Question

Why is photochemical smog more pronounced in the early morning hours? (use reactions as part of the answer)

Answer 1

Primary chemical species or primary pollutant of a photochemical smog is Nitrogen Oxides (NOx), Carbon Monoxide (CO) and several hydrocarbons compounds (from petroleum fuel, gasoline, and diesel fuel) like ethylene, formaldehyde, and acetaldehyde. In the morning time, the concentration of these primary chemical species has more concentration as compared to daytime. And that’s why the photochemical smog more pronounced in the early morning hours. As day time comes the concentration of primary pollutant (an air pollutant emitted directly from a source) decreases and the concentration of secondary pollutant decreases (not directly emitted, it forms when primary pollutant react with the atmosphere).
N₂ +O₂2NO (Primary pollutant)
2NO +O₂ 2NO₂ (Secondary Pollutant)

O (radical oxygen atom) +O₂ O₃ (Secondary Pollutant)

NO combines with atmospheric oxygen and within two hours it forms NO2 which absorbs photons from sun light and then split it into two components, Nitric Oxide, and radical Oxygen. And this single oxygen or oxygen radical is the source of the main cause of photochemical smog. This oxygen radical atom quickly combines with atmospheric oxygen (O2) molecule and forms Ozone (O3) which further react with NO (nitric oxide) to form NO2 ( nitrogen dioxide).
O₃ +NO NO₂ + O2
This reaction could be reversible depending on the amount of Nitrogen Dioxide and Nitric Oxide in the atmosphere. If the ratio of Nitrogen Dioxide is three times more than Nitric Oxide then Ozone will be formed and if this ratio is less than three to one then the Ozone will be destroyed automatically from the atmosphere and the photochemical smog could be eliminated.