Smog begins with primary pollutants, substances that may be harmless and unreactive. Secondary pollutants, formed photochemically from the primary pollutants, are responsible for the buildup of smog.
The primary pollutants include nitrogen oxides, particularly nitric
oxide and nitrogen dioxide, and organic compounds of volatile nature.
The secondary pollutants consist of peroxyacetyl nitrates (PAN),
tropospheric ozone, and aldehydes.
Sunlight causes the photo-dissociation of nitric oxide and oxygen.
Nitric oxide released into the atmosphere is quickly oxidised to nitrogen dioxide.
$$2NO+O_2\rightarrow 2NO_2$$
Sunlight causes photochemical decomposition of $$NO_2$$ into $$NO$$ and $$O$$.
$$NO_2+h\nu\rightarrow NO+O$$
Atomic oxygen is a highly reactive species that can initiate a number of reactions, one of which is the formation of ozone.
$$O_2+O+M\rightarrow O_3+M$$
Then the reaction of oxygen atoms with water takes place. This results in formation of hydroxyl radicals.
These hydroxyl radicals oxidize hydrocarbons which results in formation of hydrocarbons radicals.
Then the oxidization of hydrocarbons takes place which results in the
formation of aldehydes.
Aldehyde oxidizes to formaldehyde peroxides as well as aldehyde peroxy acids.
A second process involving the participation of unburned hydrocarbons may be depicted as:
$$O+RH\rightarrow {\dot{R}}+{\dot{RCO}}$$
$${\dot{RCO}}+O_2\rightarrow {\dot{RCO_3}}$$
$$ {\dot{RCO_3}}+NO_2\rightarrow RCO_3NO_2$$
$${\dot{CH_3COOO}}+NO_2\rightarrow CH_3COOONO_2$$
$$CH_3COOONO_2$$ i.e. peroxyacetyl nitrate (PAN) pollutant is notable for causing eye
and respiratory irritation and is responsible for
eye-watering when there is heavy pollution.