO2, it
breaks off a single oxygen radical (O)
2. The single oxygen radical helps form ozone (O3) .
3. Ozone and nitric oxide converts into O2 & NO2
4. Hydrocarbon reacts with oxygen free radical and forms different types of aldehydes and
ketones (RCO●). Some of the constituents combine with oxygen and form peroxide
radicals (RCO3
●).
5. The peroxide radical combines with oxygen to enhance the production of ozone.
6. This reaction shows the more stable formation of peroxide radicals. Some peroxide
radical combines with nitric oxide and forms nitrogen dioxide. Which again breaks into
oxygen free radicals (equation 1) and those oxygen free radicals again forms ozone
(equation 2)
7. Again peroxide radicals reacts with nitrogen dioxide and then forms Peroxyacetyl nitrate
or PAN (RCO3NO2).